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Making Measurements Using Spectrum Analyzer Mode
Measuring Amplitude Modulation with the Fast Fourier Transform Function
If the FFT stop frequency is less than the highest harmonic of the AM
modulation, than the FFT results may include aliased signals. That is, it will
include some signals that are being displayed at the wrong frequency. The
sweep time affects the sample rate and must be optimized to avoid aliasing.
The single and continuous FFT functions require a specific spectrum analyzer
setup before they can be activated. First, an AM signal is demodulated in the
time domain. In order to do this, the resolution bandwidth is widened to
include the signal sidebands within the passband of the spectrum analyzer.
Next, zero span is selected so that the spectrum analyzer operates as a
fixed-tuned receiver. Tuning is centered around the AM carrier.
The MARKER-+ AUTO FFT softkey activates the FFT function with very little
preliminary setup required. Two examples of using the FFT function are
included in this section.
l First example: uses the manual FFT functions
l Second example: uses the automatic FFT measurement.
NOTE
After the
FFT
function is used, the markers are still in
FFT
mode for use in evaluating data. Turn off
the
FFT
markers before attempting to use markers in the normal fashion. Press FFT OFF in the
FFT
menu to turn off the markers and exit the
FFT
measurement.
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