HP 8901B Operation And Calibration Manual page 244

Model 8901B
Operation
COMMENTS
lnput/Output Status
The demodulated AM is present at MODULATION OUTPUT/AUDIO INPUT (unless AUDIO
INPUT has been selected). The demodulated AM is usually present at the rear-panel AM OUTPUT
connector. (Refer
to
AM Output on page 3-147.)
Peak&/P Detector
The PEAK+ detector always detects the peak of the carrier envelope while the PEAK- detector always
detects the trough. The PEAKf/2 detector sums the peak and the trough readings and divides the
t o t a l
by
two to provide an average of the two peak values. (Do not confuse this "average" value with
the detected average value.)
Automatic Range Selection
The routine which automatically selects the modulation range contains a region of overlap between 35
and 40% AM (peak detected). When using the average or RMS detectors, ranging occurs at readings
lower than these. In general, if the modulation is reduced from above 40% into the overlap region,
only 0.1% resolution is ordinarily displayed although 0.01% resolution is available. To display the
increased resolution, press the AM key a second time. Audio Range
has
more detailed information on
the modulation ranges that can be set.
Automatic Filter Selection
When operating above 2 . 5 MHz, while using the 455 kHz IF, the upper limit of the demodulated signals
modulation rate is that of the ~ 2 0 kHz LP Filter. The 15 kHz LP Filter is automatically selected
when operating below 10 MHz or whenever the 455 kHz IF
is
selected. However, this filter may be
overridden by selecting another LP Filter. The lower limit of the modulation rate
is
determined by the
ALC response time selected. (Refer to AM ALC Response Time on page 3-139.)
Inverted Output Signal
The signal at the rear-panel AM Output is inverted relative to the Modulation Output signal.
Potential Inaccuracies
AM conditions
that
cause the carrier signal to disappear (such as 100% AM or pulse modulation) will
cause inaccuracies in measurement of FM, cPM, or input frequency, or they could cause Error 05 (FM
squelched) to be displayed when these measurements are selected.
Increased Accuracy
Accuracy of AM readings may be improved by calibrating the AM measurement (refer to AM
Calibration on page 3-141) and by taking into account the effects of residual noise (refer to Residual
Noise Effects on page 3-191).
Relative AM Depth Measurements
If AM depth is to be displayed relative to a reference, enter the value as a ratio reference using the
RATIO key. (Refer to Ratio on page 3-215.)
AM
3-137