Sum and difference frequencies are produced in the Detector/Mixer. The difference frequency produces video (dc to approximately 3 MHz), since the two signals are at the same frequency. During digital resolution bandwidths the two signals are separated by 4.8 kHz.

Log Offset/Gain Compensation

Refer to function blocks L and M of A4 Log Amplifier Schematic Diagram (sheet 3 of 4) in the HP 856OA/61B/63A Spectrum Analyzer Component Level Information binder.

Log Offset Compensation

The gain of A4U503 is set to unity, with A4R539 and A4R540 combining for a gain of 0.5. So the gain from A4U503 pin 3 to A4U508 pin 3 should be 0.5.

Log Gain Compensation

The gain of A4U508 is nominally 6.8, measuring from pin 3 to pin 8.

To check the log offset/gain compensation circuits inject a i-10 dBm signal into J3 with the analyzer set to log mode. Measure A4U503 pin 3, vin (1) and A4U508 pin 3, vout (1) and record the results. Decrease the input level to -40 dBm and make the same measurements recording vin (2) and vout (2).

The gain is then:

(vout( l)-vout(2))

(vin( 1)-vin(2))

This gives an offset independent gain measurement.

Video MUX

The video mux switches the video output between linear, log and 4.8 kHz IF (for digital RBWs). The demod video is an unused feature. The easiest way to trouble shoot this circuit is to look for blown FETs. Bad FETs are characterized by having significant gate current. Only one of the signal lines LIN-VIDEO, IF-VIDEO or LOG-VIDEO should be high

(t15 V) at any given time. The others should be low (-15 V). Also look for a voltage drop of several volts across the gate resistors R601, R605, R609, or R613 when in either the off or on state. This indicates gate current and thus a bad FET.

IF Section 8-21