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How long is the record length for the FFT (spectral) function on my windows based oscilloscope?

Question :

How long is the record length for the FFT (spectral) function on my windows based oscilloscope?

Answer :

The spectral menu shows record length of the acquired data. In our older TDS500 scopes, that would be the length of the FFT. However, that is not the length of the FFT for the newest scopes.

The time domain gate and the resolution bandwidth control which also adjusts the gate width determine the length of the FFT. You will see a readout that shows the gate width in samples. This is the amount of acquired data that goes into the FFT. However, it is not the FFT length because the spectral analyzer also includes some zero fill at the each end of the gate.

Therefore, the FFT length is greater than the gate width. It depends on what the gate width is and what the next largest available FFT length that allows for at least 20% of zero fill.

When we export data we do not export the entire FFT length. We only export the amount of data specified by the span and center frequency controls. You can think of them as a frequency domain version of the gate because they select the set of frequency points to export.

If you press the FULL button to get full span this will export 1/2 the length of FFT data points that represent zero to 1/2 the sample rate Hz. The other half of FFT points in memory are simply complex conjugate of the first half and we do not export them. The user can recreate them from the data we do export.

If you export the FULL span data then the export menu will tell you a length. Multiply by 2 and you have the FFT length.

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FAQ ID 56631

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