Agilent Technologies 856290216 service manual Log Amplifier P/O A4 Assembly

Resolution Bandwidths Performance Tests

Most resolution bandwidth problems are a result of A5 IF assembly failures. The resolution bandwidths are adjusted in the following sequence using 300 kHz as the reference: 1 MHz, 2 MHz, 100 kHz, 30 kHz, 10 kHz, 3 kHz, 1 kHz, 300 Hz, 100 Hz, 30 Hz, 10 Hz, 3 Hz, and

1 Hz. The 3 Hz and 1 Hz bandwidths are not available with Option 103.

If the IF adjustment routine encountered an error, the previously adjusted resolution bandwidths should be working properly and default DAC values are used for the remaining resolution bandwidth settings.

If the IF bandpass adjustments and the automatic IF adjustments fail to bring the resolution bandwidths within specification, troubleshoot the A5 IF assembly.

Log Amplifier (P/O A4 Assembly)

The log amplifier on the A4 assembly performs several functions. It provides log and linear paths converting the 10.7 MHz IF signal to video. In addition it also provides offset circuitry, AM/FM demodulator circuitry, a frequency counter output, and down conversion of the 10.7 MHz IF to 4.8 kHz for use by the digital IF.

The log amp results are realized by using a wide dynamic range linear detector followed by a video log amp. The detector is used for both linear and log paths and contains a mixer that acts as the down converter mixer for the digital IF.

Caution	For troubleshooting, it is recommended that you use an active probe, such
	as an HP 85024A, and another spectrum analyzer. If an HP 1120A active
	probe is being used with a spectrum analyzer having dc coupled inputs, such
	as the HP 8566A/B, HP 8569A/B and the HP 8562A/B, either set the active
	probe for an ac-coupled output or use a dc-blocking capacitor between the
	active probe and the spectrum analyzer input. Failure to do this can result in
	damage to the spectrum analyzer or the probe.

Log Amplifier

Refer to function blocks K, L, and AE of A4 Log Amplifier Schematic Diagram (sheets 3 of 4 and 4 of 4) in the HP 8560 E-Scries Spectrum Analyzer Component Level Information. The log amplifier receives the detected video signal from the detector/mixer and outputs a voltage proportional to the log of the input voltage. The linear output is tapped off at the emitter of U501D. U507 provides input offset adjustment capability and adjusts the offset of the op amp formed by U501A, B, C, and D. Q502 is a buffer. Q501 switches in additional offset for digital RBWs. The logarithmic characteristic of the base-emitter junction of U502B is used in the feedback path to produce the logging affect. U502D is used to adjust for nonlinearities in the linear mode. R531 is used to adjust log fidelity at the top of the screen.

Use the following steps to verify proper operation of the log amplifier chain:

1.Press ICAL) and IF m3 OFF. Set the digital multimeter to read dc volts and connect the negative lead to the chassis of the HP 85623 spectrum analyzer.

2.Remove W27 from A4J3 and inject a 10.7 MHz signal of +lO dBm into A4J3.

IF Section 9-13