the CHASSIS ZANUSSI BS700 which was mounted basically in all sets
from 15 Inches to 20 Inches with little variants in that era (1987 -
1990).
The design is based on MOTOROLA AND PHILIPS ASIC TDA3301 + TDA4504 + TDA8191.
Supply is based on the well TDA4601 (Siemens)
It was very durable and gave not big problems except for some dry joint and leaked Electrolytic caps.
PHILIPS TDA4502A SMALL SIGNAL COMBINATION FOR COLOUR TV
GENERAL DESCRIPTION
The integration into a single package
of all small signal functions required for colour TV reception is
achieved in the TDA4502A. The only additional circuits required for colour TV reception are the
deflection output stages, a sound detector and amplifier, and a colour decoder.
The IC includes a vision IF amplifier and video switching circuit, AFC circuit, AGC detector with
tuner output, an integral three—level sandcastle pulse generator, fully synchronized vertical and
horizontal drive outputs and a mute circuit with external availability. A triggered vertical divider
automatically adapts to 50 or 60 Hz operating mode thereby eliminating the need for external vertical
frequency control. The sound signal must be demodulated and amplified externally.
Features
Vision IF amplifier with synchronous demodulator
AGC detector, suitable for negative modulation
AGC output to tuner
AFC circuit with ON/OFF switch
Video preamplifier
Video switch to select the internal, or an external, video signal
Horizontal synchronization circuit with two control loops
Vertical synchronization [divider system) and sawtooth generation with automatic amplitude
adjustment for 50 and 60 Hz
Transmitter identification (mute)
O Sandcastle pulse generator
QUICK REFERENCE
DATA
K7 parameter symbol min. typ. max. unit
Supply voltage (pin 7) V7 9.5 12 13.2 V
Supply current (pin 7) I7 — 125 — ‘ mA
Supply current (pin 11) ‘‘ V11 — 6.0 8.5 mA
Operating ambient temperature range Tamb -25 - + 65 °C
Storage temperature range Tstg -25 — + 150 °C
Total power dissipation Ptot — — 2.3 W
PACKAGE OUTLINE
28—lead DI L; plastic with internal heat spreader (SOT117).
TDA4502A
FUNCTIONAL DESCRIPTION
IF amplifier, synchronous demodulator and AFC
The IF amplifier (pins 8 and 9) has a symmetrical input, the impedance of which enables SAW-filtering
to be used. The synchronous demodulator and the AFC circuit share an external reference tuned
circuit (pins 20 and 21). An internal RC—network provides the necessary phase—shift for AFC operation.
The AFC circuit provides a control voltage output with a voltage swing greater than 9 V at pin 18.
In the internal and external mode the AFC can be switched OFF when pin 22 is connected to positive
supply.
AGC circuit
AGC gating is performed to reduce sensitivity of the IF amplifier to external noise. The AGC time
constant
is provided by an RC circuit pin 5. The point of tuner takeover is
preset by the voltage level at pin 1.
Video switch circuit
The IC has a video switch with two video inputs and one video output. One input is connected to the
demodulated IF signal which is also fed to the video output pin of the peritelevision connector. The
other input can be switched to an external signal which is applied to the video input of the
peritelevision connector. The video output signal of the switch is fed to pin 25 of the IC, which is
the wnchronization part and, to the colour decoder. When the video switch is in the external mode,
the synchronization circuit is switched to the external signal. The vision IF, AGC and AFC circuits
will not be affected by the switching action and will, therefore, operate in the normal mode. Gating
for the AGC detector is switched OFF when the switch is in the external mode. The first control
loop is not switched to a low time constant when weak signals are received.
Horizontal oscillator start function
The horizontal oscillator start function is achieved by applying a current of 8.5 mA to pin 11 during the
switch-on period. This current can be taken from the mains rectifier. The main supply, pin 7, can then
be obtained from the horizontal output stage. The load current of the driver has to be added to the
start current.
Horizontal synchronization
The positive video input signal is applied to pin 25. The horizontal synchronization has two control
loops which have been introduced to
generate a sandcastle pulse. By using the oscillator sawtooth, an
accurate timing of the burst-key pulse can be made. Therefore, the phase of this sawtooth pulse must
have a fixed relationship to the sync pulse.
Horizontal phase detector
The circuit has two operating conditions:
(a) Synchronized
The first loop has afixed time constant and a gated phase detector, this enables optimum
performance for co-channel interference. The VCR mode is obtained by an additional load on
pin 22.
(b) Non-synchronized
In this condition the time constant is the same as during the VCR mode.
Vertical sync pulse
The vertical sync pulse integrator will not be disturbed when the vertical sync pulses have a width of
10 ps and a separation of 22 us. This type of vertical sync pulse is generated by video tapes with
anti-copy guard.
Vertical divider system
A synchronized divider system generates the vertical sawtooth wavefonns at pin 2. The system uses an
internal frequency doubler circuit to enable the horizontal oscillator to operate at its normal line
frequency. One line period equals 2 clock pulses.
Using the divider system avoids the requirement for vertical frequency adjustment. The divider has a
discriminator window for automatic switching from 50 Hz to 60 Hz mode. When the trigger pulse
arrives before line 576 the 60 Hz mode is selected, if not, the 50 Hz mode is selected.
The divider system operates with two different reset windows to give maximum interferencel
disturbance protection. The windows are activated via an up/down counter.
The counter is increased by 1 each time the separated vertical sync pulse is within the narrow window.
When the sync pulse is not within the narrow window the counter is decreased by 1.
The operation modes of the divider system are as follows:
Mode A
Large (search) window (divider ratio between 488 and 722)
This mode is valid for the following con
ditions:
0 Divider is looking for a new transmitter
O Divider ratio found — not within the narrow window limits
0 A non—standard composite video signal is detected — when a double or enlarged vertical sync pulse is
detected after the internally generated anti-top~flutter pulse has ended. This means a vertical sync
pulse width > 10 clock pulses (50 Hz); > 12 clock pulses (60 Hz). This mode is normally activated
for video tape recorders operating in the feature trick mode
0 Up/down counter value of the divider system, operating in the narrow window mode, drops below
count 6
Mode B
Narrow window (divider ratio between 522 to 528 (60 Hz) or 622 to 628 (50 Hz))
The divider system switches over to this mode when the up/down counter has reached its maximum
value of 15 approved vertical sync pulses. When the divider operates in this mode and, a vertical sync
pulse is missing within the window, the divider is reset at the end of the window and the counter value
is decreased by 1. At a counter value below 6 the divider system switches over to the large window
mode.
The divider system also generates an anti—top-flutter pulse which inhibits the Phase 1 detector during
the vertical sync pulse. The pulse width is dependent on the divider mode. In ‘Mode A’ the start is
generated by resetting the divider. In ’Mode B’ the anti—top-flutter pulse starts at the beginning of the
first equalizing pulse. The anti—top-flutter pulse ends at count 10 for the 50 Hz mode and count 12 for
the 60 Hz mode.
The vertical blanking pulse is also generated via the divider system. The start is initiated by resetting
the divider while the blanking pulse width is at count 34, (17 lines), for the 60 Hz mode and at
count 42, (21 lines), for the 50 Hz mode. The vertical blanking pulse, at the sandcastle output
(pin 27), is generated by adding the anti—top-flutter pulse to the blanking pulse. When the divider
operates in ‘Mode B’, the vertical blanking pulse starts at the beginning of the first equalizing pulse.
The length of the vertical blanking in this condition is 21 lines in the 60 Hz mode and 25 lines in the
50 Hz mode.
RATINGS
Limiting values in accordance with the Absolute Maximum System llEC 134)
parameter symbol min. max. unit
Supply voltage Vp = V7.5 — 13.2 V
Total power dissipation Pmt - 2.3 W
Operating ambient temperature range Tamb -25 + 65 °C
Storage temperature ra
nge Tstg -25 + 150 °C
CHARACTERISTICS
Vp = V7 = 12 V; Tamb = 25 °C; unless otherwise specified; all voltages are referenced to ground
(pin 6) unless otherwise specified
parameter conditions symbol min. typ. max. unit
Supplies
Supply voltage (pin 7) V7 9.5 12.0 13.2 V
Supply current (pin 7) I7 — 125 — mA
Supply current (pin 1 1) note 1 l 11 — 6 8.5 mA
Vision IF amplifier (pins 8 and 9)
Input sensitivity at 38.9 MHz note 2 V8 40 80 120 uV
Input sensitivity at 45.75 MHz note 2 V3 — 100 ~ uV
Differential input resistance
(pin 8 to pin 9) note 3 R3_g 0.8 1.3 1.8 kfl
Differential input capacitance
(pin 8 to pin 9) note 3 C39 — 5 —- pF
Gain control range G3_g — 62 — dB
Maximum input signal V3.9 50 100 — mV
Expansion of output signal for
50 dB variation of input signal note 4 AV17 — 1 — dB
Video amplifier note 5
Output level for zero signal input note 6 V17 3.3 3.7 4.1 V
Output signal top sync level V17 1.5 1.7 1.9 V
Amplitude of video output signal
(peak-to-peak value) note 7 V17(p_p) 1.4 1.8 2.2 V
Internal bias current of output
transistor (npn emitter
follower) l17(int) 1.4 2.0 — mA
Bandwidth of demodulated
output signal B 4.0 5.0 — M Hz
Differential gain note 8 G 17 — 5 10 %
Differential phase note 8 lp — 5 10 %
SELECO (ZANUSSI) 14SC454.1 BASIC CHASSIS BS700 Power supply Description based on TDA4601d (SIEMENS)
TDA4601 Operation. * The TDA4601 device is a single in line, 9 pin chip. Its predecessor was the TDA4600 device, the TDA4601 however has improved switching, better protection and cooler running. The (SIEMENS) TDA4601 power supply is a fairly standard parallel chopper switch mode type, which operates on the same basic principle as a line output stage. It is turned on and off by a square wave drive pulse, when switched on energy is stored in the chopper transformer primary winding in the form of a magnetic flux; when the chopper is turned off the magnetic flux collapses, causing a large back emf to be produced. At the secondary side of the chopper transformer this is rectified and smoothed for H.T. supply purposes. The advantage of this ty
pe of supply is that the high chopping frequency (20 to 70 KHz according to load) allows the use of relatively small H.T. smoothing capacitors making smoothing easier. Also should the chopper device go short circuit there is no H.T. output. In order to start up the TDA4601 I.C. an initial supply of 9v is required at pin 9, this voltage is sourced via R818 and D805 from the AC side of the bridge rectifier D801, also pin 5 requires a +Ve bias for the internal logic block. (On some sets pin 5 is used for standby switching). Once the power supply is up and running, the voltage on pin 9 is increased to 16v and maintained at this level by D807 and C820 acting as a half wave rectifier and smoothing circuit. PIN DESCRIPTIONS Pin 1 This is a 4v reference produced within the I.C. Pin 2 This pin detects the exact point at which energy stored in the chopper transformer collapses to zero via R824 and R825, and allows Q1 to deliver drive volts to the chopper transistor. It also opens the switch at pin 4 allowing the external capacitor C813 to charge from its external feed resistor R810. Pin 3 H.T. control/feedback via photo coupler D830. The voltage at this pin controls the on time of the chopper transistor and hence the output voltage. Normally it runs at Approximately 2v and regulates H.T. by sensing a proportion of the +4v reference at pin 1, offset by conduction of the photo coupler D830 which acts like a variable resistor. An increase in the conduction of transistor D830 and therefor a reduction of its resistance will cause a corresponding reduction of the positive voltage at Pin 3. A decrease in this voltage will result in a shorter on time for the chopper transistor and therefor a lowering of the output voltage and vice versa, oscillation frequency also varies according to load, the higher the load the lower the frequency etc. should the voltage at pin 3 exceed 2.3v an internal flip flop is triggered causing the chopper drive mark space ratio to extend to 244 (off time) to 1 (on time), the chip is now in over volts trip condition. Pin 4 At this pin a sawtooth waveform is generated which simulates chopper current, it is produced by a time constant network R810 and C813. C813 charges when the chopper is on and is discharged when the chopper is off, by an internal switch strapping pin 4 to the internal +2v reference, see Fig 2. The amplitude of the ramp is proportional to chopper drive. In an overload condition it reaches 4v amplitude at which point chopper drive is reduced to a mark-space ratio of 13 to 1, the chip is then in over current trip. The I.C. can easily withstand a short circuit on the H.T. rail and in such a case the power supply simply squegs quietly. Pin 4 is protected by internal protection components which limit the maximum voltage at this pin to 6.5v. Should a fault occur in either of the time constant components, then the chopper transistor will probably be destroyed. Pin 5 This pin can be used for remote control on/off switching of the power supply, it is normally held at about +7v and will cause the chip to enter standby mode if it falls below 2v. Pin 6 Ground. Pin 7 Chopper switch off pin. This pin clamps the chopper drive voltage to 1.6v in order to switch off the chopper. Pin 8 Chopper base current output drive pin. Pin 9 L.T. pin, approximately 9v under start-up conditions and 16v during normal running, Current consumption of the I.C. is typically 135mA. The voltage at this pin must reach 6.7v in order for the chip to start-up.
