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 |
| active probe and the spectrum analyzer input. Failure to do this can result in |
| damage to the analyzer or the probe. |
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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 binder. 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 opamp 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
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
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.