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Modulation Analyzer
7-7 7
Measuring FM
Measuring
Pre-Emphasized FM
Inserting Audio Filters
1.
Turn off AM on the signal generator and turn on 25 kHz of FM
2. Press the FM key, the green AUTOMATIC OPERATION key, and the
PEAK+ detector key. The HP 8902A should display a reading of about
25 kHz. Notice that in the upper right comer of the instrument the
yellow FM MODULATION OUTPUT LED is lit. This indicates that the
demodulated FM signal is available at the front-panel MODULATION
OUTPUT/AUDIO INPUT port. (Connecting another audio analyzer
to this port will enable you to make audio measurements on the
demodulated signal while monitoring modulation with the
H I '
8902A.)
deviation at a
1
kHz rate.
To accomodate your pre-emphasized FM signals, the HP 8902A is equipped
with four separate de-emphasis filters. (Each FM DE-EMPHASIS key is
labeled with a time constant corresponding to the 3 dB comer frequency of
that de-emphasis filter.)
3.
Press the 75ps FM DE-EMPHASIS key (if the PRE-DISPLAY key is not lit,
press it too). The display now shows the level of the demodulated signal
after it has been de-emphasized. (The de-emphasized FM signal is now
available at the front-panel MODULATION OUTPUT/AUDIO INPUT port.)
Notice that the display now shows the level of the demodulated
FM signal before it is de-emphasized. The demodulated signal is
now being applied to the de-emphasis filter after it is measured
by the HP 8902A. (The de-emphasized signal is still present at
the MODULATION OUTPUT/AUDIO INPUT port, however.)
function. Repeat step 3 to select any of the four de-emphasis filters.
4.
Press the PRE-DISPLAY key again to turn Pre-Display off.
5. Press the 75ps key again to turn
off the FM De-emphasis
To remove undesired signals such as harmonics, noise, and spurs
from the demodulated signal, insert the selectable high-pass and low-
pass audio filters when making modulation or audio measurements.
To minimize overshoot from square-wave modulation, such as
frequency shift keying
(FSK),
select the >20 kHz Bessel Filter.
bandwidth of 300 Hz to 3 kHz. Diagram 1-2 shows the typical frequency
response of each audio filter.
6.
Press the 300 Hz key and the 3 kHz key to set
a
typical measurement
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