Semiconductor
circuit for supplying power to electrical equipment, comprising a
transformer having a primary winding connected, via a parallel
connection of a collector-emitter path of a transistor with a first
capacitor, to both outputs of a rectifier circuit supplied, in turn, by a
line a-c voltage; said transistor having a base controlled via a second
capacitor by an output of a control circuit acted upon, in turn by the
rectified a-c line voltage as actual value and by a reference voltage;
said transformer having a first secondary winding to which the
electrical equipment to be supplied is connected; said transformer
having a second secondary winding with one terminal thereof connected to
the emitter of said transistor and the other terminal thereof connected
to an anode of a first diode leading to said control circuit; said
transformer having a third secondary winding with one terminal thereof
connected, on the one hand, via a series connection of a third capacitor
with a first resistance, to the other terminal of said third secondary
winding and connected, on the other hand, to the emitter of said
transistor, the collector of which is connected to said primary winding;
a point between said third capacitor and said first resistance being
connected to the cathode of a second diode; said control circuit having
nine terminals including a first terminal delivering a reference voltage
and connected, via a voltage divider formed of a third and fourth
series-connected resistances, to the anode of said second diode; a
second terminal of said control circuit serving for zero-crossing
identification being connected via a fifth resistance to said cathode of
said second diode; a third terminal of said control-circuit serving as
actual value input being directly connected to a divider point of said
voltage divider forming said connection of said first terminal of said
control circuit to said anode of said second diode; a fourth terminal of
said control circuit delivering a sawtooth voltage being connected via a
sixth resistance to a terminal of said primary winding of said
transformer facing away from said transistor; a fifth termi
nal of said
control circuit serving as a protective input being connected, via a
seventh resistance to the cathode of said first diode and, through the
intermediary of said seventh resistance and an eighth resistance, to the
cathode of a third diode having an anode connected to an input of said
rectifier circuit; a sixth terminal of said control circuit carrying
said reference potential and being connected via a fourth capacitor to
said fourth terminal of said control circuit and via a fifth capacitor
to the anode of said second diode; a seventh terminal
of said control circuit establishing a potential for pulses controlling
said transistor being connected directly and an eighth terminal of said
control circuit effecting pulse control of the base of said transistor
being connected through the intermediary of a ninth resistance to said
first capacitor leading to the base of said transistor; and a ninth
terminal of said control circuit serving as a power supply input of said
control circuit being connected both to the cathode of said first diode
as well as via the intermediary of a sixth capacitor to a terminal of
said second secondary winding as well as to a terminal of said third
secondary winding.
Description:
The
invention relates to a blocking oscillator type switching power supply
for supplying power to electrical equipment, wherein the primary winding
of a transformer, in series with the emitter-collector path of a first
bipolar transistor, is connected to a d-c voltage obtained by
rectification of a line a-c voltage fed-in via two external supply
terminals, and a secondary winding of the transformer is provided for
supplying power to the electrical equipment, wherein, furthermore, the
first bipolar transistor has a base controlled by the output of a
control circuit which is acted upon in turn by the rectified a-c line
voltage as actual value and by a set-point transmitter, and wherein a
starting circuit for further control of the base of the first bipolar
transistor is provided.
Such a blocking oscillator switching
power supply is described in the German periodical, "Funkschau" (1975)
No. 5, pages 40 to 44. It is well known that the purpose of such a
circuit is to supply electronic equipment, for example, a television
set, with stabilized and controlled supply voltages. Essential for such
switching power supply is a power switching transistor i.e. a bipolar
transistor with high switching speed and high reverse voltage. This
transistor therefore constitutes an important component of the control
element of the control circuit. Furthermore, a high operating frequency
and a transformer intended for a high operating frequency are provided,
because generally, a thorough separation of the equipment to be supplied
from the supply naturally is desired. Such switching power supplies may
be constructed either for synchronized or externally controlled
operation or for non-synchronized or free-running operation. A blocking
converter is understood to be a switching power supply in which power is
delivered to the equipment to be supplied only if the switching
transistor establishing the connection between the primary coil of the
transformer and the rectified a-c voltage is cut off. The power
delivered by the line rectifier to the primary coil of the transformer
while the switching transistor is open, is interim-stored in the
transformer and then delivered to the consumer on the secondary side of
the transformer with the switching transistor cut off.
In the
blocking converter described in the aforementioned reference in the
literature, "Funkschau" (1975), No. 5, Pages 40 to 44, the power
switching transistor is connected in the manner defined in the
introduction to this application. In addition, a so-called starting
circuit is provided. Because several diodes are generally provided in
the overall circuit of a blocking oscillator according to the definition
provided in the introduction hereto, it is necessary, in order not to
damage these diodes, that due to the collector peak current in the case
of a short circuit, no excessive stress of these diodes and possibly
existing further sensitive circuit parts can occur.
Considering
the operation of a blocking oscillator, this means that, in the event of
a short circuit, the number of collector current pulses per unit time
must be reduced. For this purpose, a control and regulating circuit is provided.
Simultaneously, a starting circuit must bring the blocking converter
back to normal operation when the equipment is switched on, and after
disturbances, for example, in the event of a short circuit. The starting
circuit shown in the literature reference "Funkschau" on Page 42
thereof, differs to some extent already from the conventional d-c
starting circuits. It is commonly known for all heretofore known
blocking oscillator circuits, however, that a thyristor or an equivalent
circuit replacing the thyristor is essential for the operation of the
control circuit.
It is accordingly an object of the invention to
provide another starting circuit. It is a further object of the
invention to provide a possible circuit for the control circuit which is
particularly well suited for this purpose. It is yet another object of
the invention to provide such a power supply which is assured of
operation over the entire range of line voltages from 90 to 270 V a-c,
while the secondary voltages and secondary load variations between
no-load and short circuit are largely constant.
With the foregoing and other objects in view, there is
provided, in accordance with the invention, a blocking oscillator-type
switching power supply for supplying power to electrical equipment
wherein a primary winding of a transformer, in series with an
emitter-collector path of a first bipolar transistor, is connected to a
d-c voltage obtained by rectification of a line a-c voltage fed-in via
two external supply terminals, a secondary winding of the transformer
being connectible to the electrical equipment for supplying power
thereto, the first bipolar transistor having a base controlled by the
output of a control circuit acted upon, in turn, by the rectified a-c
line voltage as actual value and by a set-point transmitter, and
including a starting circuit for further control of the base of the
first bipolar transistor, including a first diode in the starting
circuit having an anode directly connected to one of the supply
terminals supplied by the a-c line voltage and a cathode connected via a
resistor to an input serving to supply power to the control circuit,
the input being directly connected to a cathode of a second diode, the
second diode having an anode connected to one terminal of another
secondary winding of the transformer, the other secondary winding having
another terminal connected to the emitter of the first bipolar
transmitter.
In
accordance with another feature of the invention, there is provided a
second bipolar transistor having the same conduction type as that of the
first bipolar transistor and connected in the starting circuit with the
base thereof connected to a cathode of a semiconductor diode, the
semiconductor diode having an anode connected to the emitter of the
first bipolar transistor, the second bipolar transistor having a
collector connected via a resistor to a cathode of the first diode in
the starting circuit, and having an emitter connected to the input
serving to supply power to the control circuit and also connected to the
cathode of the second diode which is connected to the other secondary
winding of the transformer.
In accordance with a further feature
of the invention, the base of the second bipolar transistor is connected
to a resistor and via the latter to one pole of a first capacitor, the
anode of the first diode being connected to the other pole of the first
capacitor.
In accordance with an added feature of the invention,
the input serving to supply power to the control circuit is connected
via a second capacitor to an output of a line rectifier, the output of
the line rectifier being directly connected to the emitter of the first
bipolar transistor.
In accordance with an additional feature of
the invention, the other secondary winding is connected at one end to
the emitter of the first bipolar transistor and to a pole of a third
capacitor, the third capacitor having another pole connected, on the one
hand, via a resistor, to the other end of the other secondary winding
and, on the other hand, to a cathode of a third diode, the third diode
having an anode connected via a potentiometer to an actual value input
of the control circuit and, via a fourth capacitor, to the emitter of
the first bipolar transistor.
In accordance with yet another
feature of the invention, the control circuit has a control output
connected via a fifth capacitor to the base of the first bipolar
transistor for conducting to the latter control pulses generated in the
control circuit.
In accordance with a concomitant feature of the
invention, there is provided a sixth capacitor shunting the
emitter-collector path of the first transistor.
Other features which are considered as characteristic for the invention are set forth in the appended claim.
Although
the invention is illustrated and described herein as embodied in a
blocking oscillator type switching power supply, it is nevertheless not
intended to be limited to the details shown, since various modifications
and structural changes may be made therein without departing from the
spirit of the invention and within the scope and range of equivalents of
the claims.
The construction and method of operation of the invention, however,
together with additional objects and advantages thereof will be best
understood from the following description of specific embodiments when
read in connection with the accompanying drawings, in which:
FIGS. 1 and 2 are circuit diagrams of the blocking oscillator type switching power supply according to the invention; and
FIG. 3 is a circuit diagram of the control unit RS of FIGS. 1 and 2.
Referring
now to the drawing and, first, particularly to FIG. 1 thereof, there is
shown a rectifier circuit G in the form of a bridge current, which is
acted upon by a line input represented by two supply terminals 1' and
2'. Rectifier outputs 3' and 4' are shunted by an emitter-collector path
of an NPN power transistor T1 i.e. t
he
series connection of the so-called first bipolar transistor referred to
hereinbefore with a primary winding I of a transformer Tr. Together
with the inductance of the transformer Tr, the capacitance C1 determines
the frequency and limits the opening voltages of the switch embodied by
the first transistor T1. A capacitance C2, provided between the base of
the first transistor T1 and the control output 7,8 of a control circuit
RS, separates the d-c potentials of the control or regulating circuit
RS and the switching transistor T1 and serves for addressing this
switching transistor T1 with pulses. A resistor R1 provided at the
control output 7,8 of the control circuit RS is the negative-feedback
resistor of both output stages of the control circuit RS. It determines
the maximally possible output pulse current of the control circuit RS. A
secondary winding II of the transformer Tr takes over the power supply
of the control circuit, in steady state operation, via the diode D1. To
this end, the cathode of this diode D1 is directly connected to a power
supply input 9 of the control circuit RS, while the anode thereof is
connected to one terminal of the secondary winding II. The other
terminal of the secondary winding II is connected to the emitter of the
power switching transistor T1.
The cathode of the diode D1 and,
therewith, the power supply terminal 9 of the control circuits RS are
furthermore connected to one pole of a capacitor C3, the other pole of
which is connected to the output 3' of the rectifier G. The capacitance
of this capacitor C3 thereby smoothes the positive half-wave pulses and
serves simultaneously as an energy storage device during the starting
period. Another secondary windi
ng
III of the transformer Tr is connected by one of the leads thereof
likewise to the emitter of the first transistor T1, and by the other
lead thereof via a resistor R2, to one of the poles of a further
capacitor C4, the other pole of which is connected to the
first-mentioned lead of the other secondary winding III. This second
pole of the capacitor C4 is simultaneously connected to the output 3' of
the rectifier circuit G and, thereby, via the capacitor C3, to the
cathode of the diode D1 driven by the secondary winding II of the
transformer Tr as well as to the power supply input 9 of the control
circuit RS and, via a resistor R9, to the cathode of a second diode D4.
The second pole of the capacitor C4 is simultaneously connected directly
to the terminal 6 of the control circuit RS and, via a further
capacitor C 6, to the terminal 4 of the control circuit RS as well as,
additionally, via the resistor R6, to the other output 4' of the
rectifier circuit G. The other of the poles of the capacitor C4 acted
upon by the secondary winding II is connected via a further capacitor C5
to a node, which is connected on one side thereof, via a variable
resistor R4, to the terminals 1 and 3 of the control circuit RS, with
the intermediary of a fixed resistor R5 in the case of the terminal 1.
On the other side of the node, the latter and, therefore, the capacitor
C5 are connected to the anode of a third diode D2, the cathode of which
is connected on the one hand, to the resistor R2 mentioned hereinbefore
and leads to the secondary winding III of the transformer Tr and, on the
other hand, via a resistor R3 to the terminal 2 of the control circuit
RS.
The nine terminals of the control circuit RS have the following purposes or functions:
Terminal
1 supplies the internally generated reference voltage to ground i.e.
the nominal or reference value required for the control or regulating
process;
Terminal 2 serves as input for the oscillations provided
by the secondary winding III, at the zero point of which, the pulse
start of the driving pulse takes place;
Terminal 3 is the control
input, at which the existing actual value is communicated to the
control circuit RS, that actual value being generated by the rectified
oscillations at the secondary winding III;
Terminal 4 is
responsive to the occurrence of a maximum excursion i.e. when the
largest current flows through the first transistor T1 ;
Terminal 5
is a protective input which responds if the rectified line voltage
drops too sharply; Terminal 6 serves for the power supply of the control
process and, indeed, as ground terminal;
Terminal 7 supplies the
d-c component required for charging the coupling capacitor C2 leading
to the base of the first transistor T1 ;
Terminal 8 supplies the control pulse required for the base of the first transistor T1 ; and
Terminal 9 serves as the first terminal of the power supply of the control circuit RS.
Further details of the control circuit RS are described hereinbelow.
