WEGA COLOR 3016 CRT TUBE PHILIPS A66-410X.

 PHILIPS  A66-410X.110° COLOUR TELEVISION PICTURE TUBE

Three-gun temperature -compensated shadow-mask rectangular colour television tube
with electrostatic focus, magnetic deflection and convergence, metal-backed three colour phosphor dot screen and internal magnetic shield. A high white luminance is obtained at near unity current ratio. Bein¢ temperature compensated, the shadow-mask makes for optimum field purity and good uniformity during warm-up. The design is such that minimum occurence of the moire effect is ensured. The tube has a reinforced envelope and therefore no separate safety screen is necessary. Typically, a legible picture will appear within 5 s.

 

 

 

PHILIPS CRT TUBE A66-410X ELECTRON GUN TECHNOLOGY IMPROVEMENT BY
CATHODE-RAY TUBE HAVING AN ASTIGMATIC LENS ELEMENT IN ITS ELECTRON GUN:
A cathode-ray tube, in particular a shadow mask colour tube, having at least one electron gun which comprises at least three grids. The second grid serves as an accelerating electrode. An astigmatic lens element is present in the region of the second and the third grid. As a result of this an elongate spot is formed on the picture display screen of an electron beam in the case of low beam current, while the spot remains substantially unchanged in the case of high beam current. This is of importance, inter alia, for removing moire patterns in a shadow mask colour tube without it being necessary to choose a particular distance of the mask holes.



1. A cathode ray tube for color television, comprising:

2. A cathode ray tube as claimed in claim 1 wherein said second grid comprises said astigmatic lens element and a circular aperture.

3. A cathode ray tube as claimed in claim 1 wherein said third grid comprises said astigmatic lens element and a circular aperture.

Description:

The invention relates to a cathode-ray tube comprising at least one electron gun for producing at least one electron beam and a picture display screen, each electron gun comprising a cathode and at least three grids, the second grid serving as an accelerating electrode.

In such a cathode-ray tube the electron beam is usually focused substantially on the picture display screen so that a spot is formed at that region which causes the relevant part of the picture display screen to luminesce. The electron beam scans the picture display screen according to a particular pattern so that the spot moves over the picture display screen. The size of the spot depends, inter alia, on the current strength in the beam. In order to obtain a greater brightness of the luminescing part of the picture display screen, a larger beam current is necessary, which goes hand in hand with a larger spot. The picture display screen is usually scanned along lines, namely horizontal lines. It is possible that these lines are visible during scanning of the picture display screen. This will occur in particular with a low beam current because in that case the spot is comparatively small. In the case of an increasing beam current because in that case the spot is comparatively small. In the case of an increasing beam current, the spots of two successive lines may partly overlap each other so that the separate lines become less visible and consequently the line structure of the reproduced picture as a whole is less conspicuous.

The line structure of the picture may give rise to difficulties in particular in a cathode-ray tube for displaying colour pictures which comprises a colour selection electrode having systematically arranged apertures. In such a cathode-ray tube a number of electron beams are produced and each electron beam causes a particular luminescent substance present on the display screen of the tube to luminesce, while the colour selection electrode (usually termed the mask) prevents the electrons of said beam from reaching one of the other luminescent substances. During operation of the tube, annoying moire patterns may occur as a result of interference between the line structure of the picture and the hole structure of the mask. It is known that the occurrence of moire patterns can be reduced by choosing the mutual distance of the mask holes in a particular manner in relation to the line distance. The line distance is a function of the dimension of the picture at right angles to the scanning lines (in the case of horizontal scanning lines this is the height of the picture) and of the number of scanning lines per picture. The mutual distance of the mask holes should hence be chosen in accordance with the height of the picture and the number of scanning lines per picture.

It has now been found, however, that the choice of the mutual distance of the mask holes is subject to certain restrictions for various reasons, so that the occurrence of moire patterns cannot always be reduced by means of the distance of the mask holes. If with a given height of the picture display screen a mask is available in which a desirable distance of the mask holes is present, the distance of the mask holes in a mask for a smaller picture display screen should be smaller in comparison with the former mask, since actually the line distances are smaller. A smaller distance of the mask holes cannot be realized without the dimension of the mask holes being also made smaller, because otherwise the mask cannot fulfil its function of preventing electrons of a given electron beam from reaching one of the other luminescent substances. A mask having small holes and in addition a large number of such small holes meets with technological problems in manufacturing the mask. Difficulties may present themselves in addition when providing the luminescent substanves of the picture display screen. Moreover, for a ready functioning of the tube, the distance between the mask and the picture display screen should also be reduced when the distance of the mask holes is reduced. In connection with the tolerances occurring upon securing the mask, a reduction of said distance presents difficulties. For quite different considerations the value of the maximum deflection angle has a limiting effect upon the choice of the distance of the mask holes. When the maximum deflection angle of the beam increases, the angle at which the beam impinges on the mask at a particular place increases. During operation of the tube the mask is heated by the electrons and the suspension of the mask usually is such that under the influence of said heating it experiences an axial displacement. Moreover, in places where the brightness in the displayed picture is great, as a result of which the mask can be locally heated comparatively strongly, an axial displacement of the mask may occur locally. As a result of the axial displacement of the mask, the electron spot which is formed by an electron beam behind a certain mask hole is displaced and said displacement is larger according as the deflection angle is larger. This displacement of the spot on the picture display screen presents the possibility for the passed electrons to impinge upon a luminescent substance other than the one intended, so that a so-called mislanding will occur. With a given transmission of the mask the extent of the mislanding is the larger according as the mutual distance of the mask holes is smaller. In tubes in which a large deflection angle, for example 110°, occurs, it is therefore desirable that the mutual distance of the mask holes be large. This impedes a given choice of the mutual distance of the mask holes for checking the occurrence of moire patterns.

It is therefore of importance to check the occurrence of moire patterns in a different manner. This can be done in a simple manner by using an astigmatic beam when it is ensured that the line structure on the picture display screen is less visible. As already noted, the line structure in the case of a large beam current is already less visible than in the case of a small beam current. It should therefore be ensured that the line structure becomes less visible also in the case of a small beam current without a measure taken for that purpose resulting in undesirable effects in another respect. The line structure can be made less visible in the case of a small beam current by increasing the dimension of the spot at right angles to the scanning lines, in which case the spots of two successive lines will start overlapping each other increasingly also in the case of a small beam current. However, the dimension of the spot in the direction of the scanning lines must not be increased in order that the definition of the displayed picture in that direction be not adversely influenced. In the case of a larger beam current the dimension of the spot is already larger and it should be seen that it is not additionally increased in the direction at right angles to the scanning line, in order that the definition of the displayed picture in the direction at right angles to the scanning lines be not unfavourably influenced. In addition, the dimension of the electron beam at the area of the deflection plane in the case of a large beam current may not be noteworthily increased because otherwise the half-shadow effect of the electron beam occurring behind the mask holes would be inadmissibly increased. On the basis of these considerations the beam in the case of low beam currents should be influenced more than in the case of high beam currents and said influence must mainly result in an increase of the spot at right angles to the scanning lines.

According to the invention, an astigmatic lens element is present in the electron gun in the region of the second and the third grid. The region of the second and the third grid is to be understood to mean herein the part of the electron beam between on the one hand the part of the second grid located on the side of the first grid and on the other hand the part of the third grid which is farthest remote from the cathode. The operation of said astigmatic lens element is as follows. A cross-over of the beam is formed by the cathode. the first and the second grid which serves as an accelerating electrode. The location of the cross-over usually lies in the region from the first grid to the third grid dependent upon the current strength of the beam and the configuration of the grids. In the case of low beam currents the cross-over lies between the first and the second grid, while with increasing beam current it moves in the direction away from the cathode towards the space between the second and the third grid. As a result of the fact that the cross-over in the case of low beam current lies between the first and the second grid and as astigmatic lens element is present in the region of the second and the third grid, the beam, in the case of a low beam current, is influenced by said astigmatic lens element so that an increase of a spot at right angles to this scanning line is obtained. When the beam current increases, the cross-over moves in the direction of the astigmatic lens element so that the influence hereof on the beam is reduced. When the cross-over coincides with the optical centre of the astigmatic lens element, the beam is substantially not influenced by this. So, in general, the location where the astigmatic element is provided will depend on the location of the cross-over and the extent of cross-over displacement upon variation of the beam current in the absence of said element.

The astigmatic lens element may be constructed in various mannners: a non-rotationally symmetric aperture in the second grid through which the beam passes; a non-rotationally symmetric aperture in the third grid through which the beam passes; an extra plate having a non-rotationally symmetric aperture through which the beam passes; this extra plate is present either between the first and the second grid, namely on the side of the second grid, or between the second and the third grid; a non-rotationally symmetric profile of the plate-shaped part of the second grid at right angles to the axis of the gun in which a rotationally symmetric aperture is present through which the beam passes; a non-rotationally symmetric profile of the plate-shaped part of the third grid at right angles to the axis of the gun in which a rotationally symmetric aperture is present through which the beam passes; in the case of a third grid having a tubular part parallel to the axis of the gun and a second grid having likewise a tubular part parallel to the axis of the gun, which part is present around the tubular part of the third grid, the presence of axial apertures in said part of the third grid; in the case of a second grid having a tubular part parallel to the axis of theggun, the presence of axial apertures in said part which are covered by plates located farther away from the axis of the gun and which form an extra electrode. A non-rotationally symmetric profile of a plate-shaped part of a grid can be realised by securing one or more plates to the plate-shaped part so that actually certain parts have a larger thickness or, when the thickness remains the same, giving the plate itself a non-rotationally symmetric profile.

The astigmatic lens element is present in particular in one of the grids. This has the advantage that no separate extra elements need be provided in the gun and/or extra voltages be applied. Since it is not so easy to realize with great precision a small non-rotationally symmetric aperture in a grid, it is to be preferred to use a circular aperture. The grid therefore comprises preferably an astigmatic lens element and a circular aperture.

In order that the invention may be readily carried into effect, it will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a cathode-ray tube,

FIG. 2 is a cross-sectional view of an electron gun,

FIG. 3 is a cross-sectional view of the electron gun shown in FIG. 2,

FIG. 4 shows the dimension of the spot on the picture display screen in two mutually perpendicular directions as a function of the beam current,

FIGS. 5a and 5b show diagramatically the path of rays in a part of the cathode-ray tube,

FIG. 6 shows another embodiment of the second grid, and

FIGS. 7a, 7b and 7c show another embodiment of the second grid.

Referring now to FIG. 1, the cathode-ray tube 1 comprises a system 2 which is shown diagrammatically and comprises three electron guns for generating three electron beams. The electron beams are converged onto a shadow mask 3 by means of a convergence device (not shown) present partly inside and partly outside the tube, after which they each impinge on certain parts of a picture display screen 4. The scanning of the picture display screen is carried out by a deflection device 5 which is shown diagrammatically.

FIG. 2 is a cross-sectional view of one of the three electron guns. The gun comprises a cathode 6, a first grid 7, a second grid 8, a third grid 9 and a fourth grid 10. The first grid 7 comprises a plate-shaped part 11 having a circular aperture 12. The second grid 8 comprises a plate-shaped part 13 having a circular aperture 14 and furthermore a circular cylindrical part 15. On the side of the plate-shaped part 13 facing the third grid 9 two plates 16 and 17 in the form of a segment of a circle are present. The third grid 9 comprises a plate-shaped part 18 having a circular aperture 19 and furthermore two circular cylindrical parts 20 and 21. The fourth grid 10 is in the form of a circular cylindrical sleeve.




FIG. 3 is a cross-sectional view of the gun shown in FIG. 2 taken on the line III -- III of FIG. 2. The two plates 16 and 17 in the form of segments of a circle are present on the plate-shaped part 13 of the second grid which comprises a circular aperture 14. The plates 16 and 17 constitute an astigmatic lens element.

In a particular case, the distance between the cathode 6 and the first grid 7 is 0.12 mm, between the first grid 7 and the second grid 8 at the area of the apertures it is 0.47 mm, between the second grid 8 and the third grid 9 at the area of the apertures it is 2.25 mm and between the third grid 9 and the fourth grid 10 it is 1.5 mm. The first grid 7 at the area of the aperture is 0.13 mm thick, the second grid 8 at the area of the aperture is 0.25 mm thick, the thickness of the plates 16 and 17 is 1 mm, and the third grid 9 at the area of the aperture is 0.25 mm thick. The length of the circular cylindrical part 20 is 6 mm, of the circular cylindrical part 21 it is 16.5 mm and of the circular cylindrical sleeve 10 it is 10.0 mm. The inside diameter of the part 20 is 5.5 mm, of the part 21 it is 9.5 mm. The diameter of the aperture 12 is 0.75 mm, of the aperture 14 it is 0.75 mm and of the aperture 19 it is 1.5 mm. The distance between the plates 16 and 17 is 2 mm. In the cathode-ray tube the distance from the cathode to the picture display screen along the axis furthermore is 467 mm. This gun can be operated with the following voltages:

cathode O V first grid between O V and 165 V second grid 500 V third grid between 4400 V and 4600 V fourth grid 25,000 V

The variable voltage at the first grid serves to control the beam. The ratio of the voltages at the third and the fourth grid is chosen to be so that the beam is focused as readily as possible in the spot on the screen.

