HP 8560A, Service Manual

Page: 268 / 450

Video MUX
See function block U of A3 Interface Assembly Schematic Diagram (sheet 5 of 6).
The AUX VIDEO port and the 0 SPAN CAL function are not used. Both and
should be off at all times.
1. Press and set the HP 8560A controls as follows.
C E N T E R F R E Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...300 MHz
SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Press and SINGLE.
3. Check for a TTL high on pin 2 and a TTL low on pins 7, 10, and 15. Set
the analyzer to 2 dB/div and check for a TTL high on pin 10 and a TTL low on
pins 2, 7, and 15.
4. If the logic levels on are incorrect, check the LLOG_STB signal as follows:
a. Monitor pin 9 with an oscilloscope or logic probe. Check that a pulse is present
when switching between 10 dB/div and 2
b. Check the inputs to (pins 4 and 12) 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.”
5. Check comparators for proper outputs. The outputs should be high when
the noninverting input is greater than the threshold voltage of Vdc.
6. If and are working properly, set the REF LEVEL to 0
7. Monitor the voltage at while switching the analyzer between 10 dB/div and
2 dB/div. The voltage should switch between 0.9 and 0.5 Vdc.
8. If the voltage at is incorrect, suspect either or
9. The Video MUX will appear faulty if is shorted or leaky. Diode
clamps the voltage at to -0.4 V when in log expand with less than 0.8 V at
To confirm this failure, lift diode cathode and perform steps 1 through 7 again.
Video Filter
See function block V of A3 Interface Assembly Schematic Diagram (sheet 5 of 6).
The HP 8560A 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.
1. Press and set the HP 8560A controls to the following settings:
C E N T E R F R E Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...225 MHz
SPAN
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...550 MHz
SWEEP TIME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Uncoupled (MAN)
2. Press and
3. Step the Video BW from 3 MHz to 10 At each step, the peak-to-peak deviation of the
noise should decrease.
Section 7-15