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Theory of Operation
1780R-Series Service Manual
3–39
Diagrams 14 & 15 SCH Phase Timing & Sync Locked Oscillator
The 1780R-Series displays the horizontal sync to subcarrier phase relationship as
a polar plot on its Vectorscope CRT. It generates a subcarrier rate signal, phase
locked to the 50% point of sync that is demodulated against burst-locked
subcarrier to produce a dot on the vectorscope. By comparing this sync dot to the
burst vector, the SCH phase of the signal can determined.
The SCH circuit consists of four major blocks. The Sync 50% Detector amplifies
and clamps the displayed (internal) video signal. The backporch and sync tip
levels of this signal are sampled and the voltage half way between, which
corresponds to the 50% sync point, is applied to one input of a comparator. The
amplified video is applied to the remaining input so that the comparator outputs
an edge when the 50% sync point occurs.
The Sync Locked Oscillator consists of a 4 × F
SC
oscillator that is divided by
four and harmonically locked to the edge output by the 50% Sync Detector.
Overview