Measuring Amplitude Modulation with the Fast Fourier Transform Function

A Fourier transform, transforms time domain data (zero span) into the frequency domain. The fast Fourier transform (FFT) function of the spectrum analyzer allows measurements of amplitude modulation (AM). It is commonly used to measure AM at rates that cannot be measured in the normal frequency domain due to spectrum analyzer limitations on narrow resolution bandwidths. For a given AM rate, the FFT function can generate a trace faster than using the frequency domain for the equivalent spectrum analyzer measurement.

The FFT function calculates the magnitude of each frequency component from a block of time-domain samples of the input signal. It uses a flat top filter response. This implementation is a post-detection Fourier transform and it cannot be used to resolve continuous wave or carrier signals.

When (MEAS~USER), FFT Menu, and SINGLE FFT are pressed, sample-detection mode is selected and a sweep is taken to obtain a sample of the input signal. Then the spectrum analyzer executes a series of computations on the time data to produce the frequency-domain results. CDNTINUS FFT can be used instead of SINGLE FFT and the spectrum analyzer will be put in continuous sweep mode with an FFT being performed at the end of each sweep.

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