This invention relates in general to wave signal tuners and
in particular to an improved continuous type tuner for the UHF
television band.
Under present allocations there are two rather
widely spaced bands in the radio frequency spectrum which are reserved
for television broadcasting. The first, a relatively low frequency band,
is designated the VHF band and it accommodates twelve channels; five
having frequency assignments between 54 and 88 megacycles and seven
between 174 and 216 megacycles. The second band is the relatively high
frequency UHF band which accommodates seventy television channels at 6
megacycle intervals between 470 and 890 megacycles.
In view of
the relatively few (12) channels in the VHF band, either a turret or a
switch type tuner, that is, a tuner having a discrete-stop or position
for each channel, is feasible. Insofar as UHF is concerned, however, a
discrete-stop tuner is obviously impractical because of the number of
positions (70) that would be required. While tuning strips tailored to
individual UHF channels are available for use in turret type VHF tuners,
the total number of UHF and VHF stations that can be accommodated is
limited, of course, to the number of strips which may be accommodated on
the turret.
In view of the aforementioned mechanical
considerations, the prior art has invariably resorted to a continuous
type tuner for receivers designed to accommodate the entire UHF band.
The frequency determining circuits for such tuners, however, pose
special design problems since conventional lumped constant circuit
elements, which ordinarily suffice at VHF, do not function properly at
UHF. This is due to the fact that the physical dimensions of such
components become an appreciable fraction of the wavelength of UHF
signals, and particularly is this the case in the upper reaches of the
UHF band. This, in turn, dictates recourse to distributed constant
elements, such as tunable transmission lines, for use in the frequency
determining circuits.
A conventional tuned-line UHF tuner of the
type above-mentioned comprises one or more RF preselector stages, a
vacuum tube oscillator stage and a mixer circuit which develops an IF or
difference frequency signal by heterodyning a selected RF signal with
the oscillator signal. It is conventional practice to use substantially
identical quarter-wave transmission line elements, which are tuned by
rotatably supported capacitor electrodes, in each of the preselector
stages while employing a tunable half-wave line in the oscillator stage.
While the operating frequency of the oscillator throughout most of its
range is primarily controlled by the tuning capacitor, it is also
conventional prior art practice to employ separate trimmer capacitors to
insure that the upper and lower limits of the UHF range can be readily
tuned. Specifically, when the tuning capacitor is positioned for minimum
capacitance, one trimmer capacitor is adjusted to tune the oscillator
to the high frequency end of the band. On the other hand, when the
tuning capacitor is positioned for maximum capacitance, a second trimmer
capacitor is adjusted so as to establish the lower frequency limit of
the oscillator. In like fashion, the upper tuning range of the
preselector stages is determined by a separate trimmer capacitor in each
stage. All of these expedients, while effective, are undesirably costly
and complex, both as to component requirements and assembly and
alignment procedures in production.
It is therefore a principal object of the invention to provide a new and improved multi-stage UHF television tuner.
It is also an object of the invention to provide a UHF tuner construction which requires a minimum number of component parts.
It is another object of the invention to provide a continuous UHF television tuner of a unique and economical construction.
A
continuously adjustable UHF tuner constructed in accordance with the
invention comprises a housing which has a plurality of compartments each
of which includes a signal translating stage. A control shaft extends
through each of the compartments and is rotatably supported by the end
walls of the housing. The tuner also includes a corresponding plurality
of tunable frequency selector circuits, one for each stage and each
comprising an inductor having an electrical length which approaches one
quarter of a wavelength at the high frequency end of the UHF band. Each
tunable circuit further includes a capacitor having a stationary
electrode constituted by an extension of the inductor and an assigned
pair of spaced electrodes which are affixed to the control shaft for
rotational displacement from a position overlapping and embracing the
stationary electrode to a position remote therefrom. All of the
displaceable electrodes have a substantially identical configuration and
at least one of each of the displaceable electrode pairs has an
adjustable tab for establishing, in conjunction with its assigned
stationary electrode, the principal tuning capacitance for its
associated frequency selector circuits at the high frequency end of the
UHF band.