The
capacity C3 smoothes the positive half-wave pulses which are provided
by the secondary winding II, and simultaneously serves as an energy
storage device during the starting time. The secondary winding III
generates the control voltage and is simultaneously used
as
feedback. The time delay stage R2 /C4 keeps harmonics and fast
interference spikes away from the control circuit RS. The resistor R3 is
provided as a voltage divider for the second terminal of the control
circuit RS. The diode D2 rectifies the control pulses delivered by the
secondary winding III. The capacity C5 smoothes the control voltage. A
reference voltage Uref, which is referred to ground i.e. the potential
of terminal 6 is present at the terminal 1 of the control circuit RS.
The resistors R4 and R5 form a voltage divider of the input-difference
control amplifier at the terminal 3. The desired secondary voltage can
be set manually via the variable resistor R4. A time-delay stage R6 /C6
forms a sawtooth rise which corresponds to the collector current rise of
the first bipolar transistor T1 via the primary winding I of the
transformer Tr. The sawtooth present at the terminal 4 of the control
circuit RS is limited there between the reference voltage 2 V and 4 V.
The voltage divider R7 /R8 (FIG. 2), brings to the terminal 5 of the
control circuit RS the enabling voltage for the drive pulse at the
output 8 of the control circuit RS.
The diode D4, together with
the resistor R9 in cooperation with the diode D1 and the secondary
winding II, forms the starting circuit provided, in accordance with the
invention. The operation thereof is as follows:
After the
switching power supply is switched on, d-c voltages build up at the
collector of the switching transistor T1 and at the input 4 of the
control circuit RS, as a function in time of the predetermined time
constants. The positive sinusoidal half-waves charge the capacitor C3
via the starting diode D4 and the starting resistor R9 in dependence
upon the time constant R9.C3. Via the protective input terminal 5 and
the resisto
r
R11 not previously mentioned and forming the connection between the
resistor R9 and the diode D1, on the one hand, and the terminal 5 of the
control circuit RS, on the other hand, the control circuit RS is biased
ready for switching-on, and the capacitor C2 is charged via the output
7. When a predetermined voltage value at the capacitor C3 or the power
supply input 9 of the control circuit RS, respectively, is reached, the
reference voltage i.e. the nominal value for the operation of the
control voltage RS, is abruptly formed, which supplies all stages of the
control circuit and appears at the output 1 thereof. Simultaneously,
the switching transistor T1 is switched into conduction via the output
8. The switching of the transistor T1 at the primary winding T of the
transformer Tr is transformed to the second secondary winding II, the
capacity C3 being thereby charged up again via the diode D1. If
sufficient energy is stored in the capacitor C3 and if the re-charge via
the diode D1 is sufficient so that the voltage at a supply input 9 does
not fall below the given minimum operating voltage, the switching power
supply then remains connected, so that the starting process is
completed. Otherwise, the starting process described is repeated several
times.
In FIG. 2, there is shown a further embodiment of the
circuit for a blocking oscillator type switching power supply, according
to the invention, as shown in FIG. 1. Essential for this circuit of
FIG. 2 is the presence of a second bipolar transistor T2 of the type of
the first bipolar transistor T1 (i.e. in the embodiments of the
invention, an npn-transistor), which forms a further component of the
starting circuit and is connected with the collector-emitter path
thereof between the resistor R9 of the starting circuit and the current
supply input 9 of the control circuit RS. The base of this second
transistor T2 is connected to a node which leads, on the one hand, via a
resistor R10 to one electrode of a capacitor C7, the other electrode of
which is connected to the anode of the diode D4 of the starting circuit
and, accordingl
y,
to the terminal 1' of the supply input of the switching power supply G.
On the other hand, the last-mentioned node and, therefore, the base of
the second transistor T2 are connected to the cathode of a Zener diode
D3, the anode of which is connected to the output 3' of the rectifier G
and, whereby, to one pole of the capacitor C3, the second pole of which
is connected to the power supply input 9 of the control circuit RS as
well as to the cathode of the diode D1 and to the emitter of the second
transistor T2. In other respects, the circuit according to FIG. 2
corresponds to the circuit according to FIG. 1 except for the resistor
R11 which is not necessary in the embodiment of FIG. 2, and the missing
connection between the resistor R9 and the cathode of the diode D1,
respectively, and the protective input 5 of the control circuit RS.
Regarding the operation of the starting circuit according to FIG. 2,
it can be stated that the positive sinusoidal half-wave of the line
voltage, delayed by the time delay stage C7, R10 drives the base of the
transistor T2 in the starting circuit. The amplitude is limited by the
diode D3 which is provided for overvoltage protection of the control
circuit RS and which is preferably incorporated as a Zener diode. The
second transistor T2 is switched into conduction. The capacity C3 is
charged, via the serially connected diode D4 and the resistor R9 and the
collector-emitter path of the transistor T2, as soon as the voltage
between the terminal 9 and the terminal 6 of the control circuit RS i.e.
the voltage U9, meets the condition U9 <[UDs -UBE (T2)].
Because
of the time constant R9.C3, several positive half-waves are necessary
in order to increase the voltage U9 at the supply terminal 9 of the
control circuit RS to such an extent that the control circuit RS is
energized. During the negative sine half-wave, a partial energy
chargeback takes place from the capacitor C3 via the emitter-base path
of the transistor T2 of the starting circuit and via the resistor R10
and the capacitor C7, respectively, into the supply network. At
approximately 2/3 of the voltage U9, which is limited by the diode D3,
the control circuit RS is switched on. At the terminal 1 thereof, the
reference voltage Uref then appears. In addition, the voltage divider R5
/R4 becomes effective. At the terminal 3, the control amplifier
receives the voltage forming the actual value, while the first bipolar
transistor T1 of the blocking-oscillator type switching power supply is
addressed pulsewise via the terminal 8.
Because the capacitor C6
is charged via the resistor R6, a higher voltage than Uref is present at
the terminal 4 if the control circuit RS is activated. The control
voltage then discharges the capacitor C6 via the terminal 4 to half the
value of the reference voltage Uref, and immediately cuts off the
addressing input 8 of the control circuit RS. The first driving pulse of
the switching transistor T1 is thereby limited to a minimum of time.
The power for switching-on the control circuit RS and for driving the
transistor T1 is supplied by the capacitor C3. The voltage U9 at the
capacitor C3 then drops. If the voltage U9 drops below the switching-off
voltage value of the control circuit RS, the latter is then
inactivated. The next positive sine half-wave would initiate the
starting process again.
By switching the transistor T1, a voltage
is transformed in the secondary winding II of the transformer Tr. The
positive component is rectified by the diode D1, recharing of the
capacitor C3 being thereby provided. The voltage U9 at the output 9 does
not, therefore, drop below the minimum value required for the operation
of the control circuit RS, so that the control circuit RS remains
activated. The power supply continues to operate in the rhythm of the
existing conditions. In operation, the voltage U9 at the supply terminal
9 of the control circuit RS has a value which meets the condition U9
>[UDs -UBE (T2)], so that the transistor T2 of the starting circuit
remains cut off.
For the internal layout of the control circuit
RS, the construction shown, in particular, from FIG. 3 is advisable.
This construction is realized, for example, in the commercially
available type TDA 4600 (Siemens AG).
The block diagram of the control circuit according to FIG. 3
shows
the power supply thereof via the terminal 9, the output stage being
supplied directly whereas all other stages are supplied via Uref. In the
starting circuit, the individual subassemblies are supplied with power
sequentially. The d-c output voltage potential of the base current gain
i.e. the voltage for the terminal 8 of the control circuit RS, and the
charging of the capacitor C2 via the terminal 7 are formed even before
the reference voltage Uref appears. Variations of the supply voltage U9
at terminal 9 and the power fluctuations at the terminal 8/terminal 7
and at the terminal 1 of the control circuit RS are leveled or smoothed
out by the voltage control. The temperature sensitivity of the control
circuit RS and, in particular, the uneven heating of the output and
input stages and input stages on the semiconductor chip containing the
control circuit in monolithically integrated form are intercepted by the
temperature compensation provided. The output values are constant in a
specific temperature range. The message for blocking the output stage,
if the supply voltage at the terminal 9 is too low, is given also by
this subassembly to a provided control logic.
The outer voltage divider of the terminal 1 via the r
esistors
R5 and R4 to the control tap U forms, via terminal 3, the variable side
of the bridge for the control amplifier formed as a differential
amplifier. The fixed bridge side is formed by the reference voltage Uref
via an internal voltage divider. Similarly formed are circuit portions
serving for the detection of an overload short circuit and circuit
portions serving for the "standby" no-load detection, which can be
operated likewise via terminal 3.
Within a provided trigger
circuit, the driving pulse length is determined as a function of the
sawtooth rise at the terminal 4, and is transmitted to the control
logic. In the control logic, the commands of the trigger circuit are
processed. Through the zero-crossing identification at input 2 in the
control circuit RS, the control logic is enabled to start the control
input only at the zero point of the frequency oscillation. If the
voltages at the terminal 5 and at the terminal 9 are too low, the
control logic blocks the output amplifier at the terminal 8. The output
amplifier at the terminal 7 which is responsible for the base charge in
the capacitor C2, is not touched thereby.
The base current gain
for the transistor T1 i.e. for the first transistor in accordance with
the definition of the invention, is formed by two amplifiers which
mutually operate on the capacitor C2. The roof inclination of the base
driving current for the transistor T1 is impressed by the collector
current simulation at the terminal 4 to the amplifier at the terminal 8.
The control pulse for the transistor T1 at the terminal 8 is always
built up to the potential present at the terminal 7. The amplifier
working into the terminal 7 ensures that each new switching pulse at the
terminal 8 finds the required base level at terminal 7.
Supplementing
the comments regarding FIG. 1, it should also be mentioned that the
cathode of the diode D1 connected by the anode thereof to the one end of
the secondary winding II of the transformer Tr is connected via a
resistor R11 to the protective input 5 of the control circuit RS
whereas, in the circuit according to FIG. 2, the protective input 5 of
the control circuit RS is supplied via a voltage divider R8, R7 directly
from the output 3', 4' of the rectifier G delivering the rectified line
a-c voltage, and which obtains the voltage required for executing its
function. It is evident that the first possible manner of driving the
protective input 5 can be used also in the circuit according to FIG. 2,
and the second possibility also in a circuit in accordance with FIG. 1.
The
control circuit RS which is shown in FIG. 3 and is realized in detail
by the building block TDA 4600 and which is particularly well suited in
conjunction with the blocking oscillator type switching power supply
according to the invention has 9 terminals 1-9, which have the following
characteristics, as has been explained in essence hereinabove:
Terminal
1 delivers a reference voltage Uref which serves as the
constant-current source of a voltage divider R5.R4 which supplies the
required d-c voltages for the differential amplifiers provided for the
functions control, overload detection, short-circuit detection and
"standby"-no load detection. The dividing point of the voltage divider
R5 -R4 is connected to the terminal 3 of the control circuit RS. The
terminal 3 provided as the control input of RS is controlled in the
manner described hereinabove as input for the actual value of the
voltage to be controlled or regulated by the secondary winding III of
the transformer Tr. With this input, the lengths of the control pulses
for the switching transistor T1 are determined.
Via the input
provided by the terminal 2 of the control circuit RS, the zero-point
identification in the control circuit is addressed for detecting the
zero-point o
f
the oscillations respectively applied to the terminal 2. If this
oscillation changes over to the positive part, then the addressing pulse
controlling the switching transistor T1 via the terminal 8 is released
in the control logic provided in the control circuit.
A
sawtooth-shaped voltage, the rise of which corresponds to the collector
current of the switching transistor T1, is present at the terminal 4 and
is minimally and maximally limited by two reference voltages. The
sawtooth voltage serves, on the one hand as a comparator for the pulse
length while, on the other hand, the slope or rise thereof is used to
obtain in the base current amplification for the switching transistor
T1, via the terminal 8, a base drive of this switching transistor T1
which is proportional to the collector current.
The terminal 7 of
the control circuit RS as explained hereinbefore, determines the
voltage potential for the addressing pulses of the transistor T2. The
base of the switching transistor T1 is pulse-controlled via the terminal
8, as described hereinbefore. Terminal 9 is connected as the power
supply input of the control circuit RS. If a voltage level falls below a
given value, the terminal 8 is blocked. If a given positive value of
the voltage level is exceeded, the control circuit is activated. The
terminal 5 releases the terminal 8 only if a given voltage potential is
present.
Forei
gn References:
DE2417628A1 1975-10-23 363/37
DE2638225A1 1978-03-02 363/49
Other References:
Grundig Tech. Info. (Germany), vol. 28, No. 4, (1981).
IBM Technical Disclosure Bulletin, vol. 19, No. 3, pp. 978, 979, Aug. 1976.
German Periodical, "Funkschau", (1975), No. 5, pp. 40 to 44.
Inventors:
Peruth, Gunther (Munich, DE) Siemens Aktiengesellschaft (Berlin and Munich, DE)
- Line deflection output (S2000)
SELECO (ZANUSSI) 14SC454.1 BASIC CHASSIS BS700 Frequency synthesizer tuning system for television receivers:
SHOWING MOTOROLA ZC84263P UCONTROL + - MOTOROLA UAA2001 PLL SY.
