Zeropad to a fixed number of samples

 is used to determine the exact size of the Fourier transform. Any number may be typed in the  box. But the numbers on the list gives the best computational speed since they are an order of two as shown below

An error message is given if the selected Fourier transform size is less than the size of the time data window. (The same is true for the reference measurement time data window if division is to be performed.)