Positive/Negative Peak Detectors

See function blocks Y and Z of A3 Interface Assembly Schematic Diagram (sheet 5 of 6).

The following information pertains to the positive peak detector and is applicable to troubleshooting the negative peak detector.

The positive peak detector consists of an input amplifier (A3U204 and A3Q210) followed by detector diodes (A3CR203 and A3CR204) and hold capacitor A3C217. Output amplifier A3Q206, Q211, and Q212 buffers the hold capacitor. Both the input and output amplifiers have a gain of one. Each amplifier has local feedback. On the output amplifier the emitter of Q212 connects to Q206’s gate. On the input amplifier the feedback goes through Q209 and Q208 back to the base of U204D. Global feedback occurs from the output amplifier through R223 back to the input amplifier U204D. The peak detector resets through Q207.

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

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

SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..500MHz

RES BW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Auto

VIDEO BW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Auto

LOG dB/DIV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 dB/div

2.If the HP 8560A does not meet the conditions in steps a through e below, the positive and negative peak detectors are probably faulty. Continue with step 3 to check the detectors.

a.The peak-to-peak deviation of the noise in NORMAL detector mode should be approximately two divisions. Note the amplitude levels of the top and bottom of the displayed noise.

b.Select POS PEAK detector mode.

c.Confirm that the noise is about one-third division peak-to-peak. The noise should also be no higher than the top of the noise level in NORMAL detector mode.

d.Select NEG PEAK detector mode. The noise should be about one-third of a division peak-to-peak. The noise should also be no lower than the bottom of the noise in NORMAL mode.

e.Select SAMPLE detector mode. Check that the noise appears between the top and bottom of the noise in NORMAL mode.

3.On the HP 8560A, connect the front-panel CAL OUTPUT to the INPUT 50R and set the controls to the following settings:

C E N T E R F R E Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...300 MHz SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OHz SWEEP TIME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...5 s DETECTORMODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . POS PEAK

4.Monitor A3TP17 and ASTP16 simultaneously with an oscilloscope.

5.Change the reference level from -10 dBm to +30 dBm and verify a voltage change at both A3TP17 and A3TP16 of 1 V to 0.6 V in 100 mV steps.

6.Check the entire range of the detector by substituting a dc source at JlOl and varying its output from 0 V to 1 V.

7.If the peak detector appears latched up, check LPOS-RST for a negative TTL level reset pulses. The reset pulses should occur every 130 ps and should be approximately 250 ns wide.

ADC/lnterface Section 7-17