The features of this invention which are believed to
be novel are set forth with particularity in the appended claims. The
invention, together with further objects and advantages thereof, may
best be understood, however, by reference to the following description
taken in conjunction with the accompanying drawings, in the several
figures of which like reference numerals identify like elements, and in
which:
FIG. 1 is an elevation view, in section, of a continuous type UHF television tuner embodying the invention;
FIG. 2 is a sectional view of the tuner taken along lines 2--2 of FIG. 1;
FIG. 3 is a detail view, partly in cross section, of one component of the UHF tuner shown in FIG. 1; and
FIG. 4 is a schematic diagram of the UHF tuner.
Referring
now specifically to FIGS. 1 and 2, the continuously adjustable UHF
tuner 10 shown therein comprises a metal housing 11 which encloses a
plurality of signal translating stages. More particularly, tuner 10
includes first and second RF preselector stages 12, 13, respectively,
separated by a compartment wall 14, and an oscillator stage 15 shielded
from preselector 13 by a wall 16. A control shaft 17 extends through the
compartments and is rotatably journaled upon bearings supported by the
end walls 18, 19 of the housing. Shaft 17 is conductively connected to
end walls 18, 19 and to compartment walls 14, 16 by a series of
grounding leaves 20 each of which has one end soldered to a housing or
compartment wall and an intermediate portion seated within an under cut
portion of shaft 17, see FIG. 2.
Preselector stage 12 includes a
tunable frequency selector circuit comprising an inductor 22 having an
electrical length which approaches one quarter of a wave length at the
high frequency end of the UHF band. One end of inductor 22 is
conductively secured to the top wall 23 of housing 11 while the other
end terminates in a planar extension 24 which is supported by a post 21
of insulating material, see FIG. 2. In this fashion inductor 22
constitutes the inner conductor of a coaxial transmission line while the
housing and bordering walls form the outer conductor.
Extension
24 serves as the stationary electrode of a tuning capacitor which also
includes a pair of spaced electrodes 26 which are soldered, staked or
otherwise conductively affixed to control shaft 17 for rotational
displacement in a plane parallel to stationary electrode 24 from a
position overlapping and embracing the stationary electrode to a
position remote therefrom. The latter position is illustrated in FIG. 2.
Electrodes 26 are of identical arcuate configuration and each includes
an adjustable tab 26', preferably struck or formed along one edge of the
electrode itself. As will be explained more completely below, tabs 26'
together with electrode 24 serve to establish the principal tuning
capacitance for preselector 12 at the high frequency end of the UHF
band. Additionally, each of electrodes 26 has a plurality of canted
knifing slots 27 to facilitate tuning preselector 12 so that it will
"track" or follow oscillator stage 15 when the latter is tuned across
the UHF band.
Preselector 12 also includes an antenna input
circuit comprising a pair of UHF antenna terminals 28 which are mounted
on a panel 25 of insulating material atop housing 11 and are coupled to
inductor 22 via a coil 29. One of terminals 28 is returned to a plane of
reference potential, housing 11, through a resistor 30 which provides a
leakage path for any static charge accumulating on the antenna.
The
tunable frequency selector circuit for preselector 13 comprises an
inductor 32 which is similar in length and configuration to inductor 22
and is coupled thereto through a window 33 in compartment wall 14.
Inductor 32 also has one end grounded to top wall 23 of the housing and a
free end formed into a planar extension 34 which is supported by a post
21 thus permitting inductor 32 to serve as the inner conductor of a
coaxial transmission line of which compartment walls 14, 16 and housing
11 constitute the outer conductor. Extension 34 is of the same size and
configuration as extension 24 and is in alignment therewith as viewed
along shaft 17.
