Agilent Technologies 856290216 service manual A5 if Assembly, Video MUX

The gain is then:

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 troubleshoot 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.

A5 IF Assembly

The input switch connects the IF to either the cal oscillator on the A4 assembly or the

10.7MHz IF output from the Al5 RF assembly. The automatic IF adjustment uses the cal oscillator on A4 at instrument turn-on and between sweeps to align the IF filters and step-gain amplifiers. During sweeps the input switch selects the 10.7 MHz IF output from A15. The LC filters are variable-bandwidth filters that provide resolution bandwidths from

30 kHz to 2 MHz. The automatic IF adjustment sets the bandwidths and center frequencies of each filter stage.

The crystal filters are variable-bandwidth filters that provide resolution bandwidths from 300 Hz to 10 kHz. The automatic IF adjustment sets the filter bandwidths and symmetry.

The step-gain amplifiers consist of the first step-gain stage, second step-gain stage, and third step-gain stage. These amplifiers provide gain when the HP 85623 spectrum analyzer reference level is changed. The amplifiers also provide gain range to compensate for variations in the IF filter gains, which change with bandwidth and environmental conditions, and band conversion loss in the front end. Fixed-gain amplifiers shift the signal levels to lower the noise of the IF chain.

The assembly has two variable attenuators. The fine attenuator provides the 0.1 dB reference level steps. The reference 15 dB attenuator provides a reference for automatic adjustment of the step-gain amplifiers and the log amplifier. The reference 15 dB attenuator also provides gain for changes in spectrum analyzer reference level.

Various buffer amplifiers provide a high-input impedance to prevent loading of the previous filter pole and a low-output impedance to drive the next filter pole.

Digital control signals from the W2 control cable, the “analog bus,” drive the control circuitry. At the beginning of each sweep, the analog bus sets each control line for instrument operation. At the end of each sweep, the analog bus sets each control line for the next portion of the automatic IF adjustment routine. IF adjustments continuously remove the effects of component drift as the spectrum analyzer temperature changes.

IF Section 9-21