HP 8901B Operation And Calibration Manual page 273

Operation
Model 8901B
Automatic Range Selection
The routine which automatically selects the modulation range contains regions of overlap between the
following displayed peak deviations:
0.35 and
.4
radians, 3 . 5 and 4 radians, and 35 and 40 radians.
When using the average or RMS detector, ranging
will
occur at readings lower than these. In general,
if the modulation is reduced from a higher range into an overlap region, the range does not change. To
range the display for increased resolution, press the QM key a second time. To set the instrument to a
selected modulation range, refer to Audio Range on page 3-183.
Potential Inaccuracies
AM depths approaching 100% cause the carrier to nearly disappear and may cause inaccurate
measurement of QM deviation or Error 05 (FM squelched).
Input/Output Status
When QM is selected, the signal at the rear-panel FM OUTPUT still represents demodulated FM,
not QM.
Increased Accuracy
Accuracy of
QM
readings may be improved by taking into account the effects of residual noise. (Refer
to Residual Noise Effects on page 3-191.)
Phase Deviation in Degrees
To display phase deviation in degrees instead of radians, enter 1.745 as a ratio reference and select
RATIO. (Refer to Ratio on page 3-215.)
Pulsed Phase Modulation
Pulsed phase modulation, such as phase shift keying, cannot be accurately demodulated or measured
by the Modulation Analyzer.
Recommended Filter Selection
Because of the high sensitivity of the
QM
measurement
to
low-frequency audio (microphonics, hum,
etc.), use of the 50
H z
or 300
H z
low-pass filter is recommended. (Neither is switched in automatically.)
3-172
+M