Connect the HP 180A/1801A Channel A input to TP 1,
the Channel B input to TP 2, and observe the waveform.
Waveform GOOD: Check T1, CR8
and associated components.
Waveform BAD: proceed to 1-c.
Chan. A: 1V/Div
Chan. B: 0.02V/Div
1-c. Connect the HP 180A/1801A/1821A to TP C (Q7-b)
Waveform GOOD: proceed to step
CR9 through CR12, Q4/Q5/Q6 and associated
Channel A:0.1 V/Div
To verify tuning stabilizer operation, place TUNING
STABILIZER to OFF. Channel B signal becomes highly
Waveform GOOD: Check Q7
unstable, and the CRT display on the analyzer shifts.
Waveform BAD: Check Q8 and associated components
and repeat 1-a after completing repairs.
Transistors Q1/Q2/Q3 form an oscillator-amplifier whose
operational state is determined by the sampler circuit
Sampler gate diodes CR9 through CR12 are normally
output. When the analyzer is not phase locked there is
biased off by voltages applied through R10 and R11.
no correlation between consecutive 100 kHz pulses from
During the one nanosecond off time of CR8, gating
the pulse generator and the signal being received from
pulses from the pulse generator turn on CR9 through
CR12 simultaneously. When the gate diodes are turned
on, the voltage developed across R9 by the first local
oscillator signal is coupled through to place a charge on
C16 and C17. Since the sampler gate is conducting for
only one nanosecond, C16 and C17 can charge only to
approximately 15% of the dc level across R9. A
feedback path from operational amplifier Q4/Q5/Q6
completes charging of C16 and C17 to 100% of the
voltage level which appeared across R9 during the
Zener diodes CR14 and CR15
establish the bias voltage for the sampler gate diodes.
When the first local oscillator is operating at a frequency
which is exactly harmonically related to the 100 kHz
reference signal, the 100 kHz pulses from the pulse
generator turn on the sampler gate diodes in time
coincidence with the negative-going positive half cycles
of the first local oscillator output signal. When the
samples taken from the first local oscillator are taken at
the same point on the negative-going signal each time
the gate is opened, the voltage appearing across R9 is
the same each time the gate is opened, and the charge
on C16 and C17 remains constant. With these idealized
conditions the output of the sampler to the search
oscillator-amplifier would be a steady dc level at the
junction of CR14 and CR15.
developed across R9 is sampled at a different point on
the signal slope. This results in a change in the level of
the charge on C16 and C17 and a corresponding
SERVICE SHEET 5
change in the sampler output to the search oscillator-
200-310 MHz Voltage Tuned Oscillator