HP 8901B Operation And Calibration Manual page 249

Operation Model 8901B
Next, one modulator is left on and the other is toggled on and off at a 10 kHz rate. Since the
RF
signal is switched between full on and half on, the resultant carrier modulation is very near 33.33%.
(The exact depth is derived from the measurements that characterize the individual modulators.) This
modulation is then measured by the AM demodulator. (Both peak detectors are used, and their average
is computed.)
The Modulation Analyzer compares the modulation depth (static measurements) with the demodulated
AM (toggled measurements) and computes its accuracy error as follows:
Demodulated A M - Demodulated Residual A M
Computed A M
A M Calibration Factor (%) =
x
100
All segments of the calibration are carefully monitored. If any segment is out of limits, the calibration
is aborted and Error 08 is displayed.
MODULATOR A BUFFER A
Simplified Block Diagram
of
the AM Calibrator
COMMENTS
Default Calibration Factor
If the instrument's non-volatile memory is erased, 100.00% is used as the default calibration factor.
Updating the AM Calibration Factor
Whenever AM calibration is performed, the instrument updates the stored AM calibration factor.
3-144 AM Calibration