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 Q220 and Q219 should be off at all times.

1. Press (PRESET) and set the HP 8560A controls as follows.

C E N T E R F R E Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...300 MHz SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OHz

2.Press [SWEEP) and SINGLE.

3.Check for a TTL high on A3U104 pin 2 and a TTL low on U104 pins 7, 10, and 15. Set the analyzer to 2 dB/div and check for a TTL high on A3U104 pin 10 and a TTL low on A3U104 pins 2, 7, and 15.

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 pulse is present when switching between 10 dB/div and 2 dB/div.

b.Check the inputs to A3U104 (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 A3U109A/C/D for proper outputs. The outputs should be high when the noninverting input is greater than the threshold voltage of +2.4 Vdc.

6.If A3U104 and A3U109 are working properly, set the REF LEVEL to 0 dBm.

7.Monitor the voltage at A3TP14 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 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 JlOl. To confirm this failure, lift diode A3CR109’s 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 [PRESET) 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 (SWEEP) and SINGLE.

3.Step the Video BW from 3 MHz to 10 kHz. At each step, the peak-to-peak deviation of the noise should decrease.

ADC/lnterface Section 7-15