HP 8901B Operation And Calibration Manual page 272

Model 8901B Operation
MEASUREMENT TECHNIQUE
The OM on the IF
is first demodulated by a frequency discriminator. The amplitude of the
discriminator's output is proportional
to
the frequency deviation. The demodulated signal is filtered
and passed through an integrator, which extracts the phase information. The integrator's output is
detected and displayed as phase deviation in radians. The integrator's output is made available at the
MODULATION OUTPUT/AUDIO INPUT.
[ - <
[
~ M ~ L ~ T ~ L I T ~
FM
- D E M O D U L A T O R 7
FM
VOLTMETER
AND DISPLAY
AUDIO
DETECT0
RS
INPUT
I
OM Measurement Block Diagram
COMMENTS
Frequency and Distortion Measurements
The demodulated signal's frequency and distortion can also be measured. (Refer to
Audio
Measure-
ments which begins on page 3-197.)
Relative Measurements
If
@M deviation 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.)
Peak Detector
The PEAK+ detector always detects the upward carrier phase excursion while the PEAK- detector
always detects the downward carrier phase excursion. The PEAK*/2 detector sums the upward and
downward phase-excursion 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.)
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 upper limit of the modulation rate is
that of the >20 kHz LP FILTER.
(PM 3-171