"
A method for tuning a television receiver having automatic
frequency control to the carrier frequency of a selected broadcast
channel with an associated channel number including generating a
variable frequency signal by means of a local oscillator,
generating a reference frequency signal by means of a reference
oscillator, and generating a local oscillator correction signal for
matching an intermediate frequency signal derived from said local
oscillator signal and the carrier frequency signal with a
predetermined nominal intermediate frequency signal, said method
being characterized by the use of a microcomputer and comprising:
generating
binary signals representing first and second digital tune words,
said digital tune words representing a selected channel;
storing said first and second digital tune words in a first data memory in said microcomputer;
reading
said first and second digital tune words from said first memory
and generating a divided-down local oscillator frequency by the use
of said first digital tune word and a divided-down reference
oscillator frequency by the use of said second digital tune word;
comparing
said divided-down local oscillator and refer
ence frequencies and
generating a control signal representative of the difference in
frequency of said divided-down local oscillator and reference
frequencies;
coupling said control signal to said local
oscillator for causing it to be locked to the frequency of said
received carrier signal;
mixing the local oscillator frequency signal and the carrier frequency signal to generate an intermediate frequency signal;
comparing
said intermediate frequency signal with said predetermined
nominal intermediate frequency signal and providing a tuning voltage
to said microcomputer, said tuning voltage being indicative of the
magnitude and direction of a tuning error between said
intermediate frequency signal and said predetermined nominal
intermediate frequency signal;
incrementally adjusting the
reference oscillator frequency by means of a tuning signal provided
to said reference oscillator by said microcomputer in response to
said tuning voltage;
detecting when the incrementally
changing, divided-down reference oscillator frequency causes the
intermediate frequency signal to pass said predetermined nominal
intermediate frequency signal; and
incrementally stepping the
divided-down reference oscillator frequency back a predetermined
number of steps following the passage of said predetermined nominal
intermediate frequency signal by said intermediate frequency
signal in tuning said television receiver to the selected channel.
"
A
television tuning system employs a frequency synthesizer system
for establishing the tuning of the receiver. A programmable
frequency divider counter is connected between the output of a
reference oscillator and a phase comparator to which the output of
the local oscillator in the tuner also is applied. The phase
comparator output provides a tuning voltage for controlling the
tuning of the local oscillator. A microprocessor is used to control
the count of the programmable frequency divider and initially to
set a count corresponding to the selected channel in a counter
connected between the output of the local oscillator and the phase
comparator. The tuning consists of three discrete time periods.
First, a settling time to allow channel change transients to
settle; second, a short period of forced search at a relatively
rapid rate to insure proper tuning; and third, a slower rate of
step-by-step correction to accomodate for station drift and the
like during reception. This third time period is initiated either
by the passage of a fixed length of time following the start of the
forced search period or by sensing a preestablished number of
changes of state in the output of the frequency discriminator during
the forced/search period.
1. A tuning
system for the tuner of a television receiver capable of receiving a
composite television signal and including frequency discriminator
(AFT) circuit means, said system including in combination:
a reference oscillator providing a reference signal at a predetermined frequency;
a
local oscillator in the tuner providing a variable output frequency
in response to the application of a control signal thereto;
a
programmable frequency divider means having first and second inputs
coupled respectively to the output of said reference oscillator
and said local oscillator for producing signals on first and second
outputs having frequencies which are a programmable fraction of the
frequency of the signals applied to the inputs thereto;
phase
comparator means having one input coupled with the first output of
said programmable frequency divider means and having another input
coupled with the second output of said programmable frequency
divider means for developing a control signal and applying such
control signal to said local oscillator for controlling the output
frequency thereof;
counter circuit means coupled with said
programmable frequency divider means for initially setting said
divider means to a predetermined division ratio and operating to
change the programmable fraction of division thereof in accordance
with changes in the count in said counter circuit means;
control
circuit means coupled with the output of said frequency
discriminator means and further coupled with said counter circuit
means for causing said counter circuit means to count at a first
rate in a predetermined direction determined by the state of the
output signal from said discriminator means in the absence of a
predetermined signal output from said frequency discriminator means
until a predetermined maximum count is attained, thereupon resetting
said counter circuit means to a count which is a predetermined
amount less than said maximum predetermined count and continuing to
count at said first rate in the same predetermined direction from
said new count to continuously change the programmable fraction of
said frequency divider means in accordance with the state of
operation of said counter circuit means, said control means
operating in response to said predetermined signal output from the
frequency discriminator means for terminating operation of said
counter circuit means; and
further means for terminating
operation of said counter circuit means at said first rate and
causing operation thereof at a second slower rate.
2. The
combination according to claim 1 wherein said further means includes
timing means initiated into operation simultaneously with the
setting of said divider means to a predetermined division ratio,
and after a predetermined time interval said timing means producing
an output signal applied to said counter circuit means to cause
operation thereof to take place at said second slower rate.
3. The combination according to claim
1 wherein said counter circuit means includes a reversible digital
counter coupled with said programmable frequency divider, means
and said control circuit means causes said counter circuit means to
count in said predetermined direction when the output of said
frequency discriminator is of a first state and to count in the
opposite direction when the output of said frequency discriminator is
of second state; and said further means comprises means coupled with
the output of said frequency discriminator and with said counter
circuit means to take place at said second slower rate in response
to a predetermined number of changes of state of frequency
discriminator. 4. The
combination according to claim 3 further including means responsive
to the selection of a new channel in said television receiver for
resetting said further means to an initial condition of operation.
5. The combination according
to claim 4 wherein said further means comprises a search
termination counter means operative to provide an output signal
applied to said counter circuit means in response to a count
thereby of a predetermined number of changes of state of said
frequency discriminator to cause said counter circuit means to be
operated at said second slower rate.
Description:
BACKGROUND OF THE INVENTION
Both of the above mentioned pat
ents
are directed to frequency synthesizer tuning systems for use with
television receivers to enable operation of the receivers with
minimal viewer fine tuning adjustments. By the utilization of the
frequency synthesizer tuning systems of these patents, the fine
tuning adjustment which is necessary with conventional types of
television receiver tuning systems has been substantially
eliminated. The system employed in the '953 patent permits utilization
of a frequency synthesizer tuning system which correctly tunes to a
desired television station or channel even if the transmitted
signals from that station are not precisely maintained at the
proper frequencies. The '535 patent is directed to a signal seek
tuning system adaptation of the frequency synthesizer tuning system
of the '953 patent which still permits implementation of all of
the desired wide-band pull in range of the frequency synthesizer
system of the '953 patent.
The systems of the foregoing
patents operate effectively to correct automatically for frequency
offsets in a frequency synthesizer tuning system without affecting
the operation of the conventional frequency synthesizer used in the
system. The systems of these patents are in widespread use
commercially and permit direct selection, with automatic fine
tuning adjustment, of any desired VHF channel which the viewer
wishes to observe. In addition, the signal seek adaptation disclosed
in the '535 patent couples all of the advantages of the frequency
synthesizer tuning system of the '953 patent with the desirability
of providing bidirectional signal seek operation.
While the
systems disclosed in the foregoing patents operate in a highly
satisfactory manner to accomplish the desired results of accurate
tuning without the necessity of fine tuning adjustments, the
circuitry for accomplishing the desired results is somewhat
complex. It is desirable to reduce the circuit complexity and the
number of signal detectors for accomplishing these results without
compromising the accuracy of operation of the system.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide an improved tuning system for a television receiver.
It
is an additional object of this invention to provide an improved
frequency synthesizer tuning system for a television receiver.
It
is another object of this invention to provide an improved
frequency synthesizer tuning system for a television receiver which
includes a provision for adjusting the synthesizer loop for
frequency offsets in the received signal with a minimum number of
signal detectors.
It
is a further object of this invention to tune the local RF
oscillator of a television receiver to the correct frequency for a
selected channel with a frequency synthesizer tuning system, and
automatically to change the reference frequency of the synthesizer
system, or adjust the count of a programmable divider that produces a
signal that divides the frequency of the local oscillator of the
tuner, if the AFT signal produced by the AFT frequency discriminator
of the receiver is outside a predetermined range corresponding to
correct tuning.
It is still another object of this invention
to provide an improved frequency synthesizer tuning system for a
television receiver which operates to adjust the synthesizer loop for
frequency offsets in the received signal over a relatively wide
pull in range in response to the output of the receiver frequency
discriminator by changing the division ratio of a programmable
frequency divider in the reference oscillator leg or local oscillator
leg of the synthesizer loop at a first relatively high rate from
an initial nominal value to a pre-established maximum in one
direction, and then resetting the division ratio to a second nominal
value once the maximum is reached and continuing to incrementally
change the division ratio in the same direction from the second
nominal value until a properly tuned condition is indicated by the
output of the receiver AFT frequency discriminator, followed by
control at a lower rate of operation to maintain tuning during
transmitting station drifts.
In accordance with a preferred
embodiment of this invention, the frequency synthesizer tuning
system for a television receiver includes a stable reference
oscillator and a voltage controlled local oscillator in the tuner. A
programmable frequency divider is connected between the output of
the reference oscillator and one input to a phase comparator, the
other input of which is supplied by the output of the local
oscillator. The output of the phase comparator then comprises a
control signal which is supplied to the local oscillator to control
the frequency of its operation.
A
counter circuit is connected to the programmable frequency divider
for initially setting the divider to a predetermined division
ratio upon selection of a desired channel by the viewer. The
counter then operates to change the programmable fraction of the
division ratio at a first relatively high rate in a direction
controlled by the output from the receiver picture carrier
discriminator in the absence of a predetermined signal output
derived from the discriminator. A control means causes the counter
circuit to count in this direction until it is determined that a
station is tuned or a predetermined maximum count is attained if no
station is correctly tuned, thereupon resetting the counter circuit
to a count which is a predetermined amount less than the maximum
predetermined count. Counting is continued in the same predetermined
direction from the new lesser count to continuously change the
programmable fraction of the frequency divider in accordance with
the state of operation of the counter.
The
high rate operation of the counter is terminated by the control
means in response to a predetermined signal from the output of the
discriminator, indicating that a station is correctly tuned, or after
a fixed time-out interval; so that the system automatically
adjusts for frequency offsets of the received signal which
otherwise would cause the station to be mistuned if a conventional
frequency synthesizer tuning system were used. After termination of
the high rate operation of the counter, it is switched to a lower
rate operation for maintaining tuning during transmitting station
drifts.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a television receiver employing a preferred embodiment of the invention;
FIG. 2 is a detailed block diagram of a portion of the circuit of the preferred embodiment shown in FIG. 1;
FIG. 3 is a detailed circuit diagram of a portion of a circuit shown in FIG. 1;
FIG. 4 is a flow chart of the control sequence of operation of the circuit shown in FIG. 1 and 2; and
FIG.
5 shows a waveform and time/frequency chart, respectively, useful
in explaining the operation of the circuit shown in FIGS. 1, 2 and
3.
DETAILED DESCRIPTION
Referring now to the drawings,
the same reference numbers are used throughout the several figures
to designate the same or similar components.
FIG. 1 is a block diagram of a television receiver, which may be a black and white or color television receiver. Mo
st
of the circuitry of this receiver is conventional, and for that
reason it has not been shown in FIG. 1. Added to the conventional
television receiver circuitry of FIG. 1, however, is a frequency
synthesizer tuning system, in accordance with a preferred embodiment
of the invention, which is capable of automatically changing the
reference frequency when a frequency offset exists in the received
signal for a particular channel.
Transmitted composite
television signals, either received over the air or distributed by
means of a master antenna TV distribution system, are received by an
antenna 10 or on antenna input terminals to the receiver. As is
well known, these composite signals include picture and sound
carrier components and synchronizing signal components, with the
composite signal applied to an RF and tuner stage 11 of the
receiver. The stage 11 includes the conventional RF amplifiers and
tuner sections of the receiver, including a VHF oscillator section
and a UHF oscillator section. Preferably, the UHF and VHF
oscillators are voltage controlled oscillators, the freuency of
operation of which are varied in response to a tuning voltage
applied to them to effect the desired tuning of the receiver.
The
output of the RF and tuner stages 11 is applied to the remainder
of the television receiver 14, which includes the IF amplifier
stages for supplying conventional picture (video) and sound IF
signals to the video and sound processing stages of the receiver 14.
The circuitry of the receiver 14 may be of any conventional type
used to separate, amplify and otherwise process the signals for
application to a cathode ray tube 16 and to a loudspeaker 17 which
reproduce the picture and sound components, respectively, of the
received signal.
The receiver 14 also includes a conventional
AFT or automatic fine tuning discriminator circuit and
additionally may include a synch separator circ
uit
for producing an output in response to the presence of vertical
synchronizatin pulses, a picture carrier detection circuit, and an
automatic gain control (AGC) amplifier. Outputs representative of
these sensor components are shown as being coupled over a group of
lead 20 to sensory circuitry 22, which in turn couples outputs
representative of the operation of these various sensor circuits to
a microprocessor unit 23 for controlling the operation of the
microprocessor unit.
The microprocessor unit 23 is utilized
in the system of FIG. 1 for controlling the operation of a
frequency synthesizer tuning system capable of automatic offset
correction. When the viewer desires to select a new channel, he
enters the desired channel number into a channel selection keyboard
25. There are a number of different keyboards which may be employed
to accomplish this function, and the particular design is not
important to this invention. The channel selector keyboard 25 also
may include switches or keys for initiating a signal seek function
in either the "up" or "down" direction.