Preselector line 32 is tuned by a capacitor
which includes inductor extension 34 as a stationary electrode and a
pair of adjustable electrodes 36 which are conductively affixed to shaft
17 in axial alignment with electrodes 26 and displaceable over the same
limits as electrodes 26. Electrodes 36 are identical in configuration
to electrodes 26 even to the extent of having similar slots 27 and
adjustable tabs 36' which, together with stationary electrode 34,
constitute the principal tuning capacitance for preselector 13 at the
high end of the UHF band.
Preselector compartment 13 further
includes a mixer diode 35 having one lead connected to a tap on inductor
32 and a second lead protruding through an aperture 37 in compartment
wall 16 to form a coupling loop 38 which is connected to the center lead
of a feed-through capacitor 39 mounted in wall 16. An IF output coil 40
is connected between the center lead of feed-through capacitor 39 and
the center terminal of an IF output jack 41. Jack 41 is coupled to a
television receiver, now shown, via a coaxial cable 59.
As is
apparent in FIG. 1, capacitor electrodes 26, 36 are mounted
symmetrically relative to the walls of their respective compartments.
This, of course, permits a measure of control over stray capacitances by
equalizing the effects of the strays between the capacitor electrodes
and the compartment.
On the other hand, inductors 22, 32 are not
symmetrically disposed relative to their compartments in that their
center sections are offset relative to their respective extensions
24,34. Although the inductors are substantially identical in length,
inductor 32 constitutes, in effect, a mirror image of inductor 22 rather
than being identical in configuration. In this fashion their electrode
extensions 24, 34 remain centered in their respective compartments while
the inductor portions assume positions which provide a desired
magnitude of mutual coupling commensurate with the smallest feasible
opening for window 33.
Oscillator stage 15 also includes a
tunable frequency selector circuit comprising an inductor 42 having an
electrical length approaching a quarter wave length at the high
frequency end of the UHF band. The low impedance end of inductor 42 is
coupled to wall 23 of the housing through a capacitor 43 while its
opposite end is formed into a planar extension 44 which is supported by a
post 21 and disposed in alignment with preselector extensions 24, 34.
Inductor 42 together with housing 11 and walls 16, 19 form a third
capacity-tuned co-axial transmission line. Except for the fact that its
low impedance extremity is turned back to accommodate a connection to
capacitor 43, see FIG. 1, inductor 42 is substantially identical in
length and configuration to inductor 32. The tuning capacitor for the
oscillator stage comprises inductor extension 44 as a stationary
electrode and the pair of adjustable electrodes 46 which are
conductively secured to shaft 17 in alignment with preselector
electrodes 26, 36 for displacement in the same manner as those
electrodes. While they do not have the canted knifing slots found in
electrodes 26, 36, each of electrodes 46 does have a single tuning slot
47 which is located outside that area of the electrode which confronts
stationary electrode 44 and is disposed normal to the straight edge of
the electrode, see FIG. 3. In other respects, electrodes 46 are
identical in configuration to preselector electrodes 26, 36 and, in like
fashion, include adjustable tab portions 46' which cooperate with
stationary electrode 44 to establish the tuning capacitance for the
oscillator at the high frequency end of the UHF band.
As
previously noted each of preselector stages 12, 13 and oscillator 15
also employ substantially identical inductors 22, 32 and 42,
respectively. Therefore, insofar as the major components are concerned,
the three stages are identical. It is appreciated, of course, that
oscillator stage 15 must operate at a frequency which is displaced 40
megacycles from and preferably above, the operating frequency of the
preselector stages. The oscillator stage maintains this frequency
separation by virtue of capacitor 43 which is disposed in series
relation with tuning capacitor 46, 46' thereby reducing the total
capacitance of the oscillator stage and permitting tuning to a higher
frequency.
