Chapter 1 65
Making Basic Measurements
Demodulating FM Signals
Demodulating FM Signals As with amplitude modulation (see page 59) you can utilize zero span to
demodulate an FM signal. However, unlike the AM case, you cannot
simply tune to the carrier frequency and widen the resolution
bandwidth. The reason is that the envelope detector in the analyzer
responds only to amplitude variations, and there is no change in
amplitude if the frequency changes of the FM signal are limited to the
flat part of the resolution bandwidth .
On the other hand, if you tune the analyzer slightly away from the
carrier, you can utilize slope detection to demodulate the signal by
performing the following steps.
1. Determine the correct resolution ban d width.
2. Find the center of the linear portion of the filter skirt (either side).
3. Tune the analyzer to put the center point at mid screen of the
display.
4. Select zero span.
The demodulated signal is now displayed; the frequency changes have
been translated into amplitude changes., see Figure1- 58. To listen to
the signal, turn on AM demodulation and the speaker.
In this example you will demodulate a br oadcast FM signal that has a
specified 75 kHz peak deviation.
Demodulating a FM Signal Example:
Determine the correct resolution bandwidth. With a peak deviation of
75 kHz, your signal has a peak-to-peak excursion of 150kHz. So we
must find a resolution bandw idth filter with a skirt that is rea sonably
linear over that frequency range.
1. Perform a factory preset by pressing Preset, FactoryPrese t (if
present).
2. Turn on the internal 50MHz reference signal of the analyzer as
follows:
•For the E7401A, use the internal 50MHz amplitude reference
signal of the analyzer as the signal being measured. Press
Input/Output, Amptd Ref (On).
•For all other models connect a cable between the front-panel
AMPTD REF OUT to the analyzer INPUT, then press
Input/Output, Amptd Ref Out (On).
3. Set the center frequency to 50MHz by pressing FREQUENCY,
Center Freq, 50, MHz.