Due to the presence of the astigmatic lens element constituted by the plates 16 and 17, the shape of the beam in the case of low beam current is influenced and that in such manner that an increase is obtained in a vertical direction of the spot on the picture display screen. This is shown in the graph of FIG. 4 where the beam current expressed in μA is plotted on the horizontal axis and the dimensions of the spot expressed in millimetres in the centre of the picture display screen is plotted on the vertical axis. A solid line 22 relates to the vertical dimension of the spot and a broken line 23 relates to the horizontal dimension of the spot. For comparison a dot-and-dash line 24 is shown in addition which relates to a gun which is the same as the above-described gun with the only difference that the plates 16 and 17 are lacking so that no astigmatic lens element is pressed. Since the lines 22 and 23 substantially coincide in the case of high currents, a substantially circular spot is obtained; in the case of low currents a vertical spot is obtained. It appears from the position of the line 23 relative to the line 24 that, compared with the case in which no astigmatic lens element is present, the horizontal dimension of the spot has remained approximately the same or has been reduced so that the horizontal definition has remained the same or has improved. This result can be explained as follows. With a low beam current, a real cross-over is formed in the space between the first and the second grid by the cathode 6, the first grid 7, and the second grid 8. Not counting aberrations, space charge and transverse speeds upon emission, this real cross-over is substantially punctiform. As a result of the astigmatic element on the second grid, the beam in the space between the second and the third grid becomes astigmatic. Viewed from the equipotential space within the cylindrical part of the third grid, this gives rise to two virtual cross-overs, one in a vertical line and one in a horizontal line. With low beam current, said virtual cross-overs lie at some distance from each other and whent the beam current increases, said distance decreases until the virtual cross-overs substantially coincide. Then they are again substantially punctiform. This is a result of the fact that the real cross-over which, in the case of low beam current, is formed between the first and the second grid, moves in the direction of the second grid when the beam current increases and hence towards the optic centre of the astigmatic lens element. The vertical virtual line cross-over is focused onto the picture display screen by the main lens of the gun constituted by the third grid 9 and the fourth grid 10, while the horizontal virtual cross-over is focused in the area between the gun and the picture display screen. The gap formed by the plates 16 and 17 is horizontal. As a result of this, with the voltages stated and with low beam current, a virtual cross-over of the rays of the beam situated in a vertical plane is formed (horizontal line cross-over) which is situated farther away from the picture display screen than the virtual cross-over formed of the rays of the beam situated in a horizontal plane (vertical line cross-over).

FIGS. 5a and 5b show diagrammatically the path of rays in the tube from said virtual cross-over to the picture display screen. FIG. 5a shows the path of rays in a vertical plane and FIG. 5b in a horizontal plane. In FIG. 5a, 25 is the location of the virtual cross-over in the case of low beam current of the rays of the beam situated in a vertical plane and 27 is the location thereof in the case of a high beam current. In FIG. 5b, 26 is the location of the virtual cross-over in the case of a low beam current of the rays of the beam situated in a horizontal plane and 28 is the location thereof in the case of a high beam current. Since the locations 27 and 28 substantially coincide, they are shown at the same distance from the picture display screen 29. The centre of the lens constituted by the third and the fourth grid is denoted by 30. The virtual cross-over 26 is focused on the picture display screen at 31 by the lens. The virtual cross-over 25 is focused in a point 32 which is situated nearer so that a vertical extension 33-34 is formed of this on the picture display screen. In the case of high beam current focusing on the display screen in 31 takes place both in the horizontal plane and in the vertical plane. In a cathode-ray tube in which the gun does not comprise the plates 16 and 17, a virtual cross-over is formed with the same voltages by the lens effect of the second and the third grid, of which cross-over in the case of low beam current the location 35 is situated between 25 and 26 and in the case of high beam current the location is situated in 27 and 28. When the focusing is considered in a horizontal plane (FIG. 5b) it is obvious that in the absence of the plates 16 and 17 the cross-over, in the case of variation of the beam current, moves over a larger distance than in the presence of said plates so that in the latter case a smaller variation of the voltage of the third grid will be sufficient for optimum focusing. This is a favourable property, because in practice the voltage of the third grid is adjusted at a constant value.

FIG. 6 shows another embodiment of the astigmatic lens element of the second grid. In this case a plate 36 having an elongate aperture, namely a rectangular aperture 37, is present on the side of the third grid 9 on the plate-shaped part 13 comprising a circular aperture 14 of the second grid 8.

FIGs. 7a, 7b and 7c show another embodiment of the astigmatic lens element of the second grid. Figure 7a is a plan view; FIG. 7b is a cross-sectional view taken on the line VIIb -- VIIb of FIG. 7a; FIG. 7c is a cross-sectional view taken on the line VIIc -- VIIc of FIG. 7a. The second grid consists of a circular cylindrical part 38 and a plate-shaped part 39 in which an elongate bulge 40 is provided on the side of the first grid. A circular aperture 41 is present in the centre of the plate-shaped part 39. This embodiment has the advantage that the astigmatic lens element can be realized with a simple mechanical operation.


Convergence unit having three identical V-shape bent plates for shielding pole shoes:PHILIPS CRT TUBE A66-410X ELECTRON GUN TECHNOLOGY,
A color cathode-ray tube wherein a convergence unit comprises a symmetric shielding assembly in the form of three identical V-shaped bent plates, secured to each other by only three welds.

What is claimed is: 1. A color cathode-ray tube having, in an evacuated envelope, means to generate three electron beams, a display screen comprising a large number of regions luminescing in three different colors, a color selection electrode having a large number of apertures and means to converge the electron beams on the display screen, said electron beams being each associated with luminescent regions of one color by means of the color selection electrode, said convergence means comprising a cylindrical sleeve to which three pairs of ferromagnetic pole shoes extending radially inwardly are secured and in which shielding means are present to shield the magnetic fields of the said pole shoes, said shielding means including three identical V-shaped bent plates the apexes of which join each other along the axis of the said cylindrical sleeve and the first and second ends of which extend from said axis towards the wall of the said cylindrical sleeve in such manner that the space in said cylindrical sleeve is divided into three symmetrical compartments in which the said pairs of pole shoes are present, said V-shaped bent plates having respectively at their first end a lug which projects through an aperture in the cylindrical sleeve and is welded to the outer wall thereof, and have at their second ends a short projection which extends in a corresponding other aperture in the cylindrical sleeve for fixation in the tangential direction and is not welded, and further comprising means for fixation of the adjoining plates in the radial and the axial directions, said means including a projecting part at one arm of one plate and a recess in the engaging arm of the other plate. 2. A cathode-ray tube as claimed in claim 1, wherein said projection is a local bulge and the recess is a hole in which the said bulge extends. 3. A cathode-ray tube as claimed in claim 1, wherein said projection is a lug and the said recess is a slot through which the said lug extends.

Description:

The invention relates to a cathode-ray tube for displaying coloured pictures and comprising in an evacuated envelope means to generate three electron beams, a display screen comprising a large number of regions luminescing in three different colours, a colour selection electrode having a large number of apertures and means to converge the electron beams on the display screen, which electron beams are each associated with luminescent regions of one colour by means of the colour selection electrode, which convergence means comprise a cylindrical sleeve to which three pairs of ferromagnetic pole shoes extending radially inwardly are secured and in which means are present to shield the magnetic fields from the said pole shoes which consists of three V-shaped bent plates the tops of which join each other along the axis of the said cylindrical sleeve and of which the first and second ends extend from said axis towards the wall of the said cylindrical sleeve in such manner that the space in said cylindrical sleeve is divided into three compartments in which the said pairs of pole shoes are present.
Such a cathode-ray tube is described in the U.S. Pat. No. 3,689,791. In such a cathode-ray tube, three electron beams are generated which are converged on the display screen by means of three pairs of ferromagnetic pole shoes. This is carried out by means of the field between the pole shoes of each pair, which field is generated by means of an external convergence unit which is present outside the said envelope and cooperates with the pole shoes. The magnetic field in each of the pole shoes can deflect the relevant beam radially in such manner that the beams intersect each other substantially in a point at the area of the display screen.
The convergence unit in the cathode-ray tube consists of a cylindrical sleeve of non-magnetic material which has an apertured bottom. The top side of the cylindrical sleeve is open. The said pole shoes extend radially inwardly in said cylindrical sleeve. The cylindrical sleeve furthermore comprises means of a ferromagnetic material to shield the magnetic fields between the said pole shoes which generally consist of three V-shaped bent plates the tops of which join each other along the axis of the cylindrical sleeve and the ends extend in such manner towards the wall that the space in the cylindrical sleeve is divided into three identical compartments in which the said pole shoes are present.
Lugs of the ends of said V-shaped bent plates project through apertures in the cylindrical sleeve and are then welded. Hence six welds are necessary for said three plates.
The said United States Patent Specification describes a construction which reduces said number of welds from six to three, but said construction has the drawback that the systems of means to screen the magnetic fields is not symmetrical and is manufactured from two different parts.
It is the object of the invention to provide a construction which does not consist of different parts and which is symmetrical while the number of welds is restricted to three.
According to the invention, a cathode-ray tube of the type mentioned in the first paragraph is characterized in that the said V-shaped bent plates have at their first end a lug which projects through an aperture in the cylindrical sleeve and, after bending, is welded to the outer wall thereof, and have at their second end (a) means for fixing relative to the first end of the adjoining V-shaped plate in the radial and axial directions of the cylindrical sleeve, (b) a short projection which extends in an aperture in the cylindrical sleeve for fixation in a tangential direction and is not welded.
The fixation means mentioned sub (a) may consist of a projecting part at one of the said ends (for example a bulge, a lug) and a recess in the engaging end (for example a hole or slot).
It is an advantage in the manufacture that three identical V-shaped bent plates can be manufactured. Another advantage is the smaller number of welds with a smaller risk of the occurrence of welding sputters which cause separate parts in the tube, which may result in short-circuit in the means to generate electron beams.
The invention will be described in greater details with reference to the accompanying drawing, of which:
FIG. 1 shows a cathode-ray tube for displaying coloured pictures according to the invention,
FIG. 2 is an elevation of the internal convergence unit of the tube,
FIG. 3 is a sectional view according to a plane at right angles to the cylinder axis and taken on the line III--III of FIG. 5,
FIG. 4 is a sectional view analogous to FIG. 3 of another embodiment,
FIG. 5 is a sectional view taken on the line V--V of FIG. 2.
The cathode-ray tube for displaying coloured pictures shown in FIG. 1 is of the shadow mask type and comprises an evacuated glass envelope 1, a set of electron guns 2 in triangular arrangement to generate three electron beams 3, 4 and 5, a display screen 6 comprises a large number of regions luminescing in red, green and blue, respectively, a colour selection electrode 7 comprising a large number of apertures 8, and a set of deflection coils 9 for scanning the display screen 6 with the electron beams 3, 4 and 5. The electron beams 3, 4 and 5 are converged on the display screen 6 by means of an internal convergence unit 10 and an external convergence unit 11. The internal convergence unit 10 comprises a cylindrical sleeve 12 which is coaxial with the axis 13 of the cathode-ray tube.
FIG. 2 is an elevation of the internal convergence unit 10 viewed from the side of the shadow mask 7. The co-operation of the internal convergence unit 10 with the external convergence unit 11 is shown diagrammatically with an external magnetic circuit 14 having a coil 15. A current through coil 15 causes a magnetic field between the pole shoes 16 and 17 which causes a radial displacement of the electron beam 5. In a similar manner the electron beam 3 is influenced by the field between the pole shoes 18 and 19 and the electron beam 4 by the field between the pole shoes 20 and 21. In this manner the electron beams 3, 4, and 5 can be converged in one point on the display screen 6. The pairs of pole shoes are shielded relative to each other by ferromagnetic V-shaped bent plates 22, 23 and 24. It is obvious that V-shaped bent plates the top of which is more or less circular may also be used. The cylindrical sleeve 12 has a bottom plate 25 which has three apertures for passing the electron beams 3, 4 and 5. The ferromagnetic V-shaped bent plates are fixed in a tangential direction by a short projection 26 which passes through the hole 27 and bent lug 32 which also passes through hole 27 and is moreover welded.
FIG. 3 is a sectional view at right angles to the cylinder axis of the cylindrical sleeve 12 and the ferromagnetic V-shaped bent plates 22, 23 and 24 and taken on the line III--III of FIG. 5. The bulge 28 extends in hole 29 and fixes the said plate in the axial and radial directions.
In FIG. 4 the lug 30 extends through slot 31, which represents another embodiment of the fixation.
FIG. 5 is a sectional view taken on the line V--V of FIG. 2. In this case the short projection 26 extends through the hole 27.

Cathode structure comprising a heating element:
A cathode structure comprises at an end portion (1) an electron-emitting material (4) and a heating element (5) of wire (7), said cathode structure having a plurality of primary helical turns (8). These primary turns are used to form a first series of secondary turns (9) which are wound in a first direction with a pitch and which extend towards the end portion (1), and to form a second series of secondary turns (10) which extend from the end portion (1) and which have the opposite direction of winding yet the same pitch. Near the end portion (1), the first and second series of turns (9, 10) are interconnected by an arc-shaped connecting portion (12) having primary turns (8). This arc-shaped connecting portion (12) has a span S_a and a rise r_a, the ratio r_a /S_a preferably ranging from 0.3 to 0.5.

1. A cathode structure which comprises an electron-emitting material at an end portion, and in which there is a filamentary heating element comprising a plurality of primary, helical turns with which a first series of secondary turns is formed, which are wound in a first direction with a pitch and which extend in the direction of said end portion, and with which a second series of secondary turns is formed which extend from said end portion and which are wound in the opposite direction yet with the same pitch, said first and second series of turns being interconnected at the end portion by a connecting portion with primary turns, characterized in that the connecting portion is arc-shaped with a span S_a and a rise r_a, the ratio r_a /S_a ranging from 0.1 to 1.0.

