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Theory of Operation
3–80
1780R-Series Service Manual
Diagram 31 Subcarrier Regenerator
The Reference Video signal from the Waveform Monitor Reference switch
(Diagram 4) is buffered and bandpass filtered before being mixed with the
Voltage-Controlled Crystal Oscillator (VCXO) output to produce a control signal
for the VCXO. In addition, the bandpass filtered Ref Video is buffered and
envelope detected to provide a chrominance sample to one input of a coincidence
detecting one-shot; the other input is from a one-shot delayed Reference Sync
that produces a half-shot conditioned transition at backporch time.
The output of the coincidence detecting one-shot provides one input to the Pulse
Gating circuit. The second input to the Pulse Gating circuit is from a lock-detect-
ing comparator at the output of the low-pass filter. The third input is either a
resistor pulled up level (NTSC) or the +V output of the PAL burst flip-flop.
The Pulse Gating circuit drives a Pulse Generator to provide a gain controlling
pulse to the sampling transconductance amplifier. Loop bandwidth is switched
between narrow for phase lock (to integrate out noise and chrominance phase
errors on a line-by-line basis) and wide for lock acquisition when the loop is
unlocked by increasing and decreasing amplifier gain.
The output of the sampling transconductance amplifier is the control voltage for
the VCXO, which runs at 4 × F
SC
. The VCXO output is divided down by 4 to
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