386 Chapter8
ADC/Interface Section
Triggering or Video Gating Problems
7. Check that the voltage displayed on the DVM changes by 1 V for
each step of the VIDEO TRIG LEVEL.
8. If the voltage changes incorrectly, proceed as follows:
a. Check the −10 Vdc reference (A3U409 pin 4).
b. While using the front-panel knob to adjust the video trigger level,
check for the presence of pulses on A3U409 pin 15 (LDAC2).
c. While using the front-panel knob to adjust the video trigger level,
check for the presence of pulses on A3U409 pin 16 (LWRCLK).
d. Check that pulses are present on U409 pin 6 (IA0).
9. If the LWRCLK and LDAC2 signals are not correct, refer to
“Interface Strobe Select” on page 409
10.If correct trigger pulses are present at the trigger MUX output
(A3U613 pin 7), but the instrument does not appear to be sweeping,
proceed as follows:
a. Press
PRESET, SWEEP, and DLY SWP ON OFF until ON is
underlined, then
DLY SWP [ ] 30 milliseconds.
b. Using an oscilloscope, check for activity at pins 1 and 3 of
A3U615A.
c. If there is activity at pin 1 but not at pin 3 of A3U615A, suspect
A3U616 or A3U617.
d. If there is activity at pin 1 and pin 3 of A3U615A, suspect
A3U615. (Check pin 5 for activity.)
11.If there is a problem with Video Gating, proceed as follows:
a. Press
PRESET and set the spectrum analyzer as follows:
Center frequency ................................................ 300MHz
Span ............................................................................ 0Hz
Sweep time ............................................................. 150ms
b. Press TRIG, EXTERNAL, then SWEEP and GATE ON
OFF until ON is underlined.
c. Press
GATE DLY [ ] 10 milliseconds, then press GATE
LEN [] 30 milliseconds.
d. Connect a pulse/function generator (such as an 8116A) to provide
a 5 V peak-to-peak square wave (TTL level) to the spectrum
analyzer rear panel EXT/GATE TRIG INPUT and also (using a
BNC tee) to the channel 4 input of the oscilloscope (54501A).
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