2. A cathode structure as claimed in claim 1, characterized in that the ratio r_a /S_a ranges from 0.1 to 0.5.

3. A cathode structure as claimed in claim 2, characterized in that the ratio r_a /S_a ranges from 0.3 to 0.5.

4. A cathode structure as claimed in any one of claims 1 to 3, characterized in that, in the connecting portion, a diameter d_w of the wire, an internal diameter d_p of the primary turns and an average pitch P_p of the primary turns satisfy the relationship: ##EQU6## in which the argument of the sine is expressed in radials.

5. A cathode structure as claimed in claim 4, characterized in that: ##EQU7##.

6. A cathode structure as claimed in claim 5, characterized in that: ##EQU8##.

7. A cathode structure as claimed in any one of claims 1 to 3, characterized in that the diameter d_w of the wire exceeds 20 μm.

8. A cathode structure as claimed in any one of claims 1 to 3, characterized in that the internal diameter d_p of the primary turns exceeds 100 μm.

9. A cathode structure as claimed in any one of claims 1 to 3, characterized in that the span S_a of the connecting portion is smaller than 500 μm.

10. A cathode ray tube comprising an electron source which includes a cathode structure having a heating element as claimed in any one of claims 1 to 3.

Description:

BACKGROUND OF THE INVENTION
The invention relates to a cathode structure which comprises an electron-emitting material at an end portion, and in which there is a filamentary heating element comprising a plurality of primary, helical turns with which a first series of secondary turns is formed, which are wound in a first direction with a pitch and which extend in the direction of said end portion, and with which a second series of secondary turns is formed which extend from said end portion and which are wound in the opposite direction yet with the same pitch, said first and second series of turns being interconnected at the end portion by a connecting portion with primary turns.
The invention further relates to a cathode ray tube comprising an electron source which includes a cathode structure which is provided with a heating element.
Cathode structures comprising heating elements are used in electron sources for cathode ray tubes, for example, in display devices for displaying monochromatic or colour images, camera tubes, video amplifiers and oscilloscopes.
Such a cathode structure is known from the brochure "Quick-Vision CTV Picture Tube A66-410X" by L. J. G. Beriere and A. J. van IJzeren (Philips Product Note, 1973). In said document, a description is given of a tubular cathode structure in an electron gun for use in a cathode ray tube, which cathode structure comprises at an end portion a layer of an electron-emitting material to emit electrons. The cathode structure comprises a heating element which serves to heat the electron-emitting material. Said heating element comprises a wire having primary and secondary turns which is bifilarly wound in the form of a double helix. The secondary turns are built up from a first series of turns, which are wound in a first direction with a pitch and which extend in the direction of the end portion, and from a second series of turns which extend from the end portion and which are wound in the opposite direction yet with the same pitch. The first and second series of secondary turns are interconnected near to the end portion of the cathode structure by a connecting portion.
A drawback of the known cathode structure is that a number of the primary turns in the connecting portion may be short-circuited. These short-circuits occur, particularly in the primary turns in the transitions from the connecting portion to the first and second series of secondary turns. Due to the fact that the connecting portion is nearest to the electron-emitting material, the efficiency with which the heating element heats the electron-emitting material is adversely affected by these short-circuits.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a cathode structure in which a short-circuit in the primary turns of the heating element near the end portion of the cathode structure is precluded and/or an improved distribution of the primary turns of the heating element near the end portion of the cathode structure is realised, so that the efficiency of the heating element is improved.
To this end, the heating element in accordance with the invention is characterized in that the connecting portion is arc-shaped with a span S _a and a rise r _a , the ratio r _a /S _a ranging from 0.1 to 1.0.
The advantage of an arc-shaped connecting portion is that, in said connecting portion, the primary turns are arranged in a flowing line relative to each other, that is, the distances between the primary turns in the connecting portion change gradually. In addition, an arc-shaped connecting portion causes the primary turns to be flowing, particularly in the transitions from the connecting portion to the first and second series of secondary turns, so that the risk of short-circuits between the primary turns is precluded.
The known cathode structure has a so-called "flat head" (r _a /S _a ≉0), which is to be understood to mean that the connecting portion between the two transitions to the first and second series of secondary turns is situated in a plane transverse to a longitudinal axis of the cathode structure. As a result, in the known cathode structure the transitions from the connecting portion to the first and second series of secondary turns in the heating element are curved substantially.
In order to heat the electron-emitting material as effectively as possible, it is desirable that the turns of the heating element, particularly near the end portion, should be used as efficiently as possible. This can be achieved by using a wire with primary and secondary turns which is bifilarly wound in the form of a double helix instead of a single heating wire to build up the heating element. An efficient heating element is obtained if the density of the primary turns is relatively high, while electrical contact between the primary turns, particularly also in the connecting portion and in the transitions from the connecting portions to the first and the second series of secondary turns, is avoided. Given the span S _a and a rise r _a of the arc-shaped connecting portion, the primary turns can be closely spaced, while preventing a short-circuit between said primary turns by choosing the ratio r _a /S _a to be in the range from 0.1 to 1.0.
An embodiment of the heating element in accordance with the invention is characterized in that the ratio r _a /S _a ranges from 0.1 to 0.5.
If the ratio of the span to the rise of the arc-shaped connecting portion r _a /S _a is 0.5, then the connecting portion is semi-circular. If the ratio r _a /S _a is above 0.5, then the connecting portion has a pointed shape relative to the longitudinal axis of the cathode structure, so that the average distance from the primary turns of the connecting portion to the end portion of the cathode structure increases and the efficiency of the heating element decreases. If the ratio r _a /S _a is chosen in the range from 0.1 to 0.5, the connecting portion obtains a (somewhat) flattened shape, as compared to a circular connecting portion which is obtained if r _a /S _a =0.5, and the primary turns are relatively closely spaced. By flattening the connecting portion, a more efficient heat dissipation is brought about, which is necessary to heat the electron-emitting material.
An embodiment of the heating element in accordance with the invention is characterized in that the ratio r _a /S _a ranges from 0.3 to 0.5.
An optimum density of primary turns in the connecting portion of the heating element is obtained by choosing the ratio of the span to the rise of the arc-shaped connecting portion r _a /S _a to be in the range from 0.3 to 0.5.
A preferred embodiment of the heating element in accordance with the invention is characterized in that, in the connecting portion, a diameter d _w of the wire, an internal diameter d _p of the primary turns and an average pitch P _p of the primary turns satisfy the relationship: ##EQU1## in which the argument of the sine is expressed in radials.
Given a number of easily measurable parameters of the wire and the various winding ratios, the criterion which must be satisfied to preclude electrical contact between the primary turns, to be indicated by f _w ≤1.5 d _w , is contained in the above formula.
A further embodiment of the heating element in accordance with the invention is characterized in that ##EQU2##
By observing a lower limit in the formula (f _w ≥0.3 d _w ) a safe margin for the minimum distance between the primary turns in the heating element is obtained, so that, also when the heating element is in operation to heat the electron-emitting material, the primary turns do not contact each other as a result of possible thermal expansion, and hence do not cause a short-circuit between the primary turns.
A further embodiment of the heating element in accordance with the invention is characterized in that ##EQU3##
In the known cathode structure having the so-called "flat head" (r _a /S _a ≉0) short-circuits between the primary turns cannot be precluded if the various parameters of the heating wire in the formula f _w are chosen to be such that the outcome of the formula does not exceed the upper limit to be observed by the formula (f _w ≤1.0 d _w ). However, the combination of an arc-shaped connecting portion with pre-conditions for the ratio r _a /S _a as defined hereinabove, that is, in the range from 0.1 to 1.0, preferably from 0.1 to 0.5, in particular from 0.3 to 0.5, enables such values to be selected for the parameters of the heating wire (diameter d _w of the wire, internal diameter d _p of the primary turn, average pitch P _p of the primary turn and span S _a of the (arc-shaped) connecting portion) that the upper limit (f _w ≤1.0 d _w ) is not exceeded. In this manner, a low-power heating element without short-circuits is obtained which can attain high temperatures.
Within the framework of the invention, a thickness d _w for the wire in excess of 20 μm, or an internal diameter d _p of the primary turns in excess of 100 μm, or an average pitch P _p of the primary turns below 50 μm, or a span S _a of the connecting portion below 500 μm can now advantageously be used.
The above relations between the dimensions of the wire and the data about the winding ratios in the heating element enable those skilled in the art to design and use, in a simple manner, an efficient heating element for a cathode structure and hence to preclude short-circuits in the primary turns of the heating element near the end portion of the cathode structure and/or to achieve an improved distribution of the primary turns of the heating element near the end portion of the cathode structure. The inventors have recognized that, given a suitably chosen ratio of the span S _a to the rise r _a , the use of an arc-shaped connecting portion enables wire parameters and winding ratios (diameter d _w of the wire, internal diameter d _p of the primary turn and the average pitch P _p of the primary turn) in the connecting portion of the heating element to be chosen which, when r _a /S _a is chosen to be 0, i.e. a so-called "flat" head, always cause a substantial number of short-circuits between the primary turns in the connecting portion. The formula f _w provides those skilled in the art with a simple "tool" for choosing suitable wire parameters and winding ratios of the wire to be used for the connecting portion.
FIG. 2A is a schematic view, partly in cross-section, of a cathode structure in accordance with the prior art. This cathode structure comprises an end portion 1 and a cathode shaft 2 which is closed by means of a cover 3 which is partly covered by an electron-emitting material 4. In this embodiment, said cover and the part of the cathode structure cooperating with said cover form the end portion 1 of the cathode structure. The cathode shaft 2 accommodates a heating element 5 which serves to heat the electron-emitting material 4. Said heating element 5 comprises a wire 7 having primary turns 8 and secondary turns 9, 10 which is bifilarly wound in the form of a double helix and which is covered by an electrically insulating layer 6. Said secondary turns are built up of a first series of turns 9 which are wound in a first direction (i.e. counterclockwise) with a pitch and which extend towards the end portion 1, and of a second series of turns 10 which extend from the end portion 1 and which are wound in the opposite direction yet with the same pitch. The first and second series of secondary turns 9, 10 are interconnected close to the end portion 1 of the cathode structure by a connecting portion 11 having primary turns 8. This connecting portion 11 has a flat shape with respect to a longitudinal axis of the cathode structure. Above the cathode structure, there are a number of electrodes, one of which is shown in FIG. 2. The electrode 18 is commonly referred to as g ₁ -electrode and comprises an aperture 19.
CRT TUBE PHILIPS A66-410X. DEFLECTION COIL SYSTEM FOR COLOUR TELEVISION

A deflection coil system for colour television, comprising deflection coils which are subdivided into sections, an adjustable impedance being connected parallel to some of these sections for compensation of differences in the distribution of turns of the coils.

1. A deflection coil system for a colour television display tube, comprising deflection coils for electron beam deflection in the horizontal and the vertical direction, respectively, each coil being a single coil and consisting of a number of turns of a conductive wire, each of the coils for at least one of the two deflection directions being divided into a number of series-connected sections in that at least one turn which is situated between the first and the last turn of the coil is provided with a tapping, characterized in that at least one of the sections of each of the coils for at least one deflection direction, is connected in parallel with a circuit means of adjustable impedance for adjusting the dynamic convergence. 2. A deflection coil system as claimed in claim 1, characterized in that at least the first (45) or the last section (53) of the relevant coils is provided with a parallel circuit (21). 3. A deflection coil system as claimed in claim 1, characterized in that a coil section (49) which is provided with a parallel circuit extends on both sides of a mean turn (27), the angular distance α_g between said mean turn (27) and the deflection direction associated with this coil satisfying the formula: ##SPC3## 4. A deflection coil system as claimed in claim 1, characterized in that each of the sections provided with a parallel circuit comprises a number of turns substantially equal to one tenth of the total number of turns of the relevant coil. 5. A deflection coil system as claimed in claim 1, characterized in that each of the parallel circuits (21) of the horizontal deflection coils consists of a coil (59, 61, 63) of adjustable inductance, each of the parallel circuits (21) of the vertical deflection coils consisting of a variable resistor (65, 67, 69). 6. A deflection coil system as claimed in claim 1, characterized in that sections (45, 53'; 49, 49'; 53, 45') which form part of different coils are pair-wise connected in parallel. 7. A deflection coil system for a color television display tube, comprising a pair of single deflection coils for electron beam deflection in the horizontal and the vertical directions respectively, each coil comprising a plurality of turns of a conductive wire, one of said coils having a tap dividing said one coil into a plurality of series-connected sections, and means parallel coupled to at least one of the sections of each of the coils for adjusting the dynamic convergence of the tube comprising an adjustable impedance element. 8. A deflection coil system as claimed in claim 7, wherein at least an end section of the relevant coils is provided with said parallel coupled impedance element. 9. A deflection coil system as claimed in claim 7, wherein a coil section which is provided with a parallel circuit extends on both sides of a mean turn, the angular distance α_g between said mean turn and the deflection direction associated with this coil satisfying the formula: ##SPC4## 10. A deflection coil system for a color television display tube, comprising a pair of single deflection coils for electron beam deflection in the horizontal and the vertical directions respectively, each coil comprising a plurality of turns of a conductive wire, one of said coils having a tap dividing said one coil into a plurality of series-connected sections, means parallel coupled to at least one of the sections of each of the coils for adjusting the dynamic convergence of the tube comprising an adjustable impedance element, and wherein one of said sections which is provided with a parallel circuit extends on both sides of a mean turn the angular distance α_g between said mean turn and the deflection direction associated with this coil satisfying the formula: ##SPC5##

Description:

The invention relates to a deflection coil system for a colour television display tube, comprising deflection coils for electron beam deflection in the horizontal and the vertical direction, respectively, each coil consisting of a number of turns of a conductive wire, each of the coils for at least one of the two deflection directions being divided into a number of series-connected sections in that at least one turn which is situated between the first and the last turn of the coil is provided with a tapping.

