HP 8560A, Service Manual

Page: 298 / 450

Video Offset
Refer to function block P of A4 Log Amplifier Schematic Diagram (sheet 3 of 4) in the
HP Spectrum Analyzer Component Level Information
binder.
The circuit provides a programmable video offset, with a step size of 5 from -300 to
1. On the spectrum analyzer, press (SPAN),
ZERO SPAN, 1
[AMPLITUDE), 50
Turn the IF auto cal off.
2. Disconnect W27 (coax 3) from and connect a signal generator to
3. Set the signal generator controls as follows:
Amplitude . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4. Simultaneously decrease the signal generator output and spectrum analyzer reference level
in 10 steps. At each step, the signal displayed on the spectrum analyzer should be close
to the reference level.
5. Reconnect W27 (coax 3) to and cycle the analyzer power. Press
STOP REALIGN
when
it appears.
6. Press and
SINGLE
on the spectrum analyzer.
7. The offset DAC, pin 2, should now be at its default value of approximately
V. The voltage at pin 3 should be approximately 0 V for a DAC output of
2.45 V.
8. If this default offset voltage is incorrect, DAC is the most probable cause.
Video Output
1. On the spectrum analyzer, press (FREQUENCY), 300 (SPAN), 100 (Hz),
10 and
2. Connect the CAL OUTPUT to the INPUT 50R.
3. Disconnect W54 (coax 2) from Connect a test cable from to the input of an
oscilloscope.
4. Set the oscilloscope controls as follows:
Amplitude scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..0 to
Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..dc
Sweep time
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ms/DIV
5. The oscilloscope should display a 4.8 sine wave.
6. Disconnect the cable from the CAL OUTPUT and connect to the INPUT
7. Broadband noise should be displayed on the oscilloscope from approximately to
8. As the
REF
is decreased in 10 steps from -10 to -70 the noise
displayed on the oscilloscope should increase in 100 increments. If this response is not
observed, refer to “Step Gains” and “Video Offset” in this chapter.
8-16 IF Section