The low impedance end of inductor 42 is directly
connected to the output electrode or collector 49 of a grounded-base NPN
transistor oscillator 50. By employing a low impedance oscillating
device such as a transistor, a quarter-wave line or inductor is
feasible. Collector 49 is connected to a source of unidirectional
potential B+ via a decoupling choke 51, a feed-through capacitor 52
which is mounted in the top wall of housing 11, and a voltage dropping
resistor 58. The emitter electrode 53 of transistor 50 is returned to
reference potential housing 11, through a current-limiting bias resistor
54 which also serves to isolate the emitter from RF energy. The base or
control electrode 55 is connected to B+ potential through a
feed-through capacitor 56, a resistor 57 and resistor 58.
It is
recognized, of course, that a PNP transistor can be substituted for
transistor 50 simply by reversing the return connections of choke coil
51 and bias resistor 54. More particularly, such a substitution would
merely entail returning collector choke 51 to reference potential and
then connecting emitter resistor 54 through feed-through capacitor 52 to
B+.
Located within the oscillator compartment is a range or
limit control comprising a post 60 anchored to compartment wall 16 and a
stop 61 which is affixed to shaft 17 and includes a pair of abutments
62, 63 which cooperate with post 60 to confine the rotation of shaft 17
to an angular displacement of approximately 200°, the travel required by
capacitor electrodes 26, 36, 46 to tune their associated inductors
across the UHF band.
UHF tuner 10 is actuated by a viewer
control knob which is coupled to shaft 17 through a conventional gear
reduction and vernier mechanism, now shown. Initially, however, tuner 10
must be set-up or phased by a test procedure which establishes the
correct tuning range for each of the several stages. An acceptable
procedure entails energizing transistor 50 and then rotating shaft 17
counterclockwise, as viewed in FIG. 2, until abutment 62 of the limit
control encounters post 60. Transistor 50 functions as a conventional
grounded-base oscillator and develops an output signal across frequency
determining circuit 42, 44, 46. RF oscillator energy is coupled from
this circuit to mixer diode 35 through loop 38. With shaft 17 so
positioned, oscillator inductor 42 is tuned, principally by adjusting
the proximity of electrode tabs 46' to electrode 44, to a frequency near
the high end of the UHF band.
The frequency range of the
oscillator is then adjusted by coupling the output of a sweeping
generator to antenna terminals 28. In addition to an UHF signal varying
in frequency above and below UHF channel 83, the output of the sweeping
generator also includes a marker pulse which identifies the video
carrier for channel 83. This sweeping signal is coupled to inductor 22
of preselector 12 through coil 29 and from there to inductor 32 of
preselector 13 through coupling window 33. A portion of this signal is
also injected into mixer diode 35 by virtue of the tap on inductor 32.
To the output of diode 35 is externally added a pair of markers which
are separated by 41/2 megacycles and represent video and sound IF
carriers. This composite signal is then externally detected and applied
to the terminals of an oscilloscope. The displayed pattern shows the
channel 83 marker, as well as the sound and video IF carriers, and also
gives an indication of the pass band of preselector stages 12 and 13.
The frequency of oscillator 15 is adjusted for the high end of the UHF
band by positioning electrode tabs 46' relative to electrode 44 until
the channel 83 marker on the scope pattern is properly disposed in
relation to the sound and video IF markers. The pass bands of
preselectors 12, 13 are then adjusted by positioning their respective
electrode tabs 26', 36' relative to electrodes 24, 34 until a desired
pass band is displayed on the scope.
The oscillator is next
adjusted for the low end of the band by rotating tuning shaft 17 until
stop abutment 63 engages post 60. The previously described procedure is
then repeated using a sweep signal centered about UHF channel 14. The
oscillator frequency is now adjusted by inserting a tuning wand in slots
47 of electrodes 46 and positioning those electrodes relative to
electrode 44 until the scope pattern reveals proper oscillator frequency
at the low end of the band.