Contemporary colour television display tubes usually comprise three electron guns which are preferably arranged at the corners of an equilateral triangle or are adjacently arranged on one line. Each of these electron guns generates an electron beam which, after having passed through one of the apertures in a shadow mask, impinges upon a phosphor dot on a rectangular display screen, the said phosphor dot thereby luminescing a given colour. By means of magnetic fields generated in a deflection coil system, the electron beams can be deflected such that they scan the entire display screen.

The dynamic convergence of the beams is of major importance in this respect. This means that, regardless of the angle through which the beams have been deflected, the three beams must always intersect each other at the area of the shadow mask so as to cause luminescence of the correct phosphor dots. This dynamic convergence greatly depends on the distribution of the turns of the wires constituting the deflection coils. During the manufacture of the coils, deviations occur in the turn distribution which cause a difference in the dynamic convergence in individual coil systems. Such differences are not acceptable, particularly in the case of colour television display tubes having large deflection angles.

A proposal has already been made for correcting such deviations, see U.S. Pat. No. 3,169,207. According to this proposal, each of the coils is wound with a number of parallel wires, the coil being divided into a number of series-connected sections by means of tappings. The parallel wires of some of these sections are connected in parallel, and the parallel wires of other sections are connected in series. The effective turn distribution of the finished product can be influenced by varying the number of turns per section when the coil is being wound.

A drawback of this method is that the correction must be performed during winding, whilst the correction must also be rather coarse because a difference of at least one complete turn must each time be introduced. The invention has for its object to eliminate these drawbacks by providing a deflection coil system in which the effective turn distribution of the finished product can be changed while it has already been mounted on the display tube, arbitrarily small variations also being possible.

To this end a deflection coil system according to the invention is characterized in that at least one of the sections of each of the coils for at least one deflection direction is connected in parallel with a circuit of adjustable impedance.

One embodiment of the system according to the invention by means of which the convergence can be influenced on the entire display screen is characterized in that at least the first or the last section of the relevant coils is provided with a parallel circuit.

For each winding of a deflection coil the angular distance α _n between this winding and the deflection direction can be determined. From this distance the angular distance α _g of a mean turn can be calculated by means of the formula ##SPC1##

Therein, n(α ) represents the number of turns the angular distance of which amounts to α _n . It was found that the convergence in the corners of the display screen can be influenced, whilst the convergence on the axes remains substantially the same if a deflection coil system according to the invention is used which is characterized in that a coil section which is provided with a parallel circuit extends on both sides of the mean turn.

The invention will be described in detail with reference to the drawings, in which

FIG. 1 is a perspective view of a saddle-shaped deflection coil which forms part of a deflection coil system according to the invention,

FIG. 2 is a diagrammatic cross-sectional view of a ferromagnetic core having vertical saddle-shaped deflection coils,

FIG. 3 is a diagrammatic cross-sectional view of a ferromagnetic core having vertical toroidal deflection coils,

FIGS. 4 and 5 are diagrammatic views of two methods of dividing the coils of FIGS. 2 and 3 into sections in accordance with the invention, and

FIGS. 6a, 6b and 7a, 7b show a number of practical circuits for a deflection coil system according to the invention.

The saddle-shaped deflection coil shown in FIG. 1 is intended to be arranged on the outside of a display tube (not shown) so as to deflect an electron beam which is generated in the tube and whose direction of travel is denoted by an arrow 1. The coil consists of a number of turns of a conductive wire which is composed of one or more strands, preferably copper wire, which envelope a window 3. The coil comprises active parts 5 and 7 in which the wire extends mainly parallel to the direction of travel of the electrons, and a foremost coil end 9 and a rearmost coil end 11 where the wire extends substantially at right angles to the said direction of travel. The electrons are deflected substantially exclusively by the magnetic field which is generated in the active parts 5 and 7. The current required for the deflection can be supplied via connection wires 13 and 15 at the area of the rearmost coil end 11. Two turns are provided with tappings 17 and 19 that are also located at the rearmost coil end 11. As a result, the coil is divided into three sections, the first section consisting of the turns between the connection wire 13 and the first tapping 17, the second section consisting of the turns between the two tappings 17 and 19, and the third section consisting of the turns between the second tapping 19 and the connection wire 15. Connected in parallel to each of the three sections is a circuit 21 which has an adjustable impedance. Part of the deflection current which is applied to the coil via the supply wires 13 and 15 passes through a coil section while the remainder passes through the parallel circuit 21 which is associated with this section. If the impedance of the parallel circuit 21 is reduced, a larger part of the deflection current will pass through the parallel circuit. This has the same effect on the deflection of the electron beam as a reduction of the number of turns in the relevant section. In this manner continuous variation of the number of turns per section and hence of the turn distribution in the coil is possible.

FIG. 2 is a diagrammatic cross-sectional view of an angular ferromagnetic core 23 which surrounds a pair of saddle-coils, only the active parts 5 and 7 and 5' and 7' thereof being visible in FIG. 2. The coils are arranged with respect to each other such that the connection line between the two windows 3 and 3' extends horizontally and intersects the axis of the core 23. The magnetic field which generates a current which flows through these coils causes a vertical deflection of an electron beam (not shown) travelling at right angles to the plane of the drawing. The deflection direction is denoted in the FIGS. 2 to 5 by a double arrow 25. For the deflection in the horizontal direction, a complete deflection coil system is also provided with a second pair of saddle coils (not shown for the sake of clarity), which are arranged to be coaxial with the first pair and which have been turned through an angle of 90° with respect to the first pair so that the connection line between the windows of the second coil pair extends in the vertical direction.

The effect of the magnetic field which is generated by each turn of the coil on the deflection of the electrons is dependent on the location of this turn. This location is determined by measuring, in a plane perpendicular to the direction of travel of the electron beam, for example, the plane of the drawing in the FIGS. 2 to 5, the angle between the deflection direction of the coil on the one side and the connection line from the axis of the core 23 to the relevant turn on the other side.

FIG. 2 shows the angular distance α _o of the turns which are arranged nearest to the deflection direction, and the angular distance α _N of the turns which are arranged furthest from the deflection direction.

The mean angular distance α _g can be calculated from the angular distances of the turns by means of the formula: ##SPC2##

Therein, n(α ) represents the number of turns the angular distance of which amounts to α _n . The turn whose angular distance is equal to α _g is called the mean turn. In FIG. 2, this mean turn is denoted by 27 for the right-hand coil, and by 27' for the left-hand coil.

FIG. 3 is a cross-sectional view of a toroidal ferromagnetic core 23 which corresponds to FIG. 2. However, in this case the deflection coils consist of four coils 29, 31, 33 and 35 which are toroidally wound about the core. The various angular distances α _n can be determined with reference to FIG. 3 in the same manner as for saddle-coils with reference to the FIG. 2. The calculation of the mean angular distance α _g is also the same. The mean turns are denoted in FIG. 3 by 37, 39, 41 and 43.

In FIG. 4 a subdivision into sections of the saddle coils of FIG. 2 is shown diagrammatically. By means of four tappings (not shown) each of the two coils is divided into five sections 45, 47, 49, 51, 53 and 45', 47', 49', 51', 53', respectively. Each of these sections can be provided with a parallel circuit (not shown in FIG. 4). However, it appears to be particularly advantageous to provide one or more of the sections 45, 49 and 53 with a parallel circuit. The parallel circuit of the first section 45 and of the last section 53 can be utilized to influence the location of the mean turn 27. By reducing the impedance of the parallel circuit of section 45, the current flowing through this section is reduced. As regards the magnetic field which is generated by the coil this has the same effect as a reduction of the number of turns in this section and hence of the factors n(α) in the formula for α _g .

As these factors remain the same in the other sections, it is obvious that the value α _g increases as a result thereof. This means that the location of the mean turn is shifted in the direction of the last section 53. Similarly, a reduction of an impedance which is connected in parallel with section 53 will cause a shift of the mean turn 27 in the direction of the first section 45. The location of the mean turn 27 is of importance for the convergence on the entire display screen, whilst the convergence on the axes of the display screen is even determined substantially exclusively by this location. An optimum adjustment of the latter convergence can thus be obtained in the manner described above.

The convergence in the corners of the display screen is also dependent of the location of the mean turn 27. However, this convergence is also dependent of the manner in which the turns are distributed over the width of the coil. By influencing this distribution without changing the location of the mean turn 27 to any significant extent, optimum adjustment can be achieved of the convergence in the corners of the display screen without the already adjusted convergence on the axes being disturbed. To this end the section 49, extending on both sides of the mean turn, is also provided with a parallel circuit. If the impedance of this parallel circuit is varied, the factors n(α) in the formula for cos α _g vary for values of α _n which are slightly larger or smaller than α _g . The overall effect of these variations on the value of α _g is very slight, so that the location of the mean turn remains substantially the same. The turn distribution over the width of the coil, however, does change which can be clearly demonstrated by completely short-circuiting section 49. In that case the section no longer contributes to the magnetic field generated by the coil so that at this area an interruption is produced in the turns as if it were.

In a practical embodiment of saddle coils for a colour television display tube comprising three electron guns arranged on one line and having a deflection angle of 110°, each of the coils comprising 100 turns, each coil was divided by means of tappings such that each of the sections 45, 49 and 53 comprises ten turns. By providing these sections with parallel circuits and by varying the impedance thereof from zero to a value which is very large with respect to the impedance of each of the sections, it was found to be possible to correct convergence errors of 3 mm on the axes and of 5 mm in the corners of the display screen of the tube.

During the winding of the coil it can be ensured that the mean turn 27 of the uncorrected coil is always shifted in one direction with respect to the desired location. In that case the mean turn 27 must always be displaced in the same direction upon correction, so that for this purpose only section 45 or only section 53 must be provided with a parallel circuit. That one of these two sections which is not used need not even exist as a separate section in that case, so that the relevant tapping can also be omitted.

So as to achieve a saving as regards the number of tappings, the sections 47 and 51 which are not provided with a parallel circuit can also be omitted. In that case the situation shown in FIG. 5 is obtained where the coil, in accordance with FIG. 1, is divided into only three sections 45, 49 and 53.

In the simplest case the coil is wound such that the mean turn is always shifted in the direction of, for example, section 53. In that case only one tapping is made, i.e. the tapping which limits section 53, and an impedance is connected parallel to section 53. Subsequently, the convergence in the corners and on the axes of the display screen is simultaneously adjusted by variation of this impedance. Obviously, this is possible only if this combined convergence adjustment constituted an acceptable compromise.

Using FIG. 2 as a basis, the division into sections has been described for saddle coils with reference to the FIGS. 4 and 5. It is obvious that on the basis of FIG. 3 analogous figures for toroidal coils can be drawn which are subject to the same considerations. It is also possible to incorporate in one deflection coil system saddle coils for the deflection in the one direction and toroidal coils for the deflection in the other direction. This does not affect the above-mentioned considerations either.

The circuits which are connected parallel to the various sections preferably consist of variable resistors for the vertical deflection coils which are supplied with a deflection voltage of low frequency. For the horizontal deflection coils which are supplied with a deflection voltage of a much higher frequency, coils having a variable inductance are preferably used as the parallel circuit.

FIGS. 6 and 7 show some examples of circuits which can be used for deflection coil systems according to the invention.

FIG. 6a shows a pair of saddle-shaped horizontal deflection coils which are divided into sections 45, 47, 49, 51 and 53 and 45', 47', 49', 61' and 53', respectively. The coils are connected in series via a variable coil 55 which serves for compensation of mutual differences, and they are earthed in the centre of the series network. They are supplied from a known sawtooth generator 57. Connected in parallel to each of the sections 45, 49, 53, 45', 49' and 53' is a coil having a variable inductance. These coils are denoted by 59, 61, 63, 59', 61' and 63', respectively.

FIG. 6b shows an analogous circuit for saddle-shaped vertical deflection coils. The sections are numbered in the same manner as in FIG. 6a. Connected in parallel with each of the sections 45, 49, 53, 45', 49' and 53' is a variable resistor. These resistors are denoted by 65, 67, 69, 65', 67' and 69', respectively. The assembly is supplied from a known sawtooth generator 71. The voltage at the area of the centre of the series network can be adjusted by means of a potentiometer 73 which constitutes, on conjunction with the two fixed resistors 75 and 77, a voltage divider which is connected in parallel with the series network.

It appears from FIGS. 6a and 6b that, if the two deflection coils for one deflection direction are connected in series, a separate variable resistor or coil is required for each section provided with a parallel circuit. It appears that the number of variable resistors and coils can be reduced to one half by parallel connection of the deflection coils. This not only represents a saving as regards the number of components required for the circuit, but also the number of operations to be performed for adjustment is reduced.

FIGS. 7a and b show how such a circuit can be constructed for the horizontal and the vertical deflection coils, respectively. In both Figures each coil is again divided into five sections, the numbering of which corresponds to that in FIGS. 6a and b. Sections forming part of different coils are pairwise connected in parallel. As a result, the following pairs of sections are obtained: 45 and 53', 47 and 51', 49 and 49', 51 and 47', 53 and 45'. FIG. 7a shows that a coil having a variable inductance 79 is connected parallel to the first pair of sections 45, 53' of the horizontal deflection coils. Similarly, a variable coil 81 is connected parallel to the third pair of sections 49, 49', and a variable coil 83 is connected parallel to the fifth pair 53, 45'. The complete parallel network is again supplied from the sawtooth generator 57 via a variable coil 85 for compensating differences between the two deflection coils.

