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IF Section
Log Amplifier Assembly (P/O A4)
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
n.
Informatio
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
n.
Informatio
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 Offset/Gain Compensation
Refer to function blocks L and M of A4 Log Amplifier Schematic
Diagram (sheet 3 of 4) in the
Component Level Informatio
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 +10 dBm signal
into J3 with the spectrum analyzer set to log mode. Measure A4U503
pin 3, V
(1) and A4U508 pin 3, V
in
Decrease the input level to −40 dBm and make the same measurements
recording V
(2) and V
in
The gain is then:
(
v
OUT
--------------------------------------------------- -
(
v
This gives an offset-independent gain measurement.
430
HP 8560 E-Series Spectrum Analyzer
n.
(1) and record the results.
out
(2).
out
1 ( ) v
2 ( )
)
–
OUT
1 ( ) v
2 ( )
)
–
IN
IN
Chapter 8
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