Information
represented by the selection of channel numbers on the keyboard 25 is
supplied to the microprocessor unit 23 which provides output
signals over a corresponding set of leads 27 to the tuners (local
oscillators) 11 to effect the appropriate band switching control for
the tuners 11 in accordance with the particular channel which has
been selected. In addition, the keyboard 25, operating through the
microprocessor unit 23, provides output signals which operate a
channel number display 29 to provide an appropriate display of the
selected channel number to the viewer.
The
microprocessor M3870 unit 23 also processes the signals which are
used to operate the channel number display 29 through a
multiplexing circuit operation to decode the selected channel
number into a parallel encoded signal. This signal is applied to
corresponding inputs of the count-down counter or programmable
frequency divider 31 to cause the division number of the divider 31
to relate to the divided down frequency of the tuner local
oscillators connected to the input of the divider 31 through a
prescaler divider circuit 32 to the frequency of the reference
oscillator 34. Thus, the division number or division ratio of the
local oscillator frequency obtained from the output of the
programmable divider 31 is appropriately related to the frequency of
the reference crystal oscillator 34.
The
output of the oscillator 34 also is applied through a countdown
circuit or programmable frequency divider 35. Conventional frequency
synthesizer techniques are employed; and the microprocessor unit 23
automatically compensates, through appropriate code converter
circuitry, for the non-uniform channel spacing of the television
signals. It has been found most convenient to cause the programmable
frequency divider 31 to divide by numbers corresponding directly to
the oscillator frequency of the selected channel, for example, 101,
107, 113 . . . up to 931.
In accordance with the time
division multiplex operation of the microprocessor 23, the count of
the programmable frequency divider 35 initially is adjusted to a
fixed count by the application of appropriate output signals from
the microprocessor unit 23 to a point selected to be at or near the
mid-point of the operating range of the programmable frequency
divider 35. Thus, the output of the divider 35 is a stable
reference frequency (because the input is from the reference
crystal oscillator 34) which is used to establish initially and to
maintain tuning of the receiver to the selected channel.
The
output of the programmable divider 35 is applied to one of two
inputs of a phase comparator circuit 37. The other input to the
phase comparator circuit 37 is supplied from the selected one of
the VHF or UHF oscillators in the tuner stages 11 through the
programmable frequency divider 31. The phase comparator circuit 37
operates in a conventional manner to supply a DC tuning control
signal through a phase locked loop filter circuit 39 and over a
lead 40 to the oscillators in the tuner system 11 to change and
maintain their operating frequency.
With the exception of the
use of the microprocessor unit 23, the operation of the system
which has been described thus far is that of a relatively
conventional frequency synthesizer system incorporated into a
television receiver. This system is similar to the system of the
'953 patent. As in the system of that patent, the system shown in
FIG. 1, when the transmitted station or station received on a
master antenna distribution system provides the station or channel
signals at the proper frequency, operates as a relatively
conventional frequency synthesizer system. If, however, there is a
frequency offset in the received signal to cause the carrier of the
received signal to be displaced from the frequency which it should
have to some other frequency, it is possible that the system would
give the appearance of mistuning to the received station. The
microprocessor 23, operating in conjunction with the sensory
circuitry 22, is employed in conjunction with the countdown or
programmable frequency divider circuit 35 to eliminate this
disadvantage and still retain the advantages of frequency
synthesizer tuning.
Reference now should be made to FIG. 2 which shows details of t
he
interface between the keyboard 25, the microprocessor unit 23, and
the circuitry used in the frequency synthesizer portions of the
system. A commercially available microprocessor which has been used
for the microprocessor 23, and which forms the basis for the
diagramatic representation of the microprocessor in FIG. 2, is the
Matsushita Electronics Corporation MN1402 four-bit single-chip
microcomputer. This microcomputer has two, four-bit parallel input
ports labeled "A" and "B". In addition, three output ports, a five-bit
output port "C" and two four-bit output ports "D" and "E" are
provided. The internal configuration of the microcomputer 23 includes
an arithmetic logic unit (ALU), a read only memory (ROM) for
storing instructions and constants, and a random access memory
(RAM) used for data memory, arranged into four files, each file
containing 16 four-bit words. These words are selected by X and Y
registers and this memory is used, for example, for timers,
counters, etc., and also is used to hold intermediate results. To
facilitate an understanding of the operation of the system, a
portion of this memory is shown in FIG. 2 as a clock 81 and a
reversible counter 82 connected between the "B" input port and the
"D" output port. The microcomputer 23 is programmed to permit it to
operate in conjunction with the remainder of the circuits shown in
FIG. 2. The programming techniques are standard, and the
microcomputer 23 itself is a standard commercially available
circuit component.
There are several system parameters that
must be selected in the operation of the system shown in FIG. 2.
The selection of the nominal frequency of the two signals that feed
the phase comparator circuit 37 is an example. Channel selection is
provided by changing the frequency division ratio of the selector
counter 31 which divides the local oscillator signal after this
signal is passed through a prescaler circuit 32 and a divide-by-two
divider circuit 41. The nominal frequency from the programmable
frequency divider 31 (selector counter) is selected so that the
local oscillator (tuner) 11 can be set exactly on frequency for all
channels.
Since
the frequency divider 31 is able to divide only by integer
numbers, one distinct frequency possibility in the range of one KHz
is obtained, another in the range of two KHz, etc. A choice must
be made as to which of these values is optimum. Each value yields
the nominal frequency of all of the 82 channels by simply
multiplying by an appropriate integer for each channel. To simplify
the phase locked loop filtering problem by the filter 39, it is
desirable that the frequencies of the signals supplied to the phase
comparator 37 are as high as possible. This permits rapid
acquisition of a new channel along with a very clean DC control
signal to adjust the local oscillator. A trade-off for this,
however, must be made to permit fine tunning adjustment of the local
oscillator automatically to correctly tune in stations which are
off their assigned frequency, or to manually provide this feature,
if desired. The two-speed operation of the system in accordance
with the present invention allows a better trade-off to be made by
allowing rapid acquisition and then a slower speed for precise
tuning.
A compromise solution which is utilized in the circuit
of FIG. 2 is to cause the frequency division chain from the local
oscillator 11 in the tuner to the phase comparator 37 to be composed
of the fixed divide-by-256 prescaler 32, and a fixed divide-by-4
division, which is accomplished by the divider 41 at the input of
the counter 31 and a second divider 42 at the output of the counter
31. The variable frequency divider counter 31 then is loaded by
means of three latch circuits 44, 45 and 46 at an appropriate time
by the time division multiplex operation of the microcomputer 23
and a number that programs the programmable frequency divider
counter 31 to divide by the numerical value of the frequency of the
local oscillator in MHz for the channel selected. For example, if
the receiver is to be tuned to channel 2, which has a nominal local
oscillator frequency of 101 MHz, the programmable frequency
divider 31 is set to divide by 101. If the receiver is to be tuned
to channel 83, which has a nominal local oscillator frequency of
931 MHz, the programmable frequency divider 31 is set to divide by
931. In both cases, the variable divider 31 produces a 1 MHz
signal. However, because of the fixed divide-by-256 and the two
fixed divide-by-two dividers in series with the programmable divider
31, an output frequency of 976.5625 Hz is supplied from the output
of the divider 42 to the upper input of the phase comparator 37.
The
division ratio of the selector counter 31 is established by
appropriate output signals from the latch circuits 44, 45 and 46, as
mentioned above. The initial operation for changing, or maintaining,
the division ratio of the divider 31 is established by an entry of
the two digits of the selected channel number in the keyboard 25.
The microcomputer 23 operates as a time division multiplex system
for continuously monitoring the input ports and the output ports to
control the operation of the remainder of the system. The selection
of the two digits of the desired channel number is affected by a
time division multiplex iscanning of the outputs of the D output
port of microcomputer 23 and providing that information at the A
input port.
From
here the information is translated again to the D output ports to
the appropriate drivers of the channel number display circuit 29 and
to the latches 44, 45 and 46, and to a pair of similar four bit
latches 49 and 50 which control the divider ratio of the counter 35.
Although
the D output ports of the microcomputer 23 are connected in common
to all of these various portions of the circuit, the selection of
which of the latches are enabled to respond to the particular
output signals appearing on the D output ports at any given time is
effected through the C and E output ports of the microcomputer 23
in a time division multiplex fashion. A decoder circuit 52,
connected to the lowermost three outputs of the E output port of
the microcomputer 23, is used to apply unique decoding signals at
different times in the tim
e
division multiplex sequence of operation of the microcomputer 23
to the five latch circuits 44, 45, 46, 49 and 50, respectively. At
any given time in the sequence, only one of these latch circuits is
enabled for operation. A latch load signal is applied from the
upper output (EO3) at each cycle of operation of the signals
appearing on the E output port to set the latch circuit which is
enabled by the output of the decoding circuit 52 with the data
appearing on the other inputs to the latch circuit. This data
simultaneously appears on the four outputs of the D output port of
the microcomputer 23.
Thus, in rapid sequence, the latch
circuits 44, 45 and 46 are set to store the division number
corresponding to the selected channel entered onto the keyboard 25,
and the latch circuits 49 and 50 are each operated to set the
programmable divider reference counter 35 to a center or nominal
count, which is always the same upon the selection of a new channel
on the keyboard 25. Similarly, the two right-hand outputs of the C
output port (CO6 and CO5) enter the two digits of the selected
channel number in the drivers of the display circuit 29 at the
proper time in the binary encoded sequence when these digits appear
on the four-bit binary encoded representation of the D output
port. This results in a visual display of the channel number
selected.
In addition to the selection of a channel number
directly by the keyboard 25, the keyboard also may include an
additional switch 56, which is scanned in the time division multiplex
sequence to determine if the receiver is placed in a "seek" mode
of operation (when the signal seek capability is incorporated into
such a receiver). Operating in conjunction with the signal seek
switch 56 are a pair of "up" and "down" seek direction input
switches shown with a graphic representation of the seek directions
on the keyboard 25. A further provision is provided by two keys
labeled "U" and "D", which are used for "manual" fine tuning of the
receiver in the "up" or "down" directions depending upon which of
the two keys U or D has been operated. The keyboard 25 includes one
additional switch 58 which may be used to disable the automatic fine
tuning (AFT) portion of the circuit by rendering the microcomputer
insensitive to the signal output from the AFT circuit, in a manner
described more fully subsequently.
As is apparent from the
foregoing, the microcomputer 23 provides the intelligence, decision
making, and control for the system operation. It is a complete self
contained computer. The decisions or signal inputs upon which the
microcomputer 23 bases its operation include, in addition to the
inputs from the keyboard 25, inputs on sensory inputs into the B
input port and into the SNS1 and SNS0 inputs as shown in FIG. 2.
These input signals are used to provide an indication to the
microcomputer 23 of the presence or absence of a received signal;
and if the presence of such a signal is indicated, the inputs
provide a further indication of the accuracy of the tuning of the
receiver to that signal. If the system is being operated solely in a
manual mode of operation (AFT switch 58 open), the microcomputer
23 disregards all of this sensory information and tunes to the
frequency allocation of the channel selected in the manner described
above. The system will stay tuned to this condition, operating as a
conventional frequency synthesizer, whether or not a station is
present in the received signal.
When
the system is placed in its automatic mode of operation (similar
to the mode of operation of the above mentioned '953 patent), the
counter 82, integrally formed as part of the microcomputer 23,
continuously adds or subtracts one number at a time from the nominal
value or programmable division fraction entered into the programmable
frequency divider 35 at the outset of each new channel number
selection when frequency offset (mistuning) is present. The counter
82 is driven at a relatively high counting rate by clock pulses from
the clock 81 during this initial or forced search mode of
operation. Thus, automatic offset correction is provided for any
channel which is off its assigned frequency. The offset correction
automatically adjusts the frequency of the local oscillator by
changing the division ratio of the signal from the reference
oscillator 35 applied to the lower input of the phase comparator 37.
By doing this, the output of the phase comparator 37 applied to
the local oscillator 11 varies to cause the oscillator to be tuned
in the proper direction to compensate for the transmitting station
mistuning.
When the system is operating in its automatic mode
of operation, the microcomputer 23 responds to the sensor
information applied to it on its B input ports and on the S1 input
port shown in FIG. 2. These inputs are obtained from the various
outputs of the operational amplifiers shown connected to the
corresponding input ports in the detailed circuit of FIG. 3.
Depending upon whether the receiver is provided with a signal seek
feature or not, one or more of the sensory inputs of the circuit of
FIG. 3 are used. The s
ystem
shown in the drawings has a capability of correcting for frequency
offsets larger than 1.5 MHz on channels 2 and 7 and approximately 2
MHz on channels 6 and 13. The remainder of the channels have a
range between these two values.
If the receiver is not tuned
properly, the micromputer 23 executes the localized search of the
tuning range mentioned above. Since there is a necessary settling
down time for the tuning of a television receiver immediately
following selection of a new channel, a time interval of 250
milliseconds has been selected to prevent any localized search or
offset frequency correction until the expiration of this "settling
down" time period. If, at the end of this 250 millisecond time
interval, a properly tuned station is present, this is indicated by
the sensory outputs from the television receiver and no localized
search is effected to change the division ratio or programmable
divider count in the reference counter 35 for a system that also
has signal seek.
