HP 8560A, Service Manual

Page: 270 / 450

Positive/Negative Peak Detectors
See function blocks Y and Z of A3 Interface Assembly Schematic Diagram (sheet 5 of 6).
The following information pertains to the positive peak detector and is applicable to
troubleshooting the negative peak detector.
The positive peak detector consists of an input amplifier and followed
by detector diodes and and hold capacitor Output amplifier
and buffers the hold capacitor. Both the input and output amplifiers
have a gain of one. Each amplifier has local feedback. On the output amplifier the emitter of
connects to gate. On the input amplifier the feedback goes through and
Q208 back to the base of Global feedback occurs from the output amplifier through
R223 back to the input amplifier The peak detector resets through Q207.
1. Press and set the HP 8560A controls as follows:
C E N T E R F R E Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...300 MHz
SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RES BW
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Auto
VIDEO BW
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Auto
LOG dB/DIV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 dB/div
2. If the HP 8560A does not meet the conditions in steps a through e below, the positive and
negative peak detectors are probably faulty. Continue with step 3 to check the detectors.
a. The peak-to-peak deviation of the noise in NORMAL detector mode should be
approximately two divisions. Note the amplitude levels of the top and bottom of the
displayed noise.
b. Select POS PEAK detector mode.
c. Confirm that the noise is about one-third division peak-to-peak. The noise should also
be no higher than the top of the noise level in NORMAL detector mode.
d. Select NEG PEAK detector mode. The noise should be about one-third of a division
peak-to-peak. The noise should also be no lower than the bottom of the noise in
NORMAL mode.
e. Select SAMPLE detector mode. Check that the noise appears between the top and
bottom of the noise in NORMAL mode.
3. On the HP connect the front-panel CAL OUTPUT to the INPUT and set the
controls to the following settings:
C E N T E R F R E Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...300 MHz
SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SWEEP TIME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...5 s
DETECTORMODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . POS PEAK
4. Monitor and simultaneously with an oscilloscope.
5. Change the reference level from -10 to and verify a voltage change at
both and of 1 V to 0.6 V in 100 steps.
6. Check the entire range of the detector by substituting a dc source at and varying its
output from 0 V to 1 V.
7. If the peak detector appears latched up, check LPOS-RST for a negative TTL level reset
pulses. The reset pulses should occur every 130 and should be approximately 250 ns
wide.
Section 7-17