Expected Output of the FFT Power Spectrum and PSD VI in LabVIEW ?

Updated Aug 8, 2018

Reported In

Software

  • LabVