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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-S
cries
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