Theory of Operation
4–10
1720/1721
The Subcarrier Regenerator in the 1721 Vectorscope samples alternate lines and
locks to every other burst. Alternate line sampling can be defeated by moving
W987 to the W986 position, but the +V mode may not be properly phased and
there will be more phase jitter.
The Quadrature Phase Detector receives every other burst from the Reference
Switch. When first trying to lock, this may be either a positive- or negative-go-
ing, demodulated burst, since U774B has received no phasing information yet.
If the negative bursts are being received, the minus input (pin 3) of the PAL
Phasing Comparator (U348) will be zero on one line with negative-going burst
pulses on the next line. Since the reference voltage, to the comparator (pin 2), is
a positive voltage the comparator output will remain high and the Preset (pin 10)
of U774B will not be affected.
If the positive-going burst pulses are initially being received by the Quadrature
Phase Detector, pin 3 of U348 will be zero during one line and there will be a
positive-going burst pulse on the next line. This positive burst pulse will cross
the comparator threshold and its output will be negative pulses to preset U774B.
The flip-flop will be rephased, and negative-going bursts will be gated to the
Quadrature Phase Detector.
The VCXO is a phase-locked, voltage-controlled, crystal oscillator with
automatic gain control (AGC). It generates a sine wave at subcarrier frequency
(3.579545 MHz for NTSC, 4.433619 MHz for PAL), controlled by crystal Y129
and varicap CR235. The frequency is stabilized by dc feedback from the Error
Amp, U734, which changes the varicap bias when there is a frequency error.
The output is shaped into a sine wave by a Pi filter consisting of C337, L337,
and C336, to drive the Phase Detectors and the Phase Shifter. The Phase Shifter
is a goniometer, which is a variable capacitance device that shifts the phase of
the display through 360°. Subcarrier Amplifier Q434 amplifies the goniometer
output and drives the Chrominance Demodulators and the AGC Amplifier,
Q334.
The subcarrier amplitude at the collector of Q434 is rectified by CR428 and
stored in C328. The dc level on C328 controls the bias of Q222 through Q334 to
correct for any fluctuations in amplitude. The bias current for Q222 is inversely
proportional to the stored level on C328. If the amplitude rises, Q334 reduces
the gain; if the amplitude goes down, the gain increases, thus stabilizing
subcarrier amplitude.
PAL Phasing
VCXO
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