HP 8647A Operation And Service Manual page 114

The 250 to 500 MHz signal is derived by dividing the 500 to 1000 MHz
main band signal by two.
Filtering is handled by separate low-pass filters for each frequency
band.
AM is done using two separate modulators, one for the divide band
and one dual-output modulator for the main and heterodyne bands.
The AM signal from the A3 modulation section for both modulators is
summed into the ALC loop integrator.
Amplitude leveling is handled with an ALC loop to lock the level
and an amplitude DAC to set the level. The error signal from the
ALC integrator is sent to either of the two modulators along with the
summed AM signal.
The output assembly contains a calibration ROM that contains
factory-generated calibration data that is specific to the assembly.
A7 Attenuator
The attenuator assembly contains the attenuators, the
reverse-power-protection circuitry and the temperature-sense
circuitry.
The attenuator assembly contains a calibration ROM that contains
factory-generated calibration data that is specific to the assembly.
Theory of Operation 511.5