Agilent Technologies 8564EC, Service Manual

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Chapter 9 443
IF Section
Log Amplifier (P/O A4 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 85024A, and another spectrum analyzer. If an 1120A active
probe is being used with a spectrum analyzer having dc coupled inputs,
such as the 8566A/B, 8569A/B and the 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
8560 E-Series Spectrum Analyzer
Component Level Informatio
n. 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 8564EC or
8565EC spectrum analyzer.
2. Remove W27 from A4J3 and inject a 10.7 MHz signal of +10 dBm
into A4J3.
3. Set the 8564EC or 8565EC spectrum analyzer to log mode, with
a resolution bandwidth of 300 kHz and single sweep.