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Concepts
AM and FM Demodulation Concepts
AM and FM Demodulation Concepts
Demodulating an AM signal using the analyzer as a fixed tuned receiver
(Time-Domain)
The zero span mode can be used to recover amplitude modulation on a carrier signal.
The following functions establish a clear display of the waveform:
• Triggering stabilizes the waveform trace by triggering on the modulation
envelope. If the modulation of the signal is stable, video trigger synchronizes the
sweep with the demodulated waveform.
• Linear display mode should be used in amplitude modulation (AM)
measurements to avoid distortion caused by the logarithmic amplifier when
demodulating signals.
• Sweep time to view the rate of the AM signal.
• RBW and VBW are selected according to the signal bandwidth.
Demodulating an FM signal using the analyzer as a fixed tuned receiver
(Time-Domain)
To recover the frequency modulated signal, a spectrum analyzer can be used as a
manually tuned receiver (zero span). However, in contrast to AM, the signal is not
tuned into the passband center, but to one slope of the filter curve as Figure 15-23
Figure 15-23 Determining FM Parameters using FM to AM Conversion
Here the frequency variations of the FM signal are converted into amplitude
variations (FM to AM conversion) utilizing the slope of the selected bandwidth filter.
The reason we want to measure the AM component is that the envelope detector
responds only to AM variations. There are no changes in amplitude if the frequency
changes of the FM signal are limited to the flat part of the RBW (IF filter). The
resultant AM signal is then detected with the envelope detector and displayed in the
time domain.
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