HP 8560E Service Manual page 374

Diagram (sheet 5 of 6) in the
Component Level Information
The HP 8560E/EC uses digital filtering for 1 Hz to 100 Hz video
bandwidths. An RC low-pass filter is used for 300 Hz to 3 MHz video
bandwidths. Various series resistances and shunt capacitances switch
into the video filter to change its cutoff frequency.
When Gated Video is selected, the video signal is "gated" (turned on
periodically for a set duration of time). This function is shown in block
V of the block diagram as a series switch that allows the video signal to
pass only when it is closed. The actual switch, U109B/CR118, shunts
the video to ground (video signal is passed only when the switch is
open). The control circuitry for this switch is described under
"Triggering or Video Gating Problems" in this chapter. The rear panel
EXT/GATE TRIG INPUT provides the connection for triggering in the
Gated Video mode. The gate output signal is available at the rear panel
BLKG/GATE OUTPUT connector. Positive or negative edge mode, or
level mode can be selected from the front panel.
1. Press and set the HP 8560E/EC controls to the following
PRESET
settings:
Center frequency .................................................. 225 MHz
Span ...................................................................... 550 MHz
Sweep time ............................................. Uncoupled (MAN)
2. Press and
CAL IF ADJ OFF
3. Step the Video BW from 3 MHz to 10 kHz. At each step, the
peak-to-peak deviation of the noise should decrease.
4. Step the Video BW down to 1 Hz. At each step, the amplitude of the
LO feedthrough should decrease.
5. Refer to Table 7-7 on page 374
for the selected video bandwidth setting.
6. If the output of latch A3U102 is not correct, trigger an oscilloscope
on LLOG_STB (U102 pin 9) and monitor U102 pin 1 and other latch
inputs while changing the video bandwidth.
7. If the inputs are incorrect, troubleshoot the analog bus. Correct
inputs with bad outputs indicate a faulty U102.
8. Check that the outputs of A3U111A, A3U111B, and A3U107A/B/C/D
are correct for their inputs. The outputs should be high with
noninverting inputs higher than the +1.4 V threshold voltage. If a
voltage drop is noticed across these components, suspect A3CR109 or
A3Q317B. Since no dc current flows through any of the series
resistances or FETS (drain to source), no voltage drops should occur.
9. To return the HP 8560E/EC to automatic sweep, press
Chapter 7
HP 8560 E-Series Spectrum Analyzer
.
.
and check for correct latched levels
ADC/Interface Section
A3 Assembly Video Circuits
,
SWEEP
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