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 bandwidth.

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 Figure 1-58. To listen to the signal, turn on AM demodulation and the speaker.

In this example you will demodulate a broadcast 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

75kHz, your signal has a peak-to-peak excursion of 150 kHz. So we must find a resolution bandwidth filter with a skirt that is reasonably linear over that frequency range.

1.Perform a factory preset by pressing Preset, Factory Preset (if present).

2.Turn on the internal 50 MHz reference signal of the analyzer as follows:

•For the E7401A, use the internal 50 MHz 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 50 MHz by pressing FREQUENCY,

Center Freq, 50, MHz.

Chapter 1

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Agilent Technologies E7405A, E7402A, E7404A, E7403A, E7401A manual Demodulating FM Signals, Demodulating a FM Signal Example
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E7402A, E7405A, E7404A, E7401A, E7403A specifications

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