HP 8340A, Service Manual

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Model 8340A - Service
MIXER
The output of the MIXER is amplified, filtered, and sent to the
PHASE/FREQUENCY DETECTOR. The desired MIXER output is the
difference frequency and lies between 150 kHz and 6 Mhz.
PHASE/FREQUENCY DETECTOR
The PHASE/FREQUENCY DETECTOR generates a differential output
signal that is used by the LOOP AMPLIFIER as well as the PHASE
LOCK INDICATOR.
LOOP AMPLIFIER
The phase detector differential outputs are the inputs to the
LOOP AMPLIFIER. Each of the differential inputs is passed through
identical low�pass filters and a voltage divider/filter. The
output of the voltage divider is sent to a varactor diode that
tunes the
vco.
160 to 166 MHz vco
The 160 to 166 MHz
vco
is a varactor-tuned, Colpitts transistor
oscillator. A buffer transistor output provides a 160 to 166 MHz
signal that drives the MIXER.
PHASE LOCK INDICATOR
The PHASE LOCK INDICATOR senses the outputs of the phase detector
to determine when the loop is locked. The ON=LOCKED LED indicates
that a phase locked condition exists.
8-174
IOOMHz
FROM REFERENCE
LOOP
Xl.6
FROM Pll2
VCO OUT + 25
OR
VCO OUT + 600
Figure 8C-5. Phase Lock Loop 3 Operation
TO
PLL I IF
160. I 5-l 66MHz
200Hz STEPS
OR
IOllZ STEPS
Scans by HB9HCA and HB9FSX