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.

•First example: uses the manual FFT functions

•Second example: uses the automatic FFT measurement.

N O T E

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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