Chapter 8 395
ADC/Interface Section
A3 Assembly Video Circuits
9. The Video MUX will appear faulty if A3CR109 is shorted or leaky.
Diode A3CR109 clamps the voltage at A3TP14 to −0.4 V when in log
expand with less than 0.8 V at J101. To confirm this failure, lift the
cathode of diode A3CR109 and perform steps 1 through 7 again.
10.To return the spectrum analyzer to automatic sweep, press
SWEEP, SWEEP CONT SGL or press PRESET.
Video Filter
Refer to function block V of A3 Interface Assembly Schematic Diagram
(sheet 5 of 6) in the 8560 E-Series Spectrum Analyzer Component Level
Information.
The spectrum analyzer 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. The 3 MHz video
bandwidth is not available when Gated Video is selected.
When sample detection is selected, the effective video bandwidth is
limited to approximately 450 kHz by the track and hold circuitry.
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” on page 385 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
PRESET and set the spectrum analyzer controls to the
following settings:
Center frequency ..................................................... 225MHz
Span ......................................................................... 550MHz
Sweep time ................................................ Uncoupled(MAN)
2. Press
CAL and 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 8-6 on page 396 and check for correct latched levels
for the selected video bandwidth setting.
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