HP 8560E - Page 376

Chapter 7 375
ADC/Interface Section
A3 Assembly Video Circuits
A3Q210) followed by detector diodes (A3CR203 and A3CR204) and hold
capacitor A3C217. Output amplifier A3Q206, Q211, and Q212 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 Q212 connects to the gate of Q206. On the input amplifier the
feedback goes through Q209 and Q208 back to the base of U204D.
Global feedback occurs from the output amplifier through R223 back to
the input amplifier U204D. The peak detector resets through Q207.
1. Press
PRESET and set the HP 8560E/EC controls as follows:
Center frequency .................................................. 300MHz
Span ...................................................................... 500MHz
Resolution bandwidth ...............................................AUTO
Video bandwidth ........................................................AUTO
Log dB/division .....................................................10dB/div
2. If the HP 8560E/EC 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. Press
TRACE,
TRACE B, CLEAR WRITE B, VIEW B, TRACE A, MORE 1 of 3, and
DETECTOR MODES.
b. Select
DETECTOR POS PEAK 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 DETECTOR NEG PEAK 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
DETECTOR SAMPLE mode. Check that the noise appears
between the top and bottom of the noise in NORMAL mode.
3. On the HP 8560E/EC, connect the front panel CAL OUTPUT to the
INPUT 50 Ω and set the controls to the following settings:
Center frequency .................................................. 300MHz
Span .............................................................................. 0Hz
Sweep time ....................................................................... 5s
Detector mode ................................................... POSPEAK
4. Monitor A3TP17 and A3TP16 simultaneously with an oscilloscope.
5. Change the reference level from −10 dBm to +30 dBm and verify a
voltage change at both A3TP17 and A3TP16 of 0.9 V to 0.5 V in 100
mV steps.
6. Check the entire range of the detector by substituting a dc source at
J101 and varying its output from 0 V to 1 V.