FIG. 7b shows that a variable resistor 87 is connected parallel to the first pair of sections 45, 53' of the vertical deflection coils, a variable resistor 89 being connected parallel to the third pair 49, 49', and a variable resistor 91 being connected parallel to the fifth pair 53, 45'. Via a variable resistor 93 for the elimination of irregularities in the two deflection coils, the complete parallel network is again supplied by the sawtooth generator 71.

On the basis of the same considerations as described with reference to FIGS. 6 and 7 for saddle coils, it is also possible to demonstrate the desirability of pairwise parallel connection of sections forming part of different coils of the toroidal type.

CRT TUBE PHILIPS A66-410X.quadripolar field system of deflection coils Color television display device including a cathode-ray tube
A color televison display device including a cathode-ray tube whose neck supports a system of deflection coils which causes isotropic astigmatic errors upon deflection. This is eliminated with the aid of a quadripolar field to which end four windings are wound preferably as four toroid windings on the core of the deflection coil system at the area of the deflection plane. The windings are arranged pairwise opposite to each other and this in such a manner that two windings are exactly located in the gaps of the field deflection coils and the two other windings are shifted 90° in the tangential direction. Parabola currents which may have the line frequency and/or the field frequency must flow through the four windings. However, it is alternatively possible to replace the four windings by two each (thus a total of 8) in which a parabola current proportional to the line frequency flows through one pair of four windings and a parabola current proportional to the field frequency flows through the other pair of four windings.

1. A circuit for a color television display tube, said circuit comprising first and second deflection coil units adapted to receive respective deflection currents, each unit having two symmetrical coil halves which are arranged opposite to each other and defining a pair of places therebetween wherein said windings have at least a reduced winding density, the first unit being disposed 90° in the tangential direction relative to the second unit, said units being disposed around the neck of the cathode-ray tube for deflecting at least one electron beam generated in the cathode-ray tube into two orthogonal directions when the respective deflection current flows through each coil unit; means for correcting distortion including at least four windings disposed tangentially relative to one another at an angle of approximately 90° near said places respectively, switching means coupled to said windings for applying a correction current through the four windings, means coupled to said switching means for generating said correction current in accordance with the deflection current flowing through at least the first deflection coil unit, whereby the windings generate a quadripolar field which is in accordance with said deflection current at the area of the deflection plane of the electron beam. 2. A circuit as claimed in claim 1, wherein said units further comprise a core, said four windings comprising toroid coils wound on the core. 3. A circuit as claimed in claim 1, wherein said tube has three beams located on the vertices of an equilateral triangle at the area of the deflection plane, and wherein said correction current comprises the difference between a current which is substantially proportional to the square of the current flowing through the first deflection coil unit and a current which is substantially proportional to the square of the current flowing through the second deflection coil unit said difference being substantially zero for those instantaneous values of the two deflection currents at which the three beams are located on the diagonals of the tube display screen. 4. A circuit as claimed in claim 1 wherein said tube has at least one coplanar disposed beams and said correction current comprises the sum of a current which is substantially proportional to the square of the current flowing through the first deflection coil unit and a current which is substantially proportional to the square of the current flowing through the second deflection coil unit in accordance with the equation (c₁ - C₃) x² + (c₂ + c₄) Y².
in which the constants C₃ and c₄ are chosen to satisfy the equation c₄ Y² - c₃ x² = O
along the diagonals of the screen, while furthermore c_i - c₃ ≥ 0.
5. A circuit as claimed in claim 1 wherein the four windings are coupled in series and said switching means for supplying the correction current comprises a mixer circuit having an input adapted to receive a parabola signal of the frequency of the current flowing through at least one of the deflection coil units,and an output coupled to said windings. 6. A circuit as claimed in claim 5 wherein said input is adpated to receive a parabola signal of the frequency of the current flowing through the remaining deflection coil unit. 7. A circuit as claimed in claim 5, further comprising means for the elimination of asymmetries including means for applying a polarity reversible sawtooth signal of the frequency of the deflection current flowing through at least one of the deflection units to the mixer circuit input. 8. A circuit as claimed in claim 7 further comprising means for applying a polarity reversible sawtooth signal of the frequency of the deflection current flowing through the remaining deflection unit to the mixer input. 9. A system of deflection coils for use in a circuit that eliminates distortion comprising a core, and a first and a second deflection coil unit, each unit having two symmetrical coil halves which are arranged opposite to one another, the first unit being disposed 90° in the tangential direction relative to the second unit, and defining a pair of places therebetween wherein said windings having at least a reduced winding density and also defining two positions where electron beam deflection directions cross said core four toroid windings disposed on the core at said positions tangentially relative to one another at an angle of approximately 90°, the two toroid windings located opposite to each other are situated near the two places between the symmetrical coil halves of one of the two deflection coil units. 10. An in-line gun type color television picture tube apparatus comprising:
a. an in-line gun type color cathode-ray tube in which three electron guns are arranged in a line along a first axis and the spots of electron beams from these electron guns appear along the first axis on the screen of the cathode-ray tube and are converged at the center of the screen,
b. a deflection yoke which deflects and scans the electron beams so that both side spots of the three electron beams appear on the screen at an equal distance on both sides of the center spot when the three electron beams are not converged,
c. a convergence magnetic field generating means, said convergence magnetic field generating means consisting essentially of at least one pair of convergence windings which are respectively provided externally on the neck of said cathode-ray tube and are opposite from each other in reference to the axis of the cathode-ray tube and a convergence power supply which supplies the convergence current to said windings which forms said convergence magnetic field generating means forming a convergence magnetic field which shifts only said side electron beams and has a field distribution which is symetrical and linear in reference to a second and third axis, said second and third axes being positioned 90° with respect to each other and 45° with respect to said first axis, the plane formed by said second and third axes being perpendicular to the axis of the cathode-ray tube, wherein both side electron beams are symmetrically shifted in reference to the center electron beam, as a function of the deflection angle, in order to complete convergence of the beam spot trio through said convergence magnetic field.
11. An apparatus according to claim 10, wherein a deflection yoke deflects and scans the electron beams so that the distances between side spots and center spots appearing at two symmetrical positions in reference to the center of the screen are equal when three electron beams are not converged. 12. An apparatus according to claim 10, wherein said convergence magnetic field generating means is positioned such that the convergence magnetic field is induced at the deflecting position of the deflection yoke to shift symmetrically both side electron beams in reference to the center electron beam at the center of deflection. 13. An apparatus according to claim 10, wherein said at least one pair of convergence windings comprises two sets of said convergence windings arranged at positions at 90° from each other around the axis of said cathode-ray tube. 14. An apparatus according to claim 10, wherein said convergence windings are toroidally wound on a core made of a ring-shaped highly magnetic material which surrounds the neck of said cathode-ray tube about the axis of said cathode-ray tube. 15. An apparatus according to claim 14, wherein said deflection yoke is comprised of a ring-shaped yoke core surrounding the neck of the cathode-ray tube and a pair of coils which are opposed to each other and mounted on said yoke core, and wherein said yoke core is employed as the core for said convergence windings.

Description:

The invention relates to a colour television display device including a cathode-ray tube having a display screen and a system of deflection coils comprising a first and a second deflection coil unit, each unit having two symmetrical coil halves which are arranged opposite to each other, the first unit being shifted 90° in the tangential direction relative to the second unit, said system of deflection coils being slid around the neck of the cathode-ray tube for deflecting at least one electron beam generated in the cathode-ray tube into two orthogonal directions when the relevant deflection current flows through each coil unit.
The U.S. Pat. No. 3,440,483 describes a display device provided with means to correct the anistropic astigmatism. In case of anisotropic astigmatism substantially no errors in the picture formation occur along the axes of the display screen, that is to say, the horizontal and the vertical axis in the middle of the display screen which means that the system is substantially anastigmatic along the axes. On the other hand picture errors do occur along the diagonals and on either side thereof on the display screen which errors are greatest in its corners. To eliminate this astigmatism, which assumes inadmissible proportions especially in a 110° deflection colour display tube, said patent proposes to pass unequal currents through the two symmetrical coil halves of one of the two deflection units, the inequality being determined by a correction current which is the product of the instantaneous value of the two deflection currents.
This is an eminent solution. However, a drawback thereof is that a deflection coil system which is free from isotropic astigmatism is more difficult to manufacture than a deflection coil system which is free from anisotropic astigmatism.
A deflection coil system which is free from anisotropic astigmatism shows picture errors along the axes, but does not show substantially any picture errors on the diagonals. However, the difficulty to eliminate the picture errors along the axes is that there is no possibility of unequal control of the deflection currents in the two symmetrical coil halves of one of the two deflection coil units. In fact, errors along the diagonals are corrected with this kind of control, whereas exactly errors along the axes have to be corrected.
In order to be able to eliminate the errors along the axes for this isotropic astigmatism the display device according to the invention is characterized in that for the purpose of correcting the isotropic astigmatism the deflection coil system is furthermore provided with at least four windings which are provided tangentially relative to one another at an angle of approximately 90° and this in such a manner that two windings thereof, which are located opposite to each other, are situated near the two gaps between the symmetrical coil halves of one of the two deflection coil units, switching means being present to pass a correction current through the four windings, which current is dependent on a current which is mainly proportional to the square of the deflection current flowing through the first and/or on a current which is mainly proportional to the square of the current flowing through the second deflection coil unit so that the four windings generate a quadripolar field which is proportional to said currents at the area of the deflection plane of the electron beam.
The step according to the invention is based on two recognitions to be explained hereinafter.
(1) To obtain a satisfactory electron landing on the screen the correction is to be performed in the deflection plane itself. This argument particularly applies to a colour display tube of the shadow mask type.
(2) The correction is to be performed with a quadripolar field.
As will be described hereinafter, no interaction varying proportionally with the deflection occurs between correction and deflection fields when using a quadripolar field. This means that the operation of the correction field remains the same independent of the fact whether or not the deflection field is active.
Both above-mentioned requirements are satisfied with the aid of the four windings provided, according to the invention at the area of the deflection plane.
In order that the invention may be readily carried into effect, a few embodiments thereof will now be described in detail by way of example with reference to the accompanying diagrammatic drawings, in which:
FIG. 1 shows a display device provided with a three-gun cathode-ray tube and switching means for sypplying the correction current through the four windings.
FIG. 2 is an elevational side view of a deflection coil system according to the invention in which the four windings are provided as toroid windings on the core of the deflection coil system itself.
FIG. 3 is an elevational front view of the deflection coil system according to FIG. 2.
FIG. 4 is a simplified elevational front view of the system as is arranged on the neck of the cathode-ray tube, in which only the four toroid windings with their series connection and the position of the three beams on the vertices of an equilateral triangle are indicated at the area of the deflection plane in the neck of the cathode-ray tube.
FIG. 5 shows the same elevational front view as that in FIG. 4 in which, however, the three beams are co-planar.
FIG. 6 serves to explain the fact that when the correction is not performed in the deflection plane itself, a mislanding of the electrons on the screen is the result thereof.
FIG. 7 shows the area of the screen S of a cathode-ray tube of the shadow mask type when the three sources of the electron beams are placed on the vertices of an equilateral triangle according to FIG. 4, and in which the picture errors occurring as a result of the isotropic astigmatism are shown.
FIG. 8 illustrates the influence of the correcting quadripolar field along the x-axis for errors as shown in FIG. 7.
FIG. 9 illustrates the influence of this quadripolar field for the errors as shown in FIG. 7 along the y-axis.
FIG. 10 shows the area of the screen S of the cathode-ray tube when the three electron sources are placed on one line as is shown in FIG. 5, and in which the occurring errors are shown.
FIG. 11 is a detailed circuit arrangement for supplying the various currents to the four windings and
FIG. 12 is a further detailed circuit arrangement when only parabolic currents are applied to the four windings.
It is to be noted that picture errors are understood to mean those deflection errors, which are referred to as astigmatism and coma. Predominantly important in this respect are the astigmatic errors since it has been attempted as much as possible in the design of the coils to eliminate the coma errors. In addition the present invention allows the designer of the deflection coil system more freedom because he may admit the astigmatic errors to a greater extent because these errors can be corrected with the aid of the quadripolar field.
FIG. 1 shows a block diagram of a colour television display device including a three-gun display tube 1 of the shadow mask type. For convenience' sake it is assumed in this Figure that the cathode-ray tube is controlled on its three cathodes by the three chrominance signals red (R), green (G) and blue (B). However, it is alternatively possible to apply the luminance signal Y to these three cathodes and to apply the three chrominance signals to separate Wehnelt cylinders not shown in FIG. 1. The display tube 1 is provided with a deflection coil system 2 which is diagrammatically shown in FIG. 1 as two flaps but will be shown in greater detail in the following Figures. Sawtooth deflection currents of line frequency are applied to this deflection coil system 2 from a line generator 3 and the vertical deflection currents are applied from a generator 4. The EHT of approximately 25 kV, denoted in FIG. 1 by V _EHT , is also applied from the generator 3, which EHT provides for the final anode voltage of the cathode-ray tube 1.
As already stated in the preamble, the deflection coil system 2 may be of the isotropic astigmatic type which means that such a coil system is free from anisotropic astigmatism. This means that picture errors will occur along the axes of the display screen and to a certain extent beyond these axes and substantially no errors will occur along the diagonals of the system of axes. Alternatively, it is of course possible to eliminate the errors as a result of the anisotropic astigmatism in a different manner, for example, with the aid of the correction method described in said prior metioned U.S. patent. The still remaining errors as a result of the isotropic astigmatism may then be corrected with the aid of the quadripolar field generated by the windings which are provided in accordance with the present invention.
To be able to correct said errors correction currents must be applied to the four windings to be described hereinafter from switching means 5 through the lines 6 and 7. The nature of these correction currents will be further described hereinafter. In this respect it is only to be noted that these correction currents must be proportional to the square of the horizontal deflection current, in this case denoted by x ² and/or the square of the vertical deflection current, in this case denoted by y ² . Particularly, when both the errors in the direction of the x-axis, which is the horizontal axis in the centre of the screen, and in the direction of the y-axis, which is the vertical axis on the centre of the screen, are to be eliminated and yet not to disturb the faultless deflection along the diagonals, the correction currents are to be chosen in accordance with the equation c ₁ x ² - c ₂ y ² ,
wherein c ₁ and c ₂ are constants which are to be chosen in such a manner that c ₁ x ² - c ₂ y ² = 0
in the vicinity of the diagonals, that is to say, the influences of the correction currents along the diagonals eliminate each other.
In the foregoing reference is made to a correction current which is proportional to the square of the horizontal and/or to the square of the vertical deflection current. In connection with non-linearities, either in the circuit arrangements used or as a result of the shape of the display screen, it may be necessary that this correction current is more proportional or less proportional to the said squares. Thus, it may be necessary under certain circumstances that also terms of, for example, x ⁴ , x ⁶ and y ⁴ , y ⁶ , etc., are included in the correction current. In practice, this means that current waveforms are used which are not purely parabolic but slightly deviate therefrom.
FIG. 1 further shows the video amplifier 8 which supplies the three chrominance signals for the cathodes of the display tube 1, the intermediate frequency amplifier 9 incorporating detectors and amplifiers which apply the video signal to the line 10 from which the three chrominance signals can be derived and which apply the synchronizing signal to the line 11 which applies field synchronizing signals through the line 12 to generator 4, and the line synchronizing signals through the line 13 to generator 3. Furthermore, FIG. 1 shows the RF-amplifier 14, which receives colour television signals from the aerial 15. Finally a line 16 leads from generator 3 to generator 5 for supplying parabolic signals of line frequency to generator 5. These parabolic signals are therefore to be assumed as control signals for the ultimate supply of the correction current proportional to x ² which is the square proportional to line or horizontal deflection current. Sawtooth signals of line frequency are applied through line 17 which, as will be further described with reference to FIG. 11, serve for possible correction of asymmetries in the horizontal deflection coil unit or of a slant arrangement of the guns in the tube 1.
Similarly parabolic signals of the frequency of the vertical deflection current are obtained from the vertical or field deflection generator 4 through the line 18. The last-mentioned parabolic signals are therefore responsible for the ultimate correction signal to be formed which is proportional to y ² being proportional to the square of the vertical deflection current. Sawtooth signals of the vertical frequency are applied through the line 19 from the generator 4, which signals likewise serve to correct asymmetries, if any, in the vertical deflection coil unit or a slant arrangement of the guns.
In order to be able to perform the desired correction as a result of the isotropic astigmatism, four windings are included in the deflection coil system 2 in accordance with the principle of the invention. These four windings may be provided directly on the neck of the display tube 1, namely below the deflection coils so that the quadripolar field generated by these four windings is indeed active at the area of the deflection plane which is the area from which the deflection of the electron beams commences. These four windings may be provided on the neck pairwise facing each other and shifted 90° in the tangential direction relative to each other. However, it is better to work with four toroid windings 20, 21, 22 and 23 as is shown in FIGS. 2, 3, 4 and 5 which coils are wound on a core 24 associated with the deflection coil system. In that case these four toroid windings are to be provided in such a manner that they are located pairwise facing each other and are shifted 90° relative to each other in the tangential direction. In addition they must be located in a certain manner relative to the deflection directions x and y so that the axes of the quadripolar field which is generated by the four toroid windings coincide with the diagonals of these x and y directions. This is shown in FIGS. 4 and 5 in which both the x- and y axes and the diagonals positioned at an angle of 45° relative thereto are shown. In this case it is to be noted that the x-y system of axes of FIGS. 4 and 5 is assumed to be in the deflection plane D, while the x-system of axes of FIGS. 7 and 10 is assumed to be at the area of the screen S.
Said locations may be obtained by winding the windings 21 and 22 at the area of the gaps between the two coil halves of the vertical deflection coil unit 25 and by winding the windings 20 and 23 opposite to each other at the area of the windows of the two coil halves of the vertical deflection coils 25 around the core 24. This means that the windings 21 and 22 are located at the area of the windows of the two coil halves of the horizontal deflection unit 26 and that the windings 20 and 23 are located at the area of the gaps between the two coil halves of this horizontal deflection unit 26. Furthermore, FIGS. 2 and 3 show that each deflection coil unit consists of two deflection coil halves, to wit the vertical deflection unit 25 consists of a deflection coil half 27 and a deflection coil half 28 while the horizontal deflection unit 26 consists of a first deflection coil half 29 and a second deflection coil half 30. It is to be noted that although only four windings 20 to 23 are mentioned above, each of the four windings may alternatively be replaced by two windings (that is to say, winding 20 may be replaced by two windings, winding 21 may be replaced by two windings, etc.) so that an overall number of 2 pairs of windings each constituting four windings is obtained. A current proportional to the line frequency is then to flow through one pair while a current proportional to the field frequency is to flow through the other pair.
The deflection coil units themselves receive their sawtooth currents from the generators 3 and 4, respectively, and will not be further referred to because they operate in known manner.
The four toroid windings are arranged in series in the Examples according to FIGS. 4, 5 and 11, so that the desired correction currents can be applied from the generator 5 to their ends 6 and 7. However, it will be evident that these coils may alternatively be arranged in parallel or pairwise in series and each further pair in parallel. The choice thereof will depend on the number of turns for each toroid winding and therefore the current required in various cases.
FIGS. 4 and 5 show magnetic lines of force which are generated by the field of the four toroid windings 20 to 23 if a current flowing in a certain direction flows through these windings. The arrows in these lines of force show clearly that a quadripolar field is used whose axes constitute the diagonals of the x-y axis system. FIGS. 4 and 5 also show by way of the characters R, G and B the positions of the red (R), the green (G) and the blue (B) electron beams, respectively. Since in FIGS. 4 and 5 this is assumed to be at the area of the deflection plane D, the points R, G and B should be considered as fictitious points, since actually the electron beams are deflected by the action of the deflection field. In that case the deflection takes place gradually over a certain distance. Actually, it is therefore not possible to speak of a deflection plane. For the sake of simplicity, reference is always made to a deflection plane D in this respect because this does not detract from the nature of the description. In the Example of FIG. 4, the three beams are considered to be situated on vertices of an equilateral triangle, while in the Example of FIG. 5 they are considered to be situated in a plane which passes through the x-axis and the axis of tube 1. In the case of FIG. 4, forces on the electron beams R, G an B are exerted by the quadripolar field which forces are denoted by the arrows at each of the points R, G and B. It will be explained with reference to FIGS. 8 and 9 how these forces cause the desired correction to take place. The direction of the arrows as shown in FIG. 4 then corresponds to a correction as will be described with reference to FIG. 8, which correction applies for the x-direction on the screen. In that case the current which then flows through lines 6 and 7 may be considered as a positive current proportional to x ² . For the correction required along the y -axis as will be described with reference to FIG. 9, the forces should reverse their direction. This is evident from the arrows shown in FIG. 9 at the points R, G and B. Consequently, the current then flowing through the lines 6 and 7 must be proportional to - y ² which in the foregoing is expressed by the formula c ₁ x ² - c ₂ y ² . Thus FIG. 4 corresponds to FIG. 8 and FIG. 9 corresponds to a quadripolar field according to FIG. 4 in which the current which is applied through the lines 6 and 7 is of opposite sign and consequently the arrows shown in the magnetic lines of force and the forces caused thereby at the points R, G and B denoted by the arrows shown at said points must all be reversed in their direction. Furthermore FIGS. 8 and 9 diagrammatically show the quadripolar fields by means of four magnetic poles 31, 32, 33 and 34 which are shown at the ends of the diagonals and which symbolize the action of the quadripolar field at the area of the screen S. However, it is to be noted that this quadripolar field at the area of the deflection field is generated by the four toroid windings 21 to 23 and is therefore actually active in that area.
Furthermore it is to be stated that in FIG. 4 the positions of the beams R and G are located on the lower side of the x-axis while the positions of the beams R', G' and R and G in FIGS. 8 and 9 are located above the x" and x' axes, respectively. This results from the fact that reversal takes place at deflection because the beams cross each other before they impinge upon the screen so that the situation at the area of the screen S is reversed relative to the situation at the area of the deflection plane D. These points of intersection are substantially located on a sphere whose radius of curvature is determined by the picture field curvature of the deflection coil system 2. However, since the action of force by the quadripolar field is effected at the area of the deflection plane D, the direction of the arrows as shown in FIG. 4, associated with each of the points R, G and B, must be the same as those in FIG. 8 for the corresponding points. Consequently, the electron beams are indeed displaced in a manner as is shown in FIG. 8, if this Figure corresponds to FIG. 4. The same applies of course to FIG. 9 if the current direction in FIG. 4 is reversed.
Furthermore, it is to be noted that a centre C ₀ is shown in FIGS. 4, 5, 7 and 10, which centre corresponds to the axis z of the display tube 1. It is true that the FIGS. 4 and 5 are assumed to be in the deflection plane D and FIGS. 7 and 10 are assumed to be at the area of the screen S, but since the axis z passes both through the centre C ₀₁ of the deflection plane D and the centre C ₀₂ of the screen S, these centres may be considered to be corresponding. Centres C ₀ " and C ₀ ' in FIGS. 8 and 9 must, however, be considered as transformed centres, because FIGS. 8 and 9 apply to a deflected position on the screen S in the direction of the x-axis and the direction of the y-axis, respectively.
A consideration of FIGS. 7 and 8 shows that the desired correction can be obtained with the aid of the generated quadripolar field. In fact, in FIG. 7 the error along the x and y-axes is indicated for a display device in which the deflection coil system 2 exclusively has an isotropic astigmatic error. It can be seen that the desired circular form for which the three electron beams at the area of the screen S are always located on the vertices of an equilateral triangle is distorted along the axes and remains intact along the diagonals. Thus, the vertices of FIG. 7 include four circles from which it is clearly evident that the electron beams R, G and B at the area of the screen S remain located on the vertices of an equilateral triangle in spite of the deflection. This means that the dynamic radial convergence which is established in known manner by means of a separate Convergence unit with convergence currents, which have substantially the same amplitude for each of the three beams can combine the three beams in one point so that they actually cross one another at the area of the screen S.
FIG. 7 shows that as a result of the isotropic astigmatism along the x-axis the circle is extended to form an ellipse, the long axis of the ellipse being located in the y-direction which corresponds to FIG. 8. This is exactly reversed along the y-axis and then likewise the circle is extended to form an ellipse the long axis of which is, however, located in the x-direction which corresponds to FIG. 9. As already described hereinbefore, FIG. 8 corresponds to a correction in the x-direction and this correction is performed with the aid of quadripolar fields which are proportional to x ² being the square of the horizontal deflection current. In fact, this horizontal deflection current increases in the horizontal direction on either side of the y-axis passing through the centre C ₀₂ . Since the errors on the left and right-hand sides of this y-axis are the same, it follows that a current must be passed through the four windings 20 to 23 which current is mainly proportional to the square of the horizontal deflection current, that is to say, proportional to x ² . FIG. 8 shows that the points R, G and B on the ellipse are displaced along the arrows as a result of the action of force in the quadripolar field to the points R', G' and B', respectively, which are located on a circle. Due to the normally active dynamic convergence the electron beams brought to the points R', G' and B' may then be combined in the centre C ₀ , at substantially the same convergence currents, so that a satisfactory colour display is ensured. It will be evident that the desired correction is obtained because the correction current is proportional to the square of the horizontal deflection current at any point of the screen on either side of the y-axis. As will be further described with reference to FIG. 11, the extreme value of the parabolic current of line frequency used for this purpose must then be set at a zero value in the middle of the line scan period, because no correction proportional to x ² is required for this middle. This means that the extreme value of the parabola must be clamped at a zero level, or, when adaptation to the static convergence in the centre C ₀₂ is desired, to an adapted level.
The same applies to FIG. 9 because in this Figure the ellipse with its long axis is located in the x-direction and therefore FIG. 9 corresponds to the errors which occur along the y-axis in the vertical direction. Since the errors above and below the x-axis are the same, this explain the necessity to have the correction current to be proportional to y ² . Also in this case the correction will proceed smoothly when a parabolic current of field frequency is used whose extreme value is set to a zero value in the middle of the field scan period, or to a level adapted to the static convergence. Then again it is ensured that the desired correction is performed for any point on the display screen above and below the x-axis because the correction current is proportional to the square of the vertical deflection current. It follows from FIG. 9 that the points R, G and B located on an ellipse are displayed along the arrows to the points R', G' and B' due to the action of force of the quadripolar field, which points are again located on a circle and which can be centered in the centre C ₀ ' due to the normally active dynamic convergence. This again ensures that the three electron beams can be combined at substantially the same convergence currents at any point of the screen.

The aforementioned choice of C ₁ x ² -C ₂ y ² = 0 on the diagonals ensures that the situation on the diagonals is not disturbed by the correction. Said choice of C ₁ x ² -C ₁ y ² = 0 on the diagonals is, however, only necessary when both the isotropic astigmatism on the x-axis and that on the y-axis are to be eliminated. On the other hand, if, for example, for the indexing tube or the three-gun chromatron tube the errors as a result of the isotropic astigmatism are to be eliminated only on the x-axis or only on the y-axis, then it is sufficient to pass through the four toroid windings only a current which is proportional to x ² or only a current which is proportional to y ² .

It has already been stated in the preamble that the action of the correcting quadripolar field must take place in the deflection plane D. The reason for this will be described with reference to FIG. 6. This Figure diagrammatically illustrates a cross-section through the display tube 1 in which the plane K denotes the plane where the cathodes of the display tube 1 are situated, the plane D denotes the deflection plane at the area of the deflection coil system 2, M denotes the position of the mask and S denotes the position of the screen on which the phosphors have been provided. Furthermore the z-axis in FIG. 6 indicates the axis of the display tube and the point C ₀₁ corresponds to the centre C ₀₁ in FIG. 4 and FIG. 5, respectively, and the centre C ₀₂ corresponds to the centre C ₀₂ in FIGS. 7 and 10, respectively. FIG. 6 furthermore only shows one electron beam, for example, the blue (B) beam which emanates from the plane K and which normally passes through the point P so as to pass through the point C ₀₂ at the area of the screen S which point is the point of intersection of the screen S and the axis z of the display tube 1. If the electron beam were deflected in the plane D from the point P, it would meet at point Q on the screen S which point Q is the correct point because this corresponds to the blue phosphor dot which is provided on this spot on the screen S. However, the foregoing shows that a correction field is necessary to eliminate the error due to isotropic astigmatism and picture field curvature. If this correction as seen in the direction of displacement of the electrons were performed before the deflection plane D, then this means that the electron beam, starting from the point B in the plane K undergoes a displacement in advance so that it would not pass the deflection plane D at the point P, but at the point P' and would undergo a deflection in situ, so that ultimately this electron beam impinges upon the screen S at the point Q'. However, since the points Q and Q' on the screen S do not coincide, this means that there is actually a mislanding of the electrons on the screen S. In fact the landing is displaced and consequently this may result in a less saturated colour or a faulty colour rendition, since only a correct landing in the point Q on the blue phosphor dot provided in situ ensures a saturated and faultless colour rendition. This can be achieved by performing the required correction not before the deflection plane but in the deflection plane and this in such a manner that the deflection point remains the point P, but that the deflection of the electron is corrected by the quadripolar field. It is then achieved that a correct landing of the electrons on the screen S is ensured under all circumstances. Consequently, the quadripolar field produced by the four toroid windings should engage the deflection plane D. By winding in accordance with the principle of the invention the four toroid windings on the core 24 this condition is satisfied and it has been achieved that the plane of the re-adjustment of the electron beams coincides or at least approximately coincides with the deflection plane D of the deflection coil system 2, so that the landing on the screen S is not influenced. It may be noted that the normal dynamic convergence which is still required displaces the electron beams before they reach the deflection plane D, but this displacement does not result in a mislanding. The reason is that it is ensured with the aid of the principle according to the invention that the three convergence currents for the separate convergence unit have the same mutual amplitude for any point on the screen. It is true that a displacement in the landing spot is obtained (because the separate convergence unit is active between the planed K and D) but this may be adapted by a varied screen mask distance. For the described correction with the quadripolar field the displacement of the three beams R, G and B is, however, unequal. In summary it may therefore be stated that: the required displacement is split up into
(a) a part which is mutually equal for the three beams and which is formed with the aid of the separate convergence unit which, as is known, is active between the plane of the cathodes K and the deflection plane D; and
(b) a part which is mutually unequal for the three beams and which is formed with the aid of the quadripolar field active in the deflection plane D.
The four windings are always indicated hereinbefore as four toroid windings wound on the core 24. However, it is alternatively possible to adhere the four windings separately on the neck of the tube under the deflection coil system 2. Then the adjustment is, however, much more critical because the position of these four windings must exactly be determined in advance relative to the deflection coil system 2 to be provided later on. In addition there must be a possibility of shifting the deflection coil system 2 somewhat in the axial direction so as to be able to adjust the correct position of the deflection coil system 2. Winding on the core is therefore preferred because upon shifting the windings 20 to 23 are also shifted.
Although the Example of FIG. 1 always described a three-beam cathode-ray tube of the shadow mask type, it will be evident that the principle of the invention is not limited to this kind of tube along. Thus, it is alternatively possible to use a three-beam display tube of the chromatron type in which the electron beams are located in a plane passing through the x-axis and through the axis of the tube 1. (See FIG. 5). The same is alternatively possible for a three-beam display tube of the shadow mask type. In that case errors as shown in FIG. 10 will likewise occur as a result of isotropic astigmatism. This means that the electron beam B always remains in the deflection centre or either side of the x-axis, but that the beams R and G are further moved away from this centre. By generating a field of force as shown in FIG. 5 the beams G and R are displaced in accordance with the arrows shown in this Figure. This means that the beam G is displaced to the right-hand side as a result of the quadripolar field and the beam R is displaced to the left-hand side. These displacements are necessary to eliminate the errors in either side of the x-axis. Consequently the quadripolar field according to FIG. 5 is produced by a current which is proportional to the square of the vertical deflection current, that is to say, proportional to y ² .
Unlike the Examples described with reference to FIGS. 4 and 7, there is now the possibility to make the three beams R, G and B completely coincide without a separate dynamic convergence unit. In fact, as FIG. 10 shows, the blue beam B on either side of the y-axis is not displaced from the new deflection centre so that also when the same forces are exerted by the quadripolar field on the beams G and R, these beams can be made to coincide with the beam B at the area of the screen S. It follows that also the quadripolar field which is active in the x-direction must be proportionally to x ² which means that it must be generated with the aid of a parabolic current of line frequency.
The same applies to the diagonals. In fact, also in that case beam B always remains in the deflection centre, but the beams G and R are located on either side thereof (see the circles in the vertices of the screen S in FIG. 10). Also in this case a force on beam G to the right-hand side and a force on beam R to the left-hand side may ensure that these two beams are made to coincide with the beam B.
However, the required field of force in the direction of the x-axis is the smallest, it is larger along the diagonals and it is the largest on the direction of the y-axis. If it is assumed that everything on the screen S were located on a circle, the required current for the four windings 20 to 23 would have to be proportional to c ₁ x ² + c ₂ y ² .
However, the correction in the x-direction may be smaller than for the case of the circle because the beams G and R have already been displaced somewhat to the centre as a result of the isotropic astigmatic error. As a result the correction current is to be reduced by a factor of c ₃ x ² relative to the case of the pure circle.
However, in the y-direction the action of the quadripolar field must be stronger because the beams G and R are then farther away from the beam B. Consequently the correction current is to be increased by a factor of c ₄ y ² .
Consequently an overall correction current is obtained which is determined by c ₁ x ² + c ₂ y ² - c ₃ x ² + c ₄ y ²
In that case there must apply that c ₄ y ² - c ₃ x ² = 0
on the diagonals, since exclusively the correction relative to the case of the circle must be performed for these diagonals.
The last equation may be written as (c ₁ - c ₃ )x ² + (c ₂ + c ₄ )y ²
which equation with (c ₁ - c ₃ ) = c ₁ ' and (c ₂ + c ₄ ) = c ₂ '
changes into c ₁ 'x ² + c ₂ 'y ²
and in which c ₁ ' = c ₁ - c ₃ ≥0
(The zero case applies when the isotropic astigmatic error ensures that the beams R, G and B already coincide in the x-direction).
In the case of FIG. 10 this means that as a rule the overall correction current must consist of the sum of a parabolic current of line frequency and a parabolic current of field frequency. The circuit arrangement which is required for generating these currents is shown in FIG. 12 and will be described hereinafter.
It will be evident that the principle described in FIG. 10 is alternatively usable for a colour display tube of the indexing type. In fact, in such tubes the spot may not extend in a horizontal direction because this would lead to the display of unsaturated colours in an indexing tube in which the colour strips are arranged vertically on the screen. A quadripolar field according to the invention may then ensure that the sagittal picture plane for one deflection direction and the meridional picture plane for the other deflection direction coincide with the screen of the tube. Due to this additional degree of freedom in the design of the coils it is possible to start from a deflection coil having smaller residual errors.
The fact that the action of the quadripolar field on the beams is independent of the deflection performed is proved as follows:
From Maxwell's second law there follows that div B = 0 (1)
wherein B is the vectorial representation of the magnetic induction.
Equation (1) may be written as δB _x /δ _x + δB _y /δ _y + δB _z /δ _z = 0 (2)
if a three-dimensional field is supposed calculated relative to a system of axes x, y, z.
Since for the correction field a flat plane D is always used, a system of axes x, y is left as is shown in FIGS. 4 and 5. For such a flat plane δB _z /δ _z = 0
which causes equation (2) to change into δB _x /δ _x + δB _y /δ _y = 0 (3)
For the calculation to be further performed it is simpler to change into polar coordinates r and ψ for which there applies that r = √x ² + y ² and tg ψ = x/y
In this formula ψ is the angle located between the radius r and the y-axis. When furthermore the solution for the field strengths B _x and B _y in pole coordinates is given by some approximation in a field of n-poles by B _x = f (r)  sin (1 - n/2)ψ (4)
and B _y = f (r)  cos (1 - n/2)ψ (5)
then after the change-over into pole coordinates with the aid of (3) it is found that δB _x /δ _x + δB _y /δ _y = cos nψ/2 {f' (r) + (1-n/2) f (r)/r }= 0 (6)
with f' (r) = [df(r)/dr]
From this follows: f' (r) = (n/2 - 1) [f (r) /r]
with the solution f (r) = A.r (n/2 - 1) (7)
wherein A is the integration constant.
Filling in equation (7) in the equations (4) and (5) results in B _x = Ar (n/2 - 1) sin (1-n/2)ψ (8) B _y = Ar (n/2 - 1) cos (1 - n/2)ψ (9)
The values of the field strength B may be assumed to be a vector by expressing it in a complex plane as B = B _y + iB _x = Ar (n/2 - 1).sbsp.e ⁱ (1-n/2) ψ= A(r. e ^-i ψ) (n/2 - 1) (10)
It can be deduced from equation (10) that the chosen solutions in accordance with the equations (4) and (5) are correct. In fact, for n = 2, that is, for a bipolar field equation 10 changes to B = A. (10')
this means that the field strength is constant and real which is correct, if the dispersion losses are ignored a field can be seen in the y-direction (being the real axis) which has the same intensity throughout independent of the coordinates r and ψ and x and y, respectively.
In order to check the influence of the deflection for these multipolar fields to a complex auxiliary magnitude v is introduced so as to simplify equation (10) which magnitude may be expressed in the coordinates x and y in accordance with v = y - ix.
If the latter equation is written in polar coordinates it changes into V = r cosψ - r.i. sinψ - re ⁱ ψ
Filling in equation (11) in equation (10) the result is B = A (v) (n/2 - 1) (12)
In case of deflection the influence of the field must be checked in accordance with equation (12) which may be effected by assuming that a transformation equation v' = v - v _o (13)
can be set up after deflection to a point v _o which expresses the value of a new complex magnitude v' relative to the new origin v _o .
Introducing the transformation equation (13) into equation (12) yields B = A(v' + v _o ) (n/2 - 1) (14)
The influence of the multipolar field can now be checked with the aid of equation (14) for different values of n. For a quadripolar field n = 4 and equation 14 changes into B = A (v' + v _o ) (15)
Both Av' and Av _o may be considered as a pure quadripolar field. In fact, the factor n/2 - 1 which determines the nature of the field changes for n = 4 into n/2 - 1 = 1, that is to say, a quadripolar field is expressed by a power 1. Since in equation (15) both v' and v _o have the power 1 they may both be considered as a quadripolar field. The term Av _o represents a homogeneous field which exerts the same action on the three beams in magnitude and direction. The term Av' represents a quadripolar field which as regard its effect on the three beams is equal to the original quadripolar field Av. This may alternatively be expressed by stating that v _o may be considered as a new centre of a correcting quadripolar field v which varies proportionally with the deflection.
However, if a hexapolar field were chosen as a correction field, then n would have become 6. Introducing this in equation (14) results in B = A(v' + v _o ) ² = A(v' ² + v _o ² + 2v'v _o ) (16)
The factor n/2 - 1 changes into 2, that is to say, a hexapolar field is expressed by a power 2. Consequently it is found from equation (16) that in addition to the active hexapolar field Av ² and the homogenous field Av _o ² , wherein v _o may be considered as a new centre for the correcting field, a quadripolar field 2Av' is produced which is influenced in amplitude and direction by the new centre v _o because, in fact, it occurs as a product term together with v _o in equation (16). From this follows that interaction takes place between correcting hexapolar field and deflection field. A similar reasoning as for a hexapolar field etc. may be maintained for the octapolar field (fill in n = 8 in equation 14) so that this proves that only an additional quadripolar field can be used for the correction of isotropic astigmatism to be described.
FIG. 11 shows a possible embodiment of the generator 5 according to FIG. 1 for generating currents for correcting errors as described with reference to FIGS. 4 and 7. A possible signal 35 of line frequency is applied to the input terminal 16 of the generator 5, which parabola is thus proportional to the square of the horizontal deflection current, that is to say, it is proportional to x ² . This signal 35 is applied to a potentiometer 36 whose wiper is connected through a resistor 37 and a capacitor 38 of high value for the line frequency to a base electrode of a first amplifier 39. At the other end a parabola signal 40 of the vertical frequency is applied to an input terminal 18, whose signal is therefore proportional to y ² , that is to say, to the square of the vertical deflection current. This signal is applied to a potentiometer 41 whose wiper is connected likewise through a resistor 42 and a large isolation capacitor 43 to the base electrode of the amplifier 39. The constants c ₁ and c ₂ may optionally be adjusted with the aid of the wipers on the potentiometers 36 and 41, so that the condition c ₁ x ² - c ₂ y ² = 0 on the diagonals of the screen can be satisfied. The two signals are amplified as a sum signal in the amplifier 39 and are subsequently applied to a push-pull output stage which comprises the complementary pair of transistors 43" and 44 which are connected to a supply voltage of +30 Volts. The interconnected emitters of the transistors 43" and 44 are connected to the terminal 6, while the terminal 7 is connected to earth through a current feedback resistor 48. The series arrangement of the four windings 20, 21, 22 and 23 is arranged between the terminals 6 and 7. Since the same current flows through the four windings in this case, their winding sense, as shown in FIG. 4, should be such that the lines of force have a variation as is shown in this Figure. This means that windings 21 and 22 must be wound in the same sense, but windings 20 and 23 must be wound in the opposite sense on the core 24.
Unlike the afore-described pure series arrangement of the four windings 20, 21, 22 and 23 it is possible to use a series-parallel arrangement in which 21 and 22 are arranged in series and 20 and 23 are arranged in series and in which these two series arrangements are subsequently arranged in parallel, taking into account the desired generation of the quadripolar field. It is of course alternatively possible to reverse the connecting terminals of the windings 21 and 22 in the pure series arrangement as shown in FIG. 11 relative to that of the windings 20 and 23 so as to obtain also a variation of the lines of force as is shown in FIGS. 4 and 5.
FIG. 11 likewise shows clearly that it is possible to use c ₁ x ² - c ₂ y ² because the parabola signal 35 is reversed relative to the parabola signal 40, because their extreme values are directed either positively or negatively. The two signals 35 and 40 are applied through capacitors 38 and 43 to clamping diodes 38' and 43' which clamp the extreme values of these parabola signals at earth potential. These clamped signals are subsequently applied to the base of transistor 39 whose DC-adjustment is ensured by means of a potentiometer comprising a variable resistor 45 and two resistors 46 and 47 of fixed value. The desired direct current can be adjusted by means of the resistor 45.
The feedback resistor 48 which is connected through resistor 47 to the base of transistor 39 provides for the desired linearity of the circuit arrangement. In this manner it is also achieved that the circuit arrangement as seen from the terminal 6 may be considered as a current source.
Furthermore the collector line of transistor 39 includes a resistor 49 which serves to ensure the desired drive of transistors 43" and 44. The series arrangement of a resistor 51 and a capacitor 50 is connected in parallel with the series arrangement of the windings 20, 21, 22 and 23. This series arrangement serves to avoid unwanted ringing phenomena in the circuit arrangement.
FIG. 11 furthermore shows that two sawtooth signals 52 and 53 of line frequency are applied with opposite polarity to the input terminal 17. If the wiper on potentiometer 54 is in the middle, no sawtooth signal is applied to the capacitor 38 through the isolation resistor 55 and consequently no sawtooth signal of line frequency is added the parabola 35. If the wiper on the potentiometer is moved in the direction of the terminal to which the signal 52 is applied, a sawtooth of the polarity of the signal 52 is added to the signal 35. However, if this wiper is moved to the terminal to which the signal 53 is applied, a signal of the polarity of the last-mentioned signal is added to the signal 35. As already described in the preamble these sawtooth signals serve to eliminate asymmetry between the coil halves 29 and 30 for the line deflection. These sawtooth voltages may also correct a possible slant position of the electron guns which must produce the electron beams R, G and B. If there is no question of asymmetry between the horizontal coil halves 29 and 30 and if the guns have no slanted position, wiper 54 may indeed be adjusted precisely in the middle.
The same applies to the sawtooth signals 56 and 57 of vertical frequency which are applied to the terminal 19. If the wiper on potentiometer 58 is adjusted exactly in the middle, no sawtooth signal can be added to the parabola signal 40 through the resistor 59. Dependent on the movement of this wiper, a signal of the polarity of the signal 56 or of the polarity of the signal 57 may be added to the parabola 40. The supply of the sawtooth signals 56 and 57 serves to eliminate asymmetries between the vertical coil halves 27 and 28 or also in this case to correct a possible slanted position of the guns. If the said asymmetries or slanted position of the guns is not present in this case, the wiper on potentiometer 58 may be placed in its central position. If there are two pairs of four windings each, the circuit arrangement according to FIG. 11 is to be split up in two parts. The first part then supplies parabola and/or possibly sawtooth currents of line frequency to the first pair of four windings, the other part supplies parabola currents and possibly sawtooth currents of field frequency to the second pair of four windings.
FIG. 12 shows a circuit arrangement for the case where generator 5 must generate currents in accordance with the equation c ₁ 'x ² + c ₂ 'y ² , that is to say, for the elimination of errors described with reference to FIG. 10. Then the two parabola currents must have the same polarity. This follows from FIG. 12 in which the minimum of the field frequency parabola 40' points in the same direction as the line frequency parabola 35. The coefficients c ₁ ' and c ₂ ' may be adjusted with the wipers on potentiometers 36 and 41, respectively. The circuit arrangement according to FIG. 12 is a so-called magnetic clamping circuit which is extensively described in the U.S. patent application Ser. No. 43,369, filed June 4, 1970. In this case it is only stated that the terminal 6 is not DC coupled, but is coupled through an isolation capacitor 60 to the emitters of the transistors 43" and 44. The required clamping is obtained by means of an additional winding 61 which is also wound on the core 24. The mean current of the pulsatory current flowing through the transistor 43" exclusively flows through the winding 61 because coil 61 is shunted by a capacitor 62 which has a high value for field and line frequencies. Since the correct ratio to be chosen between the number of turns on the winding 61 and the overall number of turns of the four windings 20 to 23, a homogeneous field induced from winding 61 may be added to the alternating field induced from the windings 20 to 23 in the core 24, the extreme values of the parabolas being adjusted at the zero level exactly in the middle of the scan period.
The great advantage of the magnetic clamping circuit according to FIG. 12 is that clamping of the extreme value of the applied signal is exclusively dependent on the shape (in this case the parabola shape) of these signals and not on their amplitude or frequency. In the Example according to FIG. 12, the supply of the sawtooth signals 52, 53 and 56, 57 as shown in FIG. 11 is not shown. If in the Example according to FIG. 12 this is desired, the supply of these sawtooth signals may take place in a similar manner as in the circuit arrangement according to FIG. 11.

