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Model
8663A
Service
SERVICE SHEET
38
AllA FM PHASE DETECTOR
REFERENCE BLOCK DIAGRAM
7
Table 4-l. Recommended Performance Tests
After Adjustments or Repairs
Table 5-2. Post-Repair Adjustment Procedures
PRINCIPLES OF OPERATION
General
The purpose of the FM Phase Detector (AllA4) is to develop the FM
Loop Error Voltage. This is accomplished by comparing the VCO output
with a 10 MHz reference signal.
Phase Detector Circuitry
The Phase Detector Circuitry compares the phase difference between
the reference signal (FM 10 MHz Reference
Signal) and
the output of
the VCO (FM Loop N-Divider Drive), then generates a voltage
proportional to the phase error.
Both input signals are first
divided down to 100 kHz before they are compared by the FM Loop Phase
Detector, U4. The 10 MHz signal from the Reference Section is
divided by a total of 100.
This signal becomes the reference input
to the phase detector.
of 1400.
The 140 MHz VCO output is divided by a total
This signal becomes the variable input to the phase
detector.
The Phase Detector, U4, is a digital phase detector that generates
output pulse widths proportional to the phase difference between the
two input signals. If the variable 100 kHz signal is lower in
frequency or lags the reference input in phase, the output on pin 13
generates a pulse (See Figure 8-601). When the variable input is
higher in frequency or leads the input
in
phase the output on pin 2
generates a pulse (see Figure 8-602).
Figure 8-601. Phase Detector Operation
(Variable Input is Lower in Frequency than Reference S&vml)
8-601