Tracking of the oscillator across
the UHF band by the preselector stages is then checked by returning
tuning shaft 17 to the channel 83 position. Tracking is accomplished by
successively positioning control shaft 17 to tune in a series of
stations in the UHF band. More particularly, shaft 17 is rotated
clockwise, as viewed in FIG. 2, to a position corresponding to UHF
channel 75, for example, at which station a sweep signal having a
frequency centered about that channel is coupled to antenna terminals
28. Preselector stages 12, 13 are then "tracked" to the oscillator by
inserting a tuning wand alternately in the slots 27 of capacitor
electrodes 26, 36 and bending the section of the electrode adjacent the
slot, i.e., "knifing" the rotor elements, until a pattern of desired
band pass is displayed on the oscilloscope. Control shaft 17 is then
rotated to another position where the above procedure is repeated a
second time. The knifing procedure is repeated for as many channels as
is required to achieve proper tracking of the preselector circuits.
As
shown prior art trimmer type capacitors are eliminated by resort to the
disclosed electrode-tab arrangement in the frequency determining
circuits of the several stages. Moreover, a substantial economy is
achieved by forming electrodes 26, 36 and 46 from the same tool. This
procedure also eliminates any tuning discrepancies attributable to
differences in electrode size or configuration. Moreover, by forming
these electrodes from the same tool any change in electrode size or
configuration due to tool wear will not affect one stage any differently
than any other since all the electrodes will retain an identical shape
and configuration.
In another aspect, section 12, for example,
of the UHF tuner of the present invention comprises a variable capacitor
including cooperating rotor and stator capacitor plates 26 and 24
respectively, the rotor plates 26 being mounted for rotary movement with
respect to stator plate 24 between a first position wherein the
capacitance of the capacitor is at a minimum and a second position
wherein the capacitance of the capacitor is at a maximum. Each of the
rotor plates 26 has a main part and an auxiliary part 26', and only the
auxiliary part of the rotor plate 26 is opposite the stator plate 24
when the capacitor is in its minimum capacity position. Auxiliary parts
26' of rotor plate 26 is adjustable toward and away from the stator
plate 24, thereby to allow the minimum capacitance of the capacitor to
be adjusted. Auxiliary part 26' of rotor plate 26 is not opposite stator
plate 24 when the capacitor is in its maximum capacity position. The
variable capacitor further comprises means for preventing an abrupt
change in the capacitance characteristic of the capacitor at the point
where auxiliary part 26' of rotor plate 26 ceases to be opposite stator
plate 24, such means constituting the bottom tapered edge of stator
plate 24 which is non-parallel or forms an angle with the edge of
auxiliary part 26' of rotor plate 26 as the rotor is turned clockwise to
the point that auxiliary part 26' departs from confronting relationship
with stator plate 24. It will be observed that rotor plates 26 are
provided with means in the form of slots 27 to form calibrating
electrodes, and that auxiliary part 26' has a size and configuration
different from that of any of the individual calibrating electrodes.
By
the same token resort to a low impedance device for the oscillator
stage, transistor 50, permits use of substantially identical tuned
quarter-wave lines, inductors 22, 32, 42, in each frequency selector
circuit. The savings which accrue as a result of employing substantially
identical components in each of the three stages of the tuner
contribute not only to economy in component cost but also a reduction in
labor cost because of the resultant simplicity in manufacturing the
tuner.
While a particular embodiment of the present invention
has been shown and described, it is apparent that changes and
modifications may be made therein without departing from the invention
in its broader aspects. The aim of the appended claims, therefore, is to
cover all such changes and modifications as fall within the true spirit
and scope of the invention. 
The B/W Tubes Television set was powered with a External Voltage stabiliser unit for Television (portable
metal box) which relates to voltage regulators of the type employed to
supply alternating
current and a constant voltage to a load circuit from a source in which
the line voltage varies. Such regulators are frequently provided
employing saturable core reactors and condensers connected in
circuit... in such manner as to
provide a plurality of variable voltage vectors which vary in different
senses, as the line voltage varies, but which add vectorially in such
manner that t
nd substractive in respect to that in the other half. When the
supply voltage is normal the voltage provided by the single secondary
winding is zero, since there is no difference of flux in the two magnetic
circuits, and the two excitation voltages
