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Keysight InfiniiVision 3000T X-Series Oscilloscopes
User's Guide
4 FFT Spectral Analysis
FFT is used to compute the fast Fourier transform using analog input channels or a
lower math function. FFT takes the digitized time record of the specified source
and transforms it to the frequency domain. When the FFT function is selected, the
FFT spectrum is plotted on the oscilloscope display as magnitude in dBV versus
frequency. The readout for the horizontal axis changes from time to frequency
(Hertz) and the vertical readout changes from volts to dB.
Use the FFT function to find crosstalk problems, to find distortion problems in
analog waveforms caused by amplifier non-linearity, or for adjusting analog filters.
To display a FFT waveform:
1 Press the [FFT] key.
• Source 1 — selects the source for the FFT.
• Display — selects one of these FFT waveform display modes:
• Normal — this is the FFT waveform without any averaging or hold functions
applied. This is how FFT math function waveforms are displayed.
• Averaged Value — the FFT waveform is averaged the selected number of
times. Averages are calculated using a "decaying average" approximation,
where:
next_average = current_average + (new_data - current_average)/N
Where N starts at 1 for the first acquisition and increments for each
following acquisition until it reaches the selected number of averages,
where it holds.
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