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Overlap % Value - Explanation and Uses



What is the purpose of the Overlap % value in the vb parameter settings? What value should I use and what difference does it make?


The short answer is: use the default value of 50%.

When the vb instrument is taking several averages of a spectrum it calculates an FFT for each average. After each one it continues collecting more time waveform data to be used in the next FFT. So you would expect it to take 4 times as long to record 4 averages as to record one average.

But no, we can do better than that! As each section of data is fed into the FFT process it is 'windowed'. Usually this means that it is multiplied by a bell shaped Hanning window, so the signal does not have an abrupt beginning and end. However this means that the first and last sections of the data have very little effect on the FFT result - they've hardly been used. So we 'overlap' the data by re-using the end of one time waveform section as the beginning of the next. This means that it only takes 2 1/2 times longer to take 4 averages than one (assuming 50% overlap).

Note: This is a standard, internationally accepted signal processing technique.

Why Would you ever want to use other Values for the Overlap %?

If a Rectangular window is used instead of a Hanning window (e.g. when measuring transient events) then it is appropriate to use no overlap (i.e. Overlap = 0%). The other values are provided to satisfy personal preference.