FFT spectrum.vi (with zero averages) versus FFT.vi

[mfralick]

When I use FFT Spectrum.vi and FFT.vi, using the same input data and convert the FFT to single-sided data, the output spectrums do not match.  I have the FFT Spectrum set to the default of "no averaging."  If there is no averaging with the FFT Spectrum.vi, shouldn't the output be the same as FFT.vi (provided I have converted the output from FFT.vi from double-sided to single-sided)?

I could see there being some scale factor difference that I may have missed, but the spectrum shape itself is vastly different.  Is there something I'm missing? 

The reason I ask is that I have a data collector that calculates the FFT in some unknown way, and the result just happens to match the way FFT Spectrum.vi calculates the raw data (with no averaging).  It concerns me that this result does not match the FFT.vi output, or Matlab's FFT.m (which is identical to FFT.vi).

[strong@ni]

Hi mfralick,

The FFT Spectrum.vi has an input 'window' which is default to 'hanning', this means the input data is multiplied by hanning window before the FFT operation, maybe you can change it to 'Rectangle' and see if the results are the same now.

I attach a code snippet which shows the FFT Spectrum.vi and FFT.vi produce the same results.