A system with no signal seek capability is
described later that requires less sensory input but which uses a
time period where a forced search is required directly after the
settling time interval.
Upon
termination of the 250 millisecond settling down period, the
microcomputer 23 is rendered responsive to the sensory input signals on
its sensory input signal ports. In the simplest form, only the
output of the frequency discriminator 60 (FIG. 3) applied to three
comparators 61, 62 and 63 is used to provide the necessary tuning
information to the microcomputer 23. The outputs of these comparators
are applied to the B12 and B11 inputs of the microcomputer.
The
comparator 61 simply is a conventional comparator for determining
whether or not the output of the frequency discriminator is
positive or negative, as indicated in the upper waveform of FIG. 5.
The comparators 62 and 63 are each adjusted with appropriate
reference input levels to provide a narrow window centered about
the center tuning frequency (fc) of the receiver. If the tuning of
the receiver, as indicated by the output of the frequency
discriminator 60, is outside this window on either side of the
central axis shown in FIG. 5, one output condition is indicated on
the input terminal B11 of the microcomputer. Only when the tuning
frequency is within the tuning window, indicative of a properly
tuned receiver, is the appropriate input applied to the
microcomputer input terminal B11. This input overrides any other
input that may be present on the input terminal B12 and is
indicative of a properly tuned receiver. The input from the
frequency discriminator 60, as applied to the microcomputer on its
input port B12, is used to determine the direction of operation of
the counter 82 of the microcomputer for the localized search count
signals applied to the latch circuits 49 and 50 to change the count
of the reference programmable divider counter 35 on a step-by-step
basis.
The lower graph of FIG. 5 plots the relative frequency
of the local oscillator 11 to the received signal frequency with
respect to time. The various arrows are used to indicate the manner
of operation of the counter 82 in the microcomputer 23 in
conjunction with the reference counter 35 for adjusting for any
mistuning conditions which may exist after the initial station
selection has been effected in the manner described above.
If
the receiver is properly tuned, the outputs from the comparators 62
and 63 of FIG. 3 which are combined together and applied to the
input port B11 of the microcomputer 23, provide an indication that
the tuning is within the properly tuned center frequency window. As
a consequence, no further operation of the microcomputer to change
any of the outputs applied to the latch circuits 49 and 50 for the
duration of this condition is effected. On the other hand, if the
receiver is mistuned on either side of the proper tuning frequency,
the various operating characteristics shown in FIG. 5 are effected.
Assume
initially that the receiver is capable of making tuning
adjustments over a range of fc plus Δf to fc minus Δf, as indicated
in the top waveform of FIG. 5. Three specific examples of
mistuning will then be considered. Initially, assume that the local
oscillator is mistuned relative to the received signal to a
frequency f1 as shown in the lower graph of FIG. 5. In this
condition, the outout of the frequency discriminator 60 is positive
since this signal frequency lies to the lefthand side of the
center or properly tuned region of operation of the discriminator.
Under this condition of the operation, the input signal applied to
the sensor port B12 of the microcomputer 23 is such that the
microcomputer counter 82 is caused to advance in a positive
direction to change the programmable division ratio or count of the
reference counter 35 in a manner to force the output of the phase
comparator 37 to adjust the frequency of the local oscillator until
the proper tuning indicated at point B in the lower graph of FIG. 5
is reached. The time interval for accomplishing this result is
measured from the upper end of the arrow representative of the
frequency f1 to the point B.
Now assume that the receiver
mistuning is to a frequency f2 which as shown in FIG. 5 as located
on the righthand-side of the center axis fc. In this condition, the
discriminator output is negative. This is reflected in the output
of the comparator 61 applied to the input port B12 of the
microcomputer 23. The polarity of this sign
al
is identified by the microcomputer 23 to cause the counter 82 in
it to operate in the reverse direction. As this count is applied on
a step-by-step basis through the latch circuits 49 and 50 to the
reference counter 35, the division ratio or count of the reference
counter (divider) 35 is changed. As a result, the reference
oscillator signal applied to the phase comparator 37 causes the
phase comparator 37 output to drive the local oscillator frequency
in a direction opposite to that considered in the first example.
This is shown by the vector interconnecting the top of the arrow
representative of f2 to point A on the time/frequency graph of FIG.
5.
As discussed in the general discussion above, whenever the
tuning frequency reaches the narrow window on either side of fc, the
outputs of the comparators 62 and 63 provide the necessary
indication on the sensory input port terminal B11 to cause
termination of the operation of the counter 82 in the microcomputer
23. Then the reference counter 35 remains set to the count attained
just prior to the appearance of this input signal on the input port
B11 of the microcomputer 23.
A third mistuning condition can
exist, and ordinarily this condition results in an ambiguity which
cannot be corrected simply by responding to the signal polarity at
the output of the frequency discriminator. This is indicated by
the mistuned condition where the difference between the local
oscillator frequency f3 and the transmitter frequency is such that
the signal f3 lies in the range to the right of the negative
portion of the discriminator output shown in the upper waveform of
FIG. 5. In this condition, the associated sound causes the
discriminator output to be positive; so that the television
receiver normally would attempt to tune toward the next adjacent
channel and away from the properly tuned center frequency of the
channel which is desired. The output of the discriminator 60 in
this situation is the same as it was in th
e
first example considered for frequency f1; so that the counter 82
of the microprocessor 23 operates to change the count in the
reference counter 35 in a manner to cause the local oscillator
frequency to go higher toward a frequency f3 +Δf, as shown in FIG.
5.
A predetermined number of counts of the counter 82 in the
microcomputer 23 are necessary for the microcomputer to count
through the frequency range Δf, and this range is selected to be
within the pull in or operating range of the system. Once this count
has been attained, the microcomputer counter 82 immediately is
reset back to a count which corresponds to a frequency 2 Δf lower
than the frequency attained by the maximum count. This is indicated
in FIG. 5 by the frequency f3-Δf. Because the microcomputer counter
82 is limited to counting a number of counts equal to Δf, this new
frequency now is on the lefthand side of the center line fc, shown
in both waveforms of FIG. 5. This places the local oscillator
frequency at a point such that the frequency discriminator output is
the positive output shown on the lefthand-side of the upper
waveform of FIG. 5. Counting continues in the same direction as
previously. This time, however, it is in a proper direction to bring
about correct tuning; and when the center frequency is reached,
the output of the comparators 62 and 63 cause the microcomputer 23
to stop its count. The proper tuning point attained is indicated at
point C on the graph of the lower part of FIG. 5.
Because
the counter 82 of the microcomputer is limited to a maximum count
equivalent to Δf above its initial count and thereupon is reset to a
new count equivalent to 2 Δf lower than the maximum count, it is
not necessary to utilize any other sensory inputs in order to
properly tune the receiver over a wide pull in range (as much as
plus or minus 2 MHz). Only the output of the conventional frequency
discriminator 60 is used to provide the necessary sensory inputs.
The
counter 82 of the microcomputer 23 is operated by the clock 81
during the foregoing sequence of operation, immediately following the
selection of a new channel by the operation of the keyboard 25, at
a fast or high speed operation. Typically, the counter steps are
10 milliseconds per step; so that there are no initial visual
effects which can be noticed by an observer of the television
screen of the receiver being tuned. The maximum forced search
period is approximately 900 milliseconds in duration. At the end of
this time interval, a timer in the microcomputer 23 causes a
signal to be applied through the outputs of the E output port to
the decoder circuit 52 indicative of the completion of this time
interval. The decoder 52 then applies a pulse on an output lead
connected to the B13 input of the B input port of the microcomputer
23. This pulse is sensed by the microcomputer 23 and is applied to
the clock 81 to change the clock rate to a much slower rate,
approximately one-third (1/3) or one-fourth (1/4) the rate used
previously during the forced search mode of operation. This then
permits the system to accomodate station drifts which normally
occur at a very slow rate during the transmission and reception of a
television signal. As a consequence, it is possible to use more
filtering in the filter 39 on the tuning line (FIG. 1) and employ a
smaller frequency window for the channel verification sensed by
the circuitry shown in FIG. 3.
The
result is a more precise tuning from the receiver than is
otherwise possible if only a high speed operation of the clock 81
is utilized.
When the channel once again is changed by
operation of the keys in the keyboard 25 or operation of the
channel selection circuitry from a remote control unit, this new
channel input is sensed by the microcomputer 23 from the signals
applied to the A input port and the clock 81 is reset to its fast
time or the forced search mode of operation; and the process
resumes.
Instead of employing an additional decoding function
in the decoder 52, a separate decoder also could be connected to
the outputs of the D output ports to feed back the signal to the
B13 input terminal of the B input port of the microcomputer 23. The
operation of the system to change the rate or frequency of the
pulses applied by the clock 81 to the counter 82 otherwise is the
same as described above.
Although applicant has found that it
is preferable to correct for mistuning or frequency offsets by
adjusting the count or division ratio of the counter 35, such
offset adjustments also could be effected by adjusting the count in
the counter 31 in the local oscillator signal line. The operation in
such a case is the same as described above for adjusting the count
in the counter 35.
If the receiver is to be used with an
automatic signal seek mode of operation, however, additional sensory
inputs are necessary. These inputs operate in conjunction with the
output of the frequency discriminator 60. The operation of the
microcomputer 23 in controlling the count of the reference
programmable frequency counter divider 35 is the same as described
above. The additional sensory inputs simply are used in conjunction
with the outputs of the comparators 62 and 63 to signal the
microcomputer 23 to assure that tuning is to a picture channel
rather than an adjacent sound channel. This is accomplished by
utilizing the output of the synchronizing signal separator 65 which
is applied to a comparator 67 to produce an output signal to the
SNS1 sensory input of the microcomputer 23 only when vertical
synchronizing signal components are present.
In addition, the
output of a picture carrier detector 69 is applied to the input of a
comparator 70 to produce an output to the B10 sensory input of the
microcomputer 23. If the picture carrier detector 69 is producing
an output indicative of the presence of a carrier, but no output is
being obtained from the vertical synch separator 65 at the same
time, the system is mistuned to a sound carrier and the
microcomputer 23 is permitted to continue its localized search until a
properly tuned station is found. Only when there is coincidence of
signals from the picture carrier detector 69, the synch signal
separator 65, and the automatic frequency discriminator window as
determined by the comparators 62 and 63, is the microcomputer
operation terminated to indicate that a properly tuned channel is
present.
Further insurance of tuning the receiver only to a
strong signal also can be provided by the addition of an AGC
amplifier 72. This is connected to a comparator 74 coupled to the
B10 input port along with the output of the picture carrier
detector comparator 70. When the AGC amplifier 72 is used as a
sensory input, the microcomputer operation, when the system is used
in a signal seek mode, is only terminated to indicate reception of
a valid signal when that signal is strong enough to produce the
desired output from the comparator 74. The signal level which is
acceptable is set by a potentiometer 75.
It should be noted
that when the system is operated in a signal seek mode, the sensory
inputs must indicate the reception of a properly tuned signal
within a pre-established time period. If no signal is sensed by the
various sensory input circuits operating in conjunction with one
another as described above, the microcomputer 23 automatically steps
to the next channel number and repeats the sequence of operation
described above. This is when it is placed in its signal seek mode of
operation. If signal seek is not employed, the additional sensory
circuits 65, 69 and 72 are not necessary, and the inputs to the
microcomputer which are provided from these sensory circuits are not
utilized. The sensory signal input which is used both for a
receiver without a signal seek capability of operation and for a
receiver which has a signal seek mode of operation in it, is the
output of the frequency discriminator 60 operating in conjunction
with the comparators 61, 62 and 63 as described above.
As
indicated above, the wideband method of tuning precisely to an
incoming signal that is at the wrong frequency described here only
needs the frequency discriminator sensory information. The method
that uses the additional sensors described above is needed to make
this system operate compatibly with signal seek but it is not
restricted to seek operation.
For
a system that does not use signal seek operation, only the
frequency discriminator sensory input is required for proper
operation. The discriminator 60 is used for both fine tuning
direction information and to produce a frequency window to indicate
the presence of a correctly tuned station (channel verification).
Initially, after a channel change, there is a 250 millisecond
settling time, the same as the operation described above with
compatible seek. After that, however, comes a period of time where a
forced localized search is produced by the microcomputer 23. The
forced search is needed to insure that the system will correctly
tune to stations that initially may be tuned to the undesired zero
voltage crossover in the right half of the upper curve of FIG. 5.
Such signals may be within the frequency window of the discriminator
60; and if a search is not forced, this system will not correctly
tune. The compatible seek system described previously correctly
tunes the local oscillator without a forced search, because the
picture carrier detector and vertical detector do not give an output
for this situation and the system automatically goes into its search
mode of operation. However, the non-seek system does not have a
picture carrier sensor input and must be forced to search for an
initial period of time sufficient to allow the system to tune up to
its maximum frequency and then reset (loop) back to a frequency of 2
Δf lower. Then it is tuned to the positive left half portion of
the discriminator curve (FIG. 5) and the frequency window created
by the discriminator 60 is sufficient to insure proper tuning. If
the discriminator output produced by the desired incoming signal
created an initial situation that produces the correct tuning
direction information, i.e., in the left half of the curve of FIG.
