372 Chapter7
ADC/Interface Section
A3 Assembly Video Circuits
Video MUX
Refer to function block U of A3 Interface Assembly Schematic
Diagram (sheet 5 of 6) in the
HP 8560 E-Series Spectrum Analyzer
Component Level Information.
1. Press
PRESET and set the HP 8560E/EC controls as follows:
Center frequency ..................................................... 300MHz
Span .................................................................................0Hz
2. Press
SGL SWP, CAL, and IF ADJ OFF. Connect the CAL OUTPUT to
the INPUT 50 Ω connector.
3. Check for a TTL high on A3U104 pin 2 and a TTL low on U104 pin
10. Set the spectrum analyzer to 2 dB/div and check for a TTL high
on A3U104 pin 10 and a TTL low on A3U104 pin 2.
4. If the logic levels on A3U104 are incorrect, check the LLOG_STB
signal as follows:
a. Monitor A3U104 pin 9 with an oscilloscope or logic probe. Check
that a 1 microsecond, low-going pulse is present when switching
between 10 dB/div and 2 dB/div.
b. Check the inputs to A3U104 (pins 3 and 11) while switching
between 10 dB/div and 2 dB/div.
c. If the logic signals are incorrect, refer to "Analog Bus Timing" and
"Analog Bus Drivers" in this chapter.
5. Check comparators A3U109A/C for proper outputs. The outputs
should be high when the noninverting input is greater than the
threshold voltage of +1.3 Vdc.
6. If A3U104 and A3U109 are working properly, set the
AMPLITUDE
and REF LVL to 0 dBm.
7. Monitor the voltage at A3TP14 while switching the spectrum
analyzer between 10 dB/div and 2 dB/div. The voltage should switch
between 0.8 and 0.4 Vdc.
8. If the voltage at A3TP14 is incorrect, suspect either A3Q220 or
A3Q221.
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 repeat steps 1 through 7.
10.To return the HP 8560E/EC to automatic sweep, press
SWEEP,
SWEEPCONT SGL or press PRESET.
Video Filter
Refer to function block V of A3 Interface Assembly Schematic
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