Technical data

Chapter 8 133
Concepts
AM and FM Demodulation Concepts
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 is 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 8-2.
Figure 8-2 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). 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.