5, or in the right half portion that gives the correct direction
and
frequency
window information, the forced search would not be needed.
However, the forced search will produce a correct tuning situation
anyway. In these cases, the tuning either is correct to begin with
or correct tuning is reached quickly. Then, even though the forced
search is active, it simply alternates up and down through the
correct tuning point because each time the receiver is tuned a little
high in frequency, it produces a negative output from the
discriminator 60; and the tuning direction signal causes the system
to tune down in frequency.
Then,
a positive discriminator output is produced, and the system tunes
up in frequency. This continues until the forced search is removed
by time-out of the microcomputer 23 (a fraction of a second). At
such time, the receiver is correctly tuned by the frequency window of
the discriminator to be very near fc. The system cannot tune to
the undesired discriminator crossover shown in the right half
portion of FIG. 5 because the polarity of the tuning direction
signal always causes it to tune away from that point.
The
fast time or forced search operation of the system can be
terminated in a different way other than the preestablished
time-out period described above in conjunction with the operation
of the circuit shown in FIG. 2. Generally, it is desirable to build
into the system (or program into the system by means of software)
such a maximum time-out period to effect the operation which has
been described above to terminate the search and cause the clock 81
thereafter to operate in a low speed mode of operation. Termination
also can be accomplished by sensing the number of changes in the
direction sensor input applied to the B12 terminal of the B input
port to cause the search to be terminated when this direction
changes three times (or more). By doing this, any flicker that
might be observed on the screen of the television receiver is
minimized, since the forced search still takes place at the high
rate of application of clock pulses from the clock 81 to the
counter 82 in the same manner described above.
Termination
of the search, however, also may be effected by means of a search
terminate counter 78 (FIG. 3), which is advanced by pulses applied
to it each time the output of the comparator 61 changes its sign
(indicative of a change in direction for the counter 82) as applied
to it through the B12 input port, as described earlier. After three
of these changes, or some other number if desired, an output pulse
is obtained from the search terminate counter 78 and is applied to
the SNS0 input of the microcomputer 23. This causes the operation
of the clock 81 to be switched to its low speed mode of operation
to terminate the fast or "forced search" mode of operation. The
next time a new channel number is entered on the keyboard 25, a
reset pulse is applied to the search terminate counter 78 to reset
it to its original or zero count, thereby readying it for another
sequence of operation. It is apparent that the search terminate
counter 78 may not always be operated to terminate the count, since
the time-out interval which is sensed by the decode circuit 52 and
applied to the B13 input port of the microcomputer 23 may occur
before there are three changes of direction of the search. In any
event, the next time a new channel number is entered into the
keyboard 25, the search terminate counter 78 is reset; so that it
is irrelevant whether this counter reaches a full count or not to
effect the termination of the forced search operation of the
system.
FIG. 4 shows the control sequence of the system which
is stored in the ROM (Read Only Memory) of the microcomputer 23.
The microcomputer 23 operates by always running through the flow
sequence, via loops L1, L2 and L3. Loop L1 corresponds to a new
channel selection by two digit number entry. Loop L2 corresponds to
channel number increment or decrement by an up or down key
operation, respectively, or by seek operation. Loop L3 corresponds
to fine tuning, either manual or automatic. To obtain exact timing for
system control, the microcomputer 23 receives a standard timing
pulse from the output of the reference counter 35 divided in a
divide-by-five counter 80 and applied to the A13 input port of the
microcomputer 23. The control functions which are programmed into
the microcomputer 23, as indicated in the flow chart of FIG. 4, are
outlined in the following paragraphs.
Channel Number
Correction: An invalid two digit channel number entry (0, 1, 84,
99) is corrected. When the operation of the receiver is in the
signal seek mode, the next channel up from 83 is channel 2, and the
next lower channel from channel 2 is 83.
PLL
Control I: For a given channel number, a corresponding binary code
for the PLL selector counter 31 is derived as described
previously. For UHF channels, the local oscillator frequency
separation between two adjacent channels is 6 MHz and the code for
PLL is generated by the microcomputer 23 through means of a simple
calculation. This code then is transferred from the microcomputer
23 to the latches 44, 45 and 46 as described previously.
PLL
Control II: This routine of the microcomputer 23 is used to
transfer the fine tuning data to the latches 49 and 50 which
control the count of the reference counter 35 in the PLL circuit.
Channel
Number Display: The channel number is transferred from the
microcomputer 23 to the driver latches of the display driver
circuit 29.
Key Input Detection: The keyboard is arranged as
the matrix circuit shown in FIG. 2. ROM programming for scanning
and acknowledging a keyboard entry only after successive
indications provides protection against false entry due to contact
bounce. The four data output lines of the D output port of the
microcomputer 23 are used to transfer data to the phase lock loop
section of the circuit and to the display circuit 29, as well as for
scanning the keyboard matrix circuit.
Time Count: The
microcomputer 23 receives a basic timing pulse of approximately 200 Hz
from the output of the divider 80 and performs various controls for
each timing pulse. By way of example, sensing for the vertical
synch input (when the system is used with a signal seek capability)
on the input port SNS1 takes place every 2.5 milliseconds.
Automatic seek timing is selected to be 133 milliseconds for UHF
channels. All of these timing pulses are derived from the basic
synchronization timing pulse applied to the microcomputer on the
A13 input port from the output of the divider 80. Various other
timing values used in the microcomputer to properly time multiplex
sequence the operation are derived from this basic timing pulse.
Sensor
Input Detection: As described previously, the output of the
comparators shown in FIG. 3 reflect the status of the tuning of the
television receiver. If no signal seek mode of operation is used,
only the frequency discriminator or AFT discriminator 60 is
necessary. When a system is being used in a signal seek mode, a
proper television signal receipt is indicated by the presence of a
vertical synch signal at the output of the synch signal separator
65 and corresponding outputs are applied to the input leads B10 and
B11 (high level input signals) indicative of tuning to the
"correct tuned" frequency discriminator window and reception of a
picture carrier. As stated previously, the signal present on the B12
input lead is used to determine the direction of tuning when the
receiver is operated in its automatic mode.
Mode Detection: The
status of the seek and automatic/manual (A/M) switches are
detected. If the A/M switch (not shown) is in its automatic
position, automatic seek and offset correction are active. If only
the seek switch is on, only seek is performed. If the A/M switch is
in manual, manual fine tuning (MFT) is active.
Automatic
Mode: If the TV receiver is not properly tuned for VHF channels in
automatic, the local oscillator frequency is shifted automatically
toward proper tuning. The fine tuning data is generated in the
microcomputer 23 and is transferred to the latches 49 and 50 for the
reference counter 35 in the PLL circuit.
Manual Fine Tuning
(MFT) Control: The local oscillator frequency is shifted by pushing
the fine tuning up (U) or down (D) pushbutton or switch. This MFT
control can be applied to VHF channels as well as to UHF channels.
Channel
Up/Down: When a channel up (upward pointing arrow) or down
(downward pointing arrow) key closure in the keyboard 25 is
detected, or upon a direct access to an unused channel, this routine
is activated and the system will advance to the next channel in
the selected direction.
The foregoing embodiment of the
invention which has been described above and which is illustrated in
the drawings is to be considered illustrative of the inventi
on,
which is not limited to the specific embodiment selected for this
purpose. For example, hard-wired logic could be used to achieve the
various circuit operations which are accomplished by the
microcomputer 23 in conjunction with the other portions of the
system. The relative ease of programming and debugging the
microcomputer 23, however, make it much simpler to implement the
system operation with the microcomputer than with hard-wired logic.
With respect to the sensor circuit inputs to the system, an added
degree of operating assurance can be provided by the addition of a
sound carrier sensor in addition to the picture carrier sensor shown
in FIG. 3. If this feature is desired, the output of the
comparator for the sound carrier is combined with the outputs of
the comparators 70 and 74 at the input terminal B10 of the B input
port of the microcomputer 23. Because of the manner of the circut
operation which has been described previously, however, the addition
of a sound carrier detector to the system is not considered
necessary, even for a system operating in the signal seek mode of
operation. This is in contrast to conventional television receivers
having a signal seek operation, in which detection of the sound
carrier generally is a necessity to insure that mistuning of the
receiver to an adjacent sound carrier does not take place.
SELECO (ZANUSSI) 14SC454.1 BASIC CHASSIS BS700 - VIDEO CHROMA PROCESSING WITH TDA3300 (MOTOROLA)
TDA3300 3301 TV COLOR PROCESSOR
TDA3300 3301 TV COLOR PROCESSOR
The Decoder IC The centre
-piece of the decoder is the Motorola TDA3300B i.c. which carries out
all the luminance and U V Inputs from PAL delay line 9V Frequency nlyv Z
2RV2 100k chroma signal processing required. Features of this 40 -pin
chip include: (1) Automatic black -current control via feedback from the
RGB output circuits. (2) Peak beam current limiting to prevent blooming
on highlights - in addition to the normal beam current limit- ing
action. (3) Separate R, G and B input pins for the injection of
teletext/data signals (or on -screen display of the channel number with
frequency synthesis tuning). These signals can be varied by means of the
user brightness and con- trast controls. (4) Low dissipation - about
600mW. (5) By adding a small adaptor panel with a TDA3030A SECAM-to-PAL
converter i.c. during production the receiver is given multistandard
(PAL, SECAM and NTSC-4.43) capability.
A
block diagram of the TDA3300B i.c. is shown in Fig. 3. As with the
better known TDA3560 single -chip decoder, both the chroma and the burst
pass through the chroma delay line. The U output from this enters the
TDA3300B at pin 8, passing to the U detector and to the burst detector.
The latter is part of a phase -locked loop, the detector's output being
applied via an H/2 (half-line frequency) switch to the 4.43MHz voltage
-controlled crystal oscillator. The 4.43MHz reference oscillator's
output is applied for PAL switching, and to the U detector via a voltage
-controlled 90° phase shifter. This shifter is under the control of the
90° detec- tor which compares its output with the oscillator's output
coming via the PAL switch: when the phase shift is cor- rect, the output
from the 90° phase detector is zero. The combined effect of the two H/2
switches in the reference oscillator control loop - the two shown on
the right-hand side - cancels phase detector offsets. The outputs from
the U and V detectors include burst "flag" pulses which are used for
a.c.c., ident and colour -killing - there are two colour -killing
actions. RGB Output Stages The RGB output stages are of the class AB
type and incorporate extra circuitry for c.r.t. black -current sampl-
ing and beam limiting. Fig. 4 shows the red output stage. Under most
conditions transistor 2TR1 acts as a class A amplifier, driving the
tube's cathode via 2D5 and 2TR7. A high -value collector load resistor
(2R33) is used to reduce the dissipation in 2TR1. The stage gain is set
by the ratio of 2R40 and 2R36 to 2R25 and 2RV3, the latter setting the
drive level. For good transient response it's necessary for the
tube/base capacitance to be rapidly charged/discharged in accordance
with the signal swings. There is no problem when 2TR1 is being driven
from off to on, since the capacitance is discharged rapidly via 2D5 and
2TR1. When 2TR1 is driven from on to off however 2D5 will become reverse
biased. Under these conditions 2TR4 acts as an emitter -follower so
that the capacitance charges rapidly. Black -level stability is critical
for good results. As we've 2R46 5k6 2R51 120k 2TR7 BF493S 2C43l
Sampling circuit L -J 1k5 Field blanking J Red cathode _Tube input T"and
base 810capacitance nlrr Reference Line pedestal blanking Sample -and -
hold amplifier-ws switched on rt- Video Urn seen, the TDA3300B chip
incorporates circuitry for automatic black -current correction. Making
use of this reduces service calls and ensures constant performance
despite tube ageing or circuit misadjustment. Feedback is required, and
this is provided by the sampling circuit shown in the box with the
broken outline. Transistor 2TR7 acts as an emitter -follower between the
video output stage and the c.r.t.'s cathode. It's a low leakage type,
the components 2C40, 2D10 and 2C43 ensuring that the circuit has
negligible effect on the video signal. Since the beam current flows via
2R51, a voltage proportional to the beam current is produced across this
resistor. It's fed into the TDA3300B at pin 22. Black -current Control
For automatic black -current control the important thing is the small
beam current that flows when the tube is biased just above cut off. To
enable this current to be sampled, the TDA3300B replaces the video
signal with a fixed reference pedestal voltage for a couple of lines at
the end of each field blanking period (this pedestal can be seen as a
grey line at the top of the picture if the height control's setting is
reduced). The sample voltage at pin 22 of the i.c. is fed to one input
of a sample -and -hold amp- lifier which is switched on to sample the
input for one line only of the reference pedestal period. 2C33 acts as
the black -current control reservoir capacitor, holding the charge
acquired during the sampling time for the whole field period. This
charge is added to the video signal within the i.c., thus maintaining
the correct red gun black current. It's interesting to notice that when a
set is switched on from cold there's a momentary screen bright -up with
flyback lines as the beam current begins to flow. This is because it
takes several fields for 2C33 (and the corre- sponding capacitors in the
green and blue channels) to charge fully. Since the voltage
continuously available across 2R51 is proportional to beam current, it's
used within the i.c. for peak beam current limiting during the active
line periods. This is in addition to beam current limiting via the con-
trast control - and a crowbar trip that operates should the beam current
exceed 3mA.
