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Theory of Operation
1780R-Series Service Manual
3–83
C394 is lowered ≈1/3 for PAL instruments. U397 is a strobe-controlled
complementary output comparator. U397 provides TTL outputs, at External CW
frequency, to the Input Switch, U469, from the bandpass filtered EXT CW
signal.
The strobe signal is disabled in EXT CW. It is also used to turn off the VCXO in
NTSC instruments; U169 is disabled in PAL instruments.
Input Switch. U469 is a quad 2-input multiplexer used to select the F
SC
reference
for the input phase-lock loop. It is switched by the EXT CW control line to
select either buffered EXT CW or F
SC
as the reference source. U266C is an
Exclusive OR gate that inverts the Microprocessor-generated control signal.