HP 8901B Operation And Calibration Manual page 208

Operation
Model 8901B
COMMENTS
Acceptable IF Level Display
An IF level display between 99.9% and 100.1% is considered acceptable for all measurements.
Inputloutput Status
When
IF
LEVEL is selected, the output at MODULATION OUTPUT/AUDIO INPUT continues to
output the demodulated signal corresponding to the last modulation measurement selected as indicated
by the MODULATION OUTPUT annunciator. If the
IF
level is not loo%, the calibration of the output
is directly affected for AM and degraded for FM and (PM if the IF level is very low.
Relative Measurements
If IF level is to be displayed relative to a reference level, enter the ratio reference using the RATIO key.
(Refer to Ratio on page 3-215.)
IF Levels Less Than 100%
Error 03 (input circuits underdriven) is generated whenever the IF level is less than 100% unless the
ALC loop is open. Override this Error Message using Special Function 8. (Refer to Error Message
Disable Control on page 3-233.)
AM Depth When IF Level is Insufficient
When operating with the AM ALC loop open or when the IF level is insufficient, the displayed AM is
proportional to the actual AM depth, and the displayed IF level is proportional to the average carrier
level. The actual AM depth can be computed as the ratio of the two readings with the following formula:
x 100%
demodulated ac displayed AM
x
100%
=
AM in %
=
average carrier level displayed I F level
The Ratio feature can be used to simplify this calculation: First, measure the
IF
level, then press the
AM key and enter the
IF
level on the numeric keys. Then press the RATIO key. AM is now displayed
although the
IF
level is not optimum.
Guidelines for Measurements when AM ALC is Off
In most circumstances, when the AM ALC is off (loop open), use IF levels greater than or equal to
100% for FM measurements and less than or equal to 100% for AM measurements.
3-94 IF Level