Advertisement
Opera tion
Model 8902A
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 slims
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 o r 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
p u s
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 Measuring Receiver 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 t o FA4 Calihration on
page 3-181) and by taking into account the effects of residual noise (refer to Residirul Noisc
h'~jl,cl.s
on page 3-2 19).
Automatic Squelching.
If the RF input signal drops too low for accurate FM demodulation, the FM measurement is
automatically squelched (muted, turned
off).
In the 8902A Measuring Receiver, RF gain can be
inserted into the RF signal path
or IF
gain can be inserted into the IF path to raise the signal to the
level required for measurement.
Potential Inaccuracies.
AM conditions that cause the carrier signal to disappear (such as 100°/o AM
o r pulse modulation)
will cause inaccuracies in measurement of FM, QM, o r input frequency, o r 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 t o circuit configuration of
the Measuring Receiver. When pre-display is selected this way, it will not turn o f f automatically when
another detector is selected. Turn
o f f
pre-display (after selecting another detector) by pressing the
PRE DISPLAY key (HP-IB code: PO). Measurements could be affected by this pre-display selection.
3-
180
Advertisement
Chapters
