Third Lo Driver Amplifier - HP 8560A Service Manual

29. If the front-panel CAL OUTPUT amplitude is out of specification and cannot be brought
within specification by adjusting A15R561, CAL AMPTD, check the Calibrator AGC
Amplifier with the following steps. Refer to function block W of Al5 RF Schematic (sheet
3 of 4).
Note
The 300 MHz CAL OUTPUT signal comes from the divided down 600 MHz
which is passed through a leveling loop. The 300 MHz signal passes through
a low-pass filter for reducing higher harmonics. These harmonics can fool the
detector. The 300 MHz signal passes through a variable attenuator controlled
by PIN diode CR503 which is controlled by the feedback loop. Diode CR504
is the detector diode (the same type as CR505). Diode CR504 provides
temperature compensation between the reference voltage and the detected RF
voltage.
a. Measure the level of 300 MHz at Al5 TP505 with an active probe/spectrum analyzer
combination. If the signal is less than +2 dBm, be sure to perform the beginning steps
of this procedure.
b. If the signal at this point is correct, place a short across the PIN diode CR503.
c. If the signal level at the CAL OUTPUT is still less than -10 dBm with CR503 shorted
out, troubleshoot the RF forward path through amplifier Q505.
d. If the CAL OUTPUT signal level is greater than -10 dBm, troubleshoot the PIN
diode attenuator, the detector, or the feedback path.
30. Measure the detector voltage at A15J502 pin 14. The voltage should measure
approximately +0.3 Vdc when the CAL OUTPUT signal is at -10 dBm. This voltage
should change with adjustment of A15R561, CAL AMPTD.
31. Check that the voltage at U507A Pin 3 is +1.7 Vdc. If this voltage is not correct, there
may be a problem with the +lO V reference.
32. Measure voltage at U507B pin 5 while adjusting R561. This is the temperature-
compensated adjustable voltage reference to which the detected voltage is compared. It
should vary between +1.3 V and -0.6 V.
33. Adjust R561 to its limits and verify that the output U507B pin 7 measures approximately

Third LO Driver Amplifier

The Third LO Driver Amplifier (Q503) amplifies the 300 MHz from the 600 MHz phase-lock
loop to a sufficient level to drive the LO port of the Double Balanced Mixer. During the SIG
ID operation, diodes CR501 and CR502 turn off the 3rd LO Driver Amplifier in order to
minimize the amount of 300 MHz going to the double-balanced mixer.
1. Press
2. Use an active-probe/spectrum-analyzer combination to confirm the power level of the 300
MHz signal at the following test points: