LabVIEW

cancel
Showing results for 
Search instead for 
Did you mean: 

how do I graph an FFT output

Hi Lynn,

 

Can you please help clarify the "Output Units for FFT-Based VI's" specifically FFT.  This table is located at http://zone.ni.com/reference/en-XX/help/371361E-01/lvanls/fftbasedviunits/.

 

Because the FFT is a Double Sided spectrum, does the Non-DC Scale Factor of "N/2"  mean I should multiply my non-dc FFT individual output bins by 2 and divide that result by my sample size to achieve the individual power of each bin?

 

I'm including my VI.   Please take a look and let me know if you have any input.

 

thanks

Jeff

0 Kudos
Message 11 of 13
(466 Views)

Jeff,

 

I am not sure how to interpret the units for your signal.  After looking at some things and trying several calculations, I think you are correct about the scaling.

 

The signal appears to be a two-valued signal: +sqrt(2) or -sqrt(2).  In at least the first few hundred data points the samples are either alternating in value repeating only once before switching to the alternate value.  The rms value of this signal is sqrt(2).

 

I am surprised that the FFT shows only one dominant peak.  But there are also no bins which are empty, so perhaps the energy is spread all over the spectrum.

 

If you use the FFT Spectrum (Mag-Phase).vi, it handles the scaling internally so that the output is scaled as the same units as the input.

 

Lynn

0 Kudos
Message 12 of 13
(458 Views)

Lynn,

Thank you for looking at my VI and trying help interpret the results. I'll continue my work on this and ask questions as needed.

thanks again for trying to help out.

Jeff 

0 Kudos
Message 13 of 13
(455 Views)