Log Offset/Gain Compensation; Limiter; Isolation Amplifier; Detector/Mixer - Agilent Technologies 8564EC Service Manual

IF Section
Log Amplifier (P/O A4 Assembly)

Limiter

Refer to function block G of A4 log amplifier schematic diagram (sheet 2
of 4) in the 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 ≤100 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 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 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 Offset/Gain Compensation

Refer to function blocks L and M of A4 log amplifier schematic diagram
(sheet 3 of 4) in the 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.
Chapter 9 453