398 Chapter8
ADC/Interface Section
A3 Assembly Video Circuits
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 spectrum analyzer, connect the front-panel CAL OUTPUT to
the INPUT 50Ω 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.
7. If the peak detector appears latched up, check LPOS_RST (U422 pin
4) for a negative TTL level reset pulses. The reset pulses should
occur every 130 µs and should be approximately 250 ns wide.
8. If the reset pulses are absent, troubleshoot the Peak Detector Reset
circuitry.
9. If the reset pulses are present, check the gate of Q207. The pulses
should be positive-going from −12.7 V to −1.35 V.
10.The peak detector can be made into a unity gain amplifier by
shorting the cathode of CR203 to the anode of CR204. If the peak
detector functions normally as a unity gain amplifier, suspect Q208
or CR203 or CR204.
Peak Detector Reset
Refer to function block R of A3 Interface Assembly Schematic Diagram
(sheet 4 of 6) in the 8560 E-Series Spectrum Analyzer Component Level
Information.
1. Press
PRESET on the spectrum analyzer and set the controls as
follows:
Center frequency .................................................. 300MHz
Span .............................................................................. 0Hz
Sweep time ...................................................................... 5s
Detector mode ....................................................POSPEAK
2. Check that HHOLD (A3U526 pin 11) has 18 µs wide pulses every
128 µs.
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