Checking for interfering signals
Signals slightly outside the displayed span may interfere with demodulation. Due
to filter roll-off, the selected span may be influenced, during demodulation, by
signals within the guardband. You should check for such signals with a wider span
(1.28 times the desired span) prior to selecting your measurement span for a
demodulation measurement. The example below illustrates the results of
demodulating a signal with an interfering signal nearby.
The span shown to the left appears ideal, but the span to the right shows that
another signal (this one FM) appears in the guard band outside the desired span
The display to the left shows interference caused by demodulating 40 kHz span above,
which is affected by the signal present in the guard band. The demodulated signal to
the right shows the result if the interfering signal did not exist or existed outside the
guard band.
Analog Demodulation Concepts
20 - 7
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