Theory of Operation
1720/1721
4–9
Reference Amplifier. The output of this phase detector is an ac multiplication of
the input signals, which occurs only during the time that both of the input signals
are present. The average dc output level is proportional to the difference in
phase between the inputs.
The output is filtered to remove any chrominance and harmonics and drives
U640, which is a non-inverting, high-gain operational amplifier.
The Quadrature Phase Detector is similar to the Loop Phase Detector, except that
the carrier input signal is phase shifted by 90° by a network consisting of L750,
C548, and R647. This results in the output of the Quadrature Phase Detector
being maximum when the output of the Loop Phase Detector is zero. Since
Loop Phase Detector output is zero (phases are matched) during burst, the
Quadrature Phase Detector provides a large-amplitude pulse occurring only
during burst time. Q732 inverts and amplifies the output pulse to drive the Burst
Clamp and Lock Detector.
At burst time, C738 is charged by current through R734. The direction and
amount of current is determined by the output of the Loop Phase Detector
Amplifier, U640, with respect to ground (through R734 and Q733). The voltage
developed across R734 is the input voltage seen by the Error Amplifier.
The Subcarrier Regenerator has two different bandwidths. Wide bandwidth is
for fast lock-up and a large pull-in range. Once locked, the loop goes into a
narrow-bandwidth mode, which provides a stable reference with very little phase
jitter.
Q836 is turned on at every burst pulse, keeping C737 discharged and the output
of U734B at a low level. If burst is missing or not locked, Q836 is off during
burst time and C737 charges slowly negative until the output of U734B is high
enough to turn on the Bandwidth Switch transistor, Q632.
Q632 is off when subcarrier is locked to incoming burst. If the subcarrier is not
locked, U734B saturates Q632 and grounds a portion of the Error Amplifier
feedback to allow the Error Amplifier to rapidly change the bias on the VCXO
varicap CR235 and quickly bring the loop back into lock.
U734A is a non-inverting amplifier whose RC feedback network acts as a
low-pass filter to determine the Subcarrier Regenerator loop response. Any input
voltage to U734A is amplified and biases the VCXO varicap, CR235.
The Loop Balance control, R534, adjusts the Phase Locked Loop dc offset so
that there is no phase shift when burst amplitude changes.
Quadrature Phase
Detector and Amplifier
Burst Clamp
Lock Detector and
Bandwidth Switch
Error Amplifier
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