Agilent Technologies 8510XF Operating And Service Manual page 156

System Maintenance
Theory of Operation
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The detector signal is applied to the ALC circuit, which compares the
detector voltage with a reference voltage, and feeds back a control signal
to the amplifier. The gain of the amplifier is adjusted so that the detector
voltage matches the reference voltage. In this way, the LO input to the
>50 GHz mixers is maintained at a constant level.
5-20 8510XF Network Analyzer Systems