Number of Points used in FFT calcuations using Spectral Measurements

[Fat Controller]

Hi folks,

I'm a pretty new user of labview (Ver 7.1) and I am trying to perform spectral analysis of power systems to ensure that they comply with stated standards. I'm using PXI-1002 system with PXI-6025E DAQ cards. I am able to get the analogue data into the program and display the time and frequency domain data to screen however I require a specific resolution to the spectral analysis to comply with the standard. obviously I can set the sampling frequency but I am unable to set the number of points of the actual FFT using either the "spectral measurements" function or other specific FFT VIs.
Can the number of points be set manually or do the functions some how deside the best number of points depending on the amount of data passed to it, I've found that increasing the "Scan to read at a time" value of the AI config VI I'm using seems to increase the resolution but I don't know how the FFT functions deal with "Scan to read at a time" values that are not of 2^n.


Cheers

The Fat Controller

[Cyril]

Hi,

The answer lies in the labVIEW Help file< If you dig deep enough through the hierarchy of the Spectral Measurment Express VI's, you will end up eventually to the Power Spectrum.vi or Real FFT.vi that can be found in the Analyze>>Signal Processing>>Frequency Domain Palette (see screenshot attached)>

The computation details are given in the help for those VIs

I will let you go through those files for details, but basically, when the number of samples in the input signal is a valid power of 2, the VIs compute the fast Fourier transform using the a fast radix-2 FFT algorithm.

When the number of samples in the input sequence is not a valid power of 2 but is factorable as the product of small prime numbers, the VIs compute the discrete Fourier transform using an efficient DFT algorithm according to the type of trnasorm that is executed (i.e Real, Complex)

But the help file explains it better than me 🙂

Hope this helps,

[Fat Controller]

Thanks mate that was exactly what i was after.