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Theory of Operation
3–86
1780R-Series Service Manual
U338. RCO from U342 is the counter load for the programmable counter (÷M
and ÷N).
The input register for the programmable counter (U235 and U238) is loaded with
a count from the Microprocessor Data Bus. This count is in the form of two 8-bit
words. The word consisting of the 8 least significant bits (LSBs) is loaded into
the input register of U235 followed by the 8 most significant bits (MSBs) word
being loaded into the register for U238. Registers are loaded on the rising edge
of the RCLK, which is supplied from the Microprocessor. The two 8-bit words
are the Microprocessor-decoded setting of the front-panel PHASE SHIFT. The
contents of the input registers are loaded into the counter when the 7200 Counter
ripple carries out (RCO), and the counter starts on the next F
SC
’ clock. Clock
Enable for U238 is the ripple carry out from U235.
Ripple carry out from U238 is one input to a coincidence input for a one-shot
(U335B). The other input to this pulse stuffer one-shot is from U230B, which is
clocked by U230A. U230 is a dual, edge-triggered, D-type, TTL flip-flop. The
pulse stuffer is required to compensate for a possible missing count in the
programmable counter. The one-shot output is the reference input for the output
Phase Detector, U449.
Phase Detector, Error Amplifier, ÷4F
SC
VCXO and 4 Counter. The remaining input
to the Phase Detector is the Voltage-Controlled Crystal Oscillator (VCXO)
generated subcarrier (IF
SC
). Phase difference between the reference (R) and
feedback (V) inputs of the Phase Detector directly control the output voltage.
The output voltage of the Phase Detector drives U349B, an Integrating Error
Amplifier, whose output is the control voltage to the Varactor-controlled 4F
SC
VCXO.
Q356 is the oscillator running at 4F
SC
. U445 is an edge-triggered quad D-type,
flip-flop with complementary outputs. It divides down the 4F
SC
output of the
VCXO to F
SC
. The Q2 output of U445 drives one input of the balanced mixer to
close the phase-lock loop. U445 also serves as an output latch, which provides 0
180_ phase shift to output the IF
SC
and QF
SC
references that are used by the
Vectorscope to demodulate the color signal. In addition, outputs from U445 drive
the Test Circle Oscillator and PAL +V/–V switching.
Low-pass Filter, Error Amplifier, and Phase Detector. U256A and B are Exclusive
OR gates that are configured as a balanced mixer where the output subcarrier
reference and the intermediate subcarrier reference (F
SC
’) are mixed to derive a
difference signal. Mixer output is low-pass filtered and drives a comparator stage
(U263A) that is configured as a Hysteresis Amplifier to output a TTL error
signal that drives the feedback input of the Phase Detector, U449.