WEGA COLOR 3016 CRT TUBE PHILIPS A66-410X. GLASS FOR TELEVISION DISPLAY CATHODE-RAY TUBES:
Glass for envelopes of television display cathode-ray tubes, particularly screen glass for color television, which transmits at most 0.5 mr/h of X-ray radiation at an acceleration voltage of 40 to 45 k. volt, and which has a composition in percent by weight:

what is claimed is

1. Glass for envelopes of television display cathode-ray tubes, particularly intended for the face-plate of the tube, consisting essentially of the following in percent by weight:

2. Glass as claimed in claim 1, consisting essentially of the following in percent by weight:

Description:

The invention relates to glass for an envelope of a television display cathode-ray tube, particularly glass for the face-plate of the tube.

Particular requirements are imposed on glass for envelopes of cathode-ray tubes for the display of colored television images as compared with that for the display of monochrome television images. Such special glasses are known, for example, from the British Pat. specification No. 1,123,857 the composition of which in percent by weight lies within the following range of compositions:

SiO ₂ 62-66 BaO 11-14 Li ₂ O 0-1 MgO 0-3 Na ₂ O 7-8.5 PbO 0-2 K ₂ O 6.5-9 Al ₂ O ₃ 1-4 CaO 2-4.5 As ₂ O ₃ + Sb ₂ O ₃ 0.3-0.7 CeO ₂ 0.05-0.3

the special requirements which, as compared with glass for the envelopes for monochrome display, are imposed on glass for envelopes for color display, are connected with differences in the manufacture and in the use of these tubes. In the first place, the glass components of envelopes for color display unlike those for the monochrome display envelopes cannot be sealed by fusing them together but must be connected together with the aid of an enamel. This is connected with the fact that a shadow mask is provided in these tubes, which mask determines the path of the required three electron beams. Furthermore, an extremely fine grating-like pattern of three different luminescent substances corresponding to the apertures of the shadow mask is provided on the inner side of the screen. The requirements relative to the maximum permissible distortion of the glass are in this case much more stringent in connection therewith than for glass of envelopes for monochrome display. In addition, the temperature at which the tube must be heated during evacuation and sealing must be approximately 20° higher and the heat treatment is of a longer duration than for the tubes for monochrome display.

The glasses within the above-mentioned range are eminently satisfactory in a technological respect relative to the softening point, the quality and the thermal coefficient of expansion. For the acceleration voltages until recently used on the electron guns, the absorption of these glasses for the X-ray radiation generated during operation as a result of the electron bombardment on the glass and on the shadow mask is sufficiently great. This even applies when the tube is built in in a cabinet in direct vision construction, thus without a protective cover glass.

The requirement up till now had been that the intensity of the transmitted X-ray radiation may be at most 0.5 milliroentgen per hour (mr/h) at a maximum thickness of 11 mm of the screen glass, an acceleration voltage of 27.5 k. volt and an anode current of 300 μA in a television display tube.

There is, however, a tendency to still further increase the margin of safety to X-ray radiation transmitted by television display tubes. There is a need of a kind of glass in which at most 0.5 mr/h is transmitted at an acceleration voltage of 35 k.volt. The above-described glasses then no longer have a sufficiently high absorption and do not satisfy the stricter safety requirements. For reasons of a technological nature, the thickness of the screen cannot be increased much further than 11 mm. To obtain a sufficiently high absorption while using a glass within the above-mentioned range of compositions, the screen should be thicker by as much as 2.5 mm.

For a satisfactory processing of the glass and moulding face-plates thereof, it is necessary that the temperature dependence of the viscosity is not too great. In practice this means that the temperature difference between the softening point, which is the temperature at which the viscosity of the glass is 10 ^{7 6} poises, and the annealing point, which is the temperature at which the viscosity of the glass is 10 ^{13 4} poises, must be at least 190° C.

In connection with the conventional manufacturing technique and the very stringent requirements which are imposed on the maximum permissible distortion of the glass components during manufacture of the tube, it is necessary that glass for a color display tube has an annealing point which is not lower than 485° C.

Finally it is of importance that a glass for a color television display tube has approximately the same coefficient of expansion as that of the known glasses (approximately 99 × 10 ^-7 between 30° and 300° C.), so that a better match is obtained with the existing glasses and metal components which must be sealed on or in respectively.

In the kind of glass according to the present invention, a content of PbO is present with an approximately equal BaO content relative to the known glass. It is by no means surprising in itself that the absorption of X-ray radiation is increased as a result thereof. It was, however, not obvious that it was possible to maintain the physical properties of the glass at the same level by means of a few other modifications. Furthermore, it is also known (from U. K. Pat. specification No. 664,769) that no discoloration occurs on the glass of the face-plate due to electron bombardment, provided that the glass contains CeO ₂ and provided that the glass contains no more than 1 percent of readily reducible oxides. However, the glass according to the invention which does not satisfy the last-mentioned requirement owing to its content of PbO, does not discolor under the influence of the electron bombardment.

The range of glass compositions according to the present invention is characterized by the following limits in percent by weight:

SiO ₂ 58-67 PbO 2-7 Li ₂ O0-1 MgO 0-3 Na ₂ O2-3 Al ₂ O ₃ 1-4 K ₂ O11-14 As ₂ O ₃ + 0.3-0.7 CaO 3-4.5 CeO ₂ 0.05-0.6 BaO 11-14

the softening point of these glasses lies between 690° and 710° C.; the annealing point between 485° and 510° C. and the thermal coefficient of expansion is approximately 97 to 100 × 10 ^-7 between 30° and 300° C. The glasses according to the invention amply satisfy the above-mentioned requirement of transmitting at most 0.5 mr/h at an acceleration voltage of 35 K.volt; it was found that this amount was not yet reached at an acceleration voltage of even 44 to 45 k.volt when using these glasses. The electric resistance 9 ^is at least 10 ⁹ 4 ohm.cm and at least 10 ^{7 5} ohm.cm at 250° and 350°, respectively, while these values are 10 ^{8 5} and 10 ^{6 7} ohm.cm for the above-mentioned known glasses.

The following glass is an example of a glass suitable for the relevant purpose. It is obtained in a manner which is common practice in glass technology by melting the relevant oxides or compounds which are converted into the oxides.

SiO ₂ 59% by weight PbO 6.3% by weight. Li ₂ O0.4% by weight Al ₂ O ₃ 2.2% by weight. Na ₂ O2.4% by weight Sb ₂ O ₃ 0.3% by weight. K ₂ O12.7% by weight CeO ₂ 0.5% by weight. CaO 3.6% by weight Softening point 705°C. BaO 12.3% by weight annealing point 502°C. coefficient of expansion (30-300° C.) 97 × 10 ^-7 log 250°C. = 10.3 log 350°C. = 8.2.