Log Amplifier - HP 8560A Service Manual

A4 Assembly's Log Amplifier Circuits
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
doubles 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 analyzer or the probe.

Log Amplifier

Refer to function blocks K, L, and AE of A4 Log Amplifier Schematic Diagram in the
HP 856OA/61B/63A Spectrum Analyzer Component Level Information
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
formed by U501A, B, C, and D. Q 502 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 non-linearities 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. Set the digital multimeter to read dc Volts and connect the negative lead to the chassis of
the spectrum analyzer.
2. Remove W27 from A4J3 and inject a 10.7 MHz signal of +lO dBm into A4J3.
3. Set the analyzer to log mode, with a RBW=300kHz and single sweep.
4. Using the DMM, check the voltage at U503 pin 6.
5. Verify that this level is about -700 mV.
6. Adjust the reference level of the HP 8566A/B to place the signal at the reference level.
7. Reduce the input signal level in 10 dB steps while noting the voltage displayed on the
DMM. The voltage should increase (become less negative) at a rate of 30 mV for each
10 dB decrease in input power. Troubleshoot the A4 assembly if the signal does not
decrease properly.
8. Set the spectrum analyzer resolution bandwidth to 100 kHz to place the wide/narrow
filter in narrow mode.
9. Repeat steps 2 through 7.
8-14 IF Section
binder. The log