Filter
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amplifier. A4CR302 and A4CR303 are varactor diodes in the limiter filter and are used to
tune the filter.
Limiter
Refer to function block G of A4 Log Amplifier Schematic Diagram (sheet 2 of 4) in the
HP 8560 E-Series Spectrum Analyzer Component Level Information.
The limiter consists of 7 identical 20 dB gain stages. A “log narrow filter” is switched in
for
RBWs
5100
kHz. This filter is switched in using the control lines NARROW between
the 4th and 5th stages. During normal operation, the limiter serves to amplify even the
smallest 10.7 MHz signals up to a level sufficient to drive the LO Amplifier and subsequent
detector/mixer. This signal serves as the LO for the mixer circuitry.
Isolation Amplifier
Refer to function block H of A4 Log Amplifier Schematic Diagram (sheet 3 of 4) in the
HP 8560 E-Series Spectrum Analyzer Component Level Information.
The isolation amplifier prevents LO port to RF port feedthrough in the mixer from feeding
back to the input of the limiter and causing loop oscillations. In addition, the isolation
amplifier matches the phase of the non-limited signal path to the phase of the limited signal
path. The isolation amplifier should have a gain of about 4 dB and also has a “log narrow
filter” that is switched with the control line NARROWB.
Detector/Mixer
Refer to function block J of A4 Log Amplifier Schematic Diagram (sheet 3 of 4) in the
HP 8560 E-Series Spectrum Analyzer Component Level Information.
Sum and difference frequencies are produced in the detector/mixer. The difference frequency
produces video (dc to approximately 3 MHz), since the two signals are at the same frequency.
During digital resolution bandwidths the two signals are separated by about 4.8 kHz.
Log
Off
set/Gain
Compensation
Refer to function blocks L and M of A4 Log Amplifier Schematic Diagram (sheet 3 of 4) in the
HP 8560 E-Series Spectrum Analyzer Component Level Information.
Log Offset Compensation
The gain of A4U503 is set to unity, with A4R539 and A4R540 combining for a gain of 0.5.
Therefore, the gain from A4U503 pin 3 to A4U508 pin 3 should be 0.5.
Log Gain Compensation
The gain of A4U508 is nominally 6.8, measuring from pin 3 to pin 8. To check the log
offset/gain compensation circuits, inject a
+lOdBm
signal into J3 with the HP 85623
spectrum analyzer set to log mode. Measure A4U503 pin 3,
Vi,
(1) and A4U508 pin 3,
Vout
(1) and record the results. Decrease the input level to -40
dBm
and make the same
measurements recording
Vi,
(2) and
Vout
(2).
9-20
IF Section