Chapter 8 421
IF Section
Log Amplifier Assembly (P/O A4)
Log Amplifier Assembly (P/O A4)
The log amplifier assembly on A4 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 in the
HP 8560 E-Series 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
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. Press
CAL and IF ADJ OFF. 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 +10 dBm
into A4J3.
3. Set the spectrum analyzer to log mode, with a resolution bandwidth
of 300 kHz and single sweep.
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