HP 8901B Operation And Calibration Manual page 257

Operation Model 8901B
Peak&/P Detector
The PEAK+ detector always detects the upward carrier frequency excursion while the PEAK- detector
always detects the downward carrier frequency excursion. The PEAKf/2 detector
sums
the upward
and downward frequency-excursion readings and divides the total by two to provide an average of the
two peak values. (Do not confuse this "average" value with the detected average value.)
Inverted Output Signals
When operating with carrier frequencies below 2.5 MHz, the output signal at MODULATION
OUTPUT/AUDIO INPUT is inverted unless the 455 kHz
IF
is selected. When operating with carrier
frequencies above 2.5 MHz or when using the
455
kHz
IF,
the signal at FM OUTPUT is inverted.
Automatic Range Selection
The routine which automatically selects the modulation range contains regions of overlap between the
following displayed peak deviations:
0.35 and 0 . 4 kHz (750
ps
de-emphasis, pre-display only), 3.5 and 4 kHz, and 35 and 40 kHz.
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 FM key a second time. To set the instrument to a
selected modulation range, refer
to
Audio Range.
Default Condition
When the Modulation Analyzer is first powered up, demodulated FM, with a sensitivity of 0.01
Vpk/MHz (400 kHz peak deviation range), is available at MODULATION OUTPUT/AUDIO INPUT.
The instrument does not autorange to more sensitive ranges because the Frequency Measurement mode
has been automatically selected, which automatically disables Error
04
(audio circuits overdriven).
When Error 04 is disabled, the instrument only autoranges to less sensitive audio ranges.
Increased Accuracy
Accuracy of FM readings may be improved by using the FM Calibrator (refer to FM Calibration on
page 3-155) and by taking into account the effects of residual noise (refer to Residual Noise Effects
on page 3-191).
Automatic Squelching
If the RF input signal drops too low for accurate FM demodulation, the FM measurement is
automatically squelched (muted, turned off).
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, @M, or input frequency, or they could cause Error 05 (FM
squelched) to be displayed when these measurements are selected.
Considerations When Using the RMS Detector
During FM measurements if the RMS detector is selected (Blue Key, AVG), FM de-emphasis pre-
display is automatically selected. Pre-display must turn on at this time due to circuit configuration of
the Modulation Analyzer. When pre-display is sel ted this way, it will not turn off automatically when
DISPLAY key (HP-IB code: PO). Measurements could be affected by this pre-display selection.
another detector is selected. Turn off pre-display
(
B
er selecting another detector) by pressing the PRE
3-154
FM