HP 8901B Operation And Calibration Manual page 427

Adjustments
Model 8901B
8. Set the audio oscillator level
to
100 mVrms.
9. On the Modulation Analyzer, press
S
(Shift) AUDIO DISTN. Adjust A52R44 (NOTCH FLTR
GAIN) for a display on the Modulation Analyzer between 99.9 and 100.1%.
10. Set the audio oscillator frequency to 800 Hz.
11. On the Modulation Analyzer, press the blue key then press 400 Hz DISTN (the PEAK- key).
The Modulation Analyzer's display should read greater than 98%.
5-36 Adjustment 21