This device will accept a PAL or NTSC composite video signal and output the
three color signals, needing only a simple driver amplifier to interface to the pic-
ture tube. The provision of high bandwidth on-screen display inputs makes it
suitable for text display, TV games, cameras, etc. The TDA3301 B has user con»
trol laws, and also a phase shift control which operates in PAL, as well as NTSC.
0 Automatic Black Level Setup
0 Beam Current Limiting
0 Uses Inexpensive 4.43 MHZ to 3.58 MHz Crystal
0 No Oscillator Adjustment Required
0 Three OSD Inputs Plus Fast Blanking Input
0 Four DC, High Impedance User Controls
0 lnterlaces with TDA33030B SECAM Adaptor
0 Single 12 V Supply
0 Low Dissipation, Typically 600 mW
The brilliance control operates by adding a pedestal to the output
signals. The amplitude of the pedestal is controlled by Pin 30.
During CRT beam current sampling a standard pedestal is
substituted, its value being equivalent tothe value given by V30 Nom
Brightness at black level with V30 Nom is given by the sum of three gun
currents at the sampling level, i.e. 3x20 |.1A with 100 k reference
resistors on Pins 16, 19, and 22.
During picture blanking the brilliance pedestal is zero; therefore, the
output voltage during blanking is always the minimum brilliance black
level (Note: Signal channels are also gain blanked).
Chrominance Decoder
The chrominance decoder section of the TDA3301 B
consists of the following blocks:
Phase-locked reference oscillator;
Phase-locked 90 degree servo loop;
U and V axis decoders
ACC detector and identification detector; .
Identification circuits and PAL bistable; .
Color difference filters and matrixes with fast blanking
Circuits.
The major design considerations apart from optimum
performance were:
o A minimum number of factory adjustments,
o A minimum number of external components,
0 Compatibility with SECAM adapter TDA3030B,
0 Low dissipation,
0 Use of a standard 4.433618 Mhz crystal rather
than a 2.0 fc crystal with a divider.
The crystal VCO is of the phase shift variety in which the
frequency is controlled by varying the phase of the feedback.
A great deal of care was taken to ensure that the oscillator loop
gain and the crystal loading impedance were held constant in
order to ensure that the circuit functions well with low grade
crystal (crystals having high magnitude spurious responses
can cause bad phase jitter). lt is also necessary to ensure that
the gain at third harmonic is low enough to ensure absence of
oscillation at this frequency.
It can be seen that the
necessary 1 45°C phase shift is obtained by variable addition
ol two currents I1 and I2 which are then fed into the load
resistance of the crystal tuned circuit R1. Feedback is taken
from the crystal load capacitance which gives a voltage of VF
lagging the crystal current by 90°.
The RC network in the T1 collector causes I1 to lag the
collector current of T1 by 45°.
For SECAM operation, the currents I1 and I2 are added
together in a fixed ratio giving a frequency close to nominal.
When decoding PAL there are two departures from normal
chroma reference regeneration practice:
a) The loop is locked to the burst entering from the PAL
delay line matrix U channel and hence there is no
alternating component. A small improvement in signal
noise ratio is gained but more important is that the loop
filter is not compromised by the 7.8 kHz component
normally required at this point for PAL identification
b) The H/2 switching of the oscillator phase is carried out
before the phase detector. This implies any error signal
from the phase detector is a signal at 7.8 kHz and not dc.
A commutator at the phase detector output also driven
from the PAL bistable coverts this ac signal to a dc prior
to the loop filter. The purpose ot this is that constant
offsets in the phase detector are converted by the
commutator to a signal at 7.8 kHz which is integrated to
zero and does not give a phase error.
When used for decoding NTSC the bistable is inhibited, and
slightly less accurate phasing is achieved; however, as a hue
control is used on NTSC this cannot be considered to be a
serious disadvantage.
90° Reference Generation
To generate the U axis reference a variable all-pass network
is utilized in a servo loop. The output of the all-pass network
is compared with the oscillator output with a phase detector of
which the output is filtered and corrects the operating point of
the variable all»pass network .
As with the reference loop the oscillator signal is taken after
the H/2 phase switch and a commutator inserted before the
filter so that constant phase detector errors are cancelled.
For SECAM operation the loop filter is grounded causing
near zero phase shift so that the two synchronous detectors
work in phase and not in quadralure.
The use of a 4.4 MHz oscillator and a servo loop to generate
the required 90° reference signal allows the use of a standard,
high volume, low cost crystal and gives an extremely accurate
90° which may be easily switched to 0° for decoding AM
SECAM generated by the TDA3030B adapter.
ACC and Identification Detectors
During burst gate time the output components of the U and
also the V demodulators are steered into PNP emitters. One
collector current of each PNP pair is mirrored and balanced
against its twin giving push-pull current sources for driving the
ACC and the identification filter capacitors.
The identification detector is given an internal offset by
making the NPN current mirror emitter resistors unequal. The
resistors are offset by 5% such that the identification detector
pulls up on its filter capacitor with zero signal.
Identification
See Figure 11 for definitions.
Monochrome I1 > I2
PAL ldent. OK I1 < lg
PAL ldent_ X l1 > I2
NTSC I3 > I2
Only for correctly identified PAL signal is the capacitor
voltage held low since I2 is then greater than I1.
For monochrome and incorrectly identified PAL signals l1>l2
hence voltage VC rises with each burst gate pulse.
When V,ef1 is exceeded by 0.7 V Latch 1 is made to conduct
which increases the rate of voltage rise on C. Maximum
current is limited by R1.
When Vref2 is exceeded by 0.7 V then Latch 2 is made to
conduct until C is completely discharged and the current drops
to a value insufficient to hold on Latch 2.
As Latch 2 turns on Latch 1 must turn off.
Latch 2 turning on gives extra trigger pulse to bistable to
correct identification.
The inhibit line on Latch 2 restricts its conduction to alternate
lines as controlled by the bistable. This function allows the
SECAM switching line to inhibit the bistable operation by firing
Latch 2 in the correct phase for SECAM. For NTSC, Latch 2
is fired by a current injected on Pin 6.
lf the voltage on C is greater than 1.4 V, then the saturation
is held down. Only for SECAM/NTSC with Latch 2 on, or
correctly identified PAL, can the saturation control be
anywhere but minimum.
NTSC Switch
NTSC operation is selected when current (I3) is injected into
Pin 6. On the TDA33O1 B this current must be derived
externally by connecting Pin 6 to +12 V via a 27 k resistor (as
on TDA33OOB). For normal PAL operation Pin 40 should be
connected to +12 V and Pin 6 to the filter capacitor.
4 Color Difference Matrixing, Color Killing,
and Chroma Blanking
During picture time the two demodulators feed simple RC
filters with emitter follower outputs. Color killing and blanking
is performed by lifting these outputs to a voltage above the
maximum value that the color difference signal could supply.
The color difference matrixing is performed by two
differential amplifiers, each with one side split to give the
correct values of the -(B-Y) and -(Ft-Y) signals. These are
added to give the (G-Y) signal.
The three color difference signals are then taken to the
virtual grounds of the video output stages together with
luminance signal.
Sandcastle Selection
The TDA3301B may be used with a two level sandcastle
and a separate frame pulse to Pin 28, or with only a three level
(super) sandcastle. In the latter case, a resistor of 1.0 MQ is
necessary from + 12 V to Pin 28 and a 70 pF capacitor from
Pin 28 to ground.
Timing Counter for Sample Control
In order to control beam current sampling at the beginning
of each frame scan, two edge triggered flip-flops are used.
The output K ofthe first flip-flop A is used to clock the second
tlip-flop B. Clocking of A by the burst gate is inhibited by a count
of A.B.
The count sequence can only be initiated by the trailing
edge of the frame pulse. ln order to provide control signals for:
Luma/Chroma blanking
Beam current sampling
On-screen display blanking
Brilliance control
The appropriate flip-flop outputs ar matrixed with sandcastle
and frame signals by an emitter-follower matrix.
Video Output Sections
Each video output stage consists of a feedback amplifier in A further drive current is used to control the DC operating
which the input signal is a current drive to the virtual earth from point; this is derived from the sample and hold stage which
the luminance, color difference and on-screen display stages. samples the beam current after frame flyback.
PHILIPS TDA3653B TDA3653C Vertical deflection and guard circuit (90°):
GENERAL DESCRIPTION
The TDA3653B/C is a vertical deflection output circuit for drive of various deflection systems with currents up to
1.5 A peak-to-peak.
Features
· Driver
· Output stage
· Thermal protection and output stage protection
· Flyback generator
· Voltage stabilizer
· Guard circuit
QUICK REFERENCE DATA
Note to the quick reference data
1. The maximum supply voltage should be chosen such that during flyback the voltage at pin 5 does not exceed 60 V.
FUNCTIONAL DESCRIPTION
Output stage and protection circuit
Pin 5 is the output pin. The supply for the output stage is fed to pin 6 and the output stage ground is connected to pin 4.
The output transistors of the class-B output stage can each deliver 0.75 A maximum.
The maximum voltage for pin 5 and 6 is 60 V.
The output power transistors are protected such that their operation remains within the SOAR area. This is achieved by
the co-operation of the thermal protection circuit, the current-voltage detector, the short-circuit protection and the special
measures in the internal circuit layout.
Driver and switching circuit
Pin 1 is the input for the driver of the output stage. The signal at pin 1 is also applied via external resistors to pin 3 which
is the input of a switching circuit. When the flyback starts, this switching circuit rapidly turns off the lower output stage
and so limits the turn-off dissipation.
It also allows a quick start of the flyback generator.
External connection of pin 1 to pin 3 allows for applications in which the pins are driven separately.
Flyback generator
During scan the capacitor connected between pins 6 and 8 is charged to a level which is dependent on the value of the
resistor at pin 8 (see Fig.1).
When the flyback starts and the voltage at the output pin (pin 5) exceeds the supply voltage, the flyback generator is
activated.
The supply voltage is then connected in series, via pin 8, with the voltage across the capacitor during the flyback period.
This implies that during scan the supply voltage can be reduced to the required scan voltage plus saturation voltage of
the output transistors.
The amplitude of the flyback voltage can be chosen by changing the value of the external resistor at pin 8.
It
should be noted that the application is chosen such that the lowest
voltage at pin 8 is > 2.5 V, during normal operation.
Guard circuit
When there is no deflection current and the flyback generator is not activated, the voltage at pin 8 reduces to less than
1.8 V. The guard circuit will then produce a DC voltage at pin 7, which can be used to blank the picture tube and thus
prevent screen damage.
Voltage stabilizer
The internal voltage stabilizer provides a stabilized supply of 6 V to drive the output stage, which prevents the drive
current of the output stage being affected by supply voltage variations.
PACKAGE OUTLINES
TDA3653B: 9-lead SIL; plastic (SOT110B); SOT110-1; 1996 November 25.
TDA3653C: 9-lead SIL; plastic power (SOT131); SOT131-2 November 25.
PARAMETER SYMBOL MIN. TYP. MAX. UNIT
Supply (note 1)
Supply voltage range
pin 9 VP = V9-4 10 - 40 V
pin 6 V6-4 - - 60 V
Output (pin 5)
Peak output voltage during flyback V5-4M - - 60 V
Output current I5(p-p) - 1.2 1.5 A
Operating junction temperature range Tj -25 - +150 °C
Thermal resistance junction to mounting base
(SOT110B) Rth j-mb - 10 - K/W
(SOT131) Rth j-mb - 3.5 - K/W.
--------------------------------------------------
A SCART Connector (which stands for Syndicat des Constructeurs d'Appareils Radiorécepteurs et Téléviseurs) is a standard for connecting audio-visual equipment together. The official standard for SCART is CENELEC document number EN 50049-1. SCART is also known as Péritel (especially in France) and Euroconnector but the name SCART will be used exclusively herein. The standard defines a 21-pin connector (herein after a SCART connector) for carrying analog television signals. Various pieces of equipment may be connected by cables having a plug fitting the SCART connectors. Television apparatuses commonly include one or more SCART connectors.
Although a SCART connector is bidirectional, the present invention is concerned with the use of a SCART connector as an input connector for receiving signals into a television apparatus. A SCART connector can receive input television signals either in an RGB format in which the red, green and blue signals are received on Pins 15, 11 and 7, respectively, or alternatively in an S-Video format in which the luminance (Y) and chroma (C) signals are received on Pins 20 and 15. As a result of the common usage of Pin 15 in accordance with the SCART standard, a SCART connector cannot receive input television signals in an RGB format and in an S-Video format at the same time.This kind of connector is todays obsoleted !Consequently many commercially available television apparatuses include a separate SCART connectors each dedicated to receive input television signals in one of an RGB format and an S-Video format. This limits the functionality of the SCART connectors. In practical terms, the number of SCART connectors which can be provided on a television apparatus is limited by cost and space considerations. However, different users wish the input a wide range of different combinations of formats of television signals, depending on the equipment they personally own and use. However, the provision of SCART connectors dedicated to input television signals in one of an RGB format and an S-Video format limits the overall connectivity of the television apparatus. Furthermore, for many users the different RGB format and S-Video format are confusing. Some users may not understand or may mistake the format of a television signal being supplied on a given cable from a given piece of equipment. This can result in the supply of input television signals of an inappropriate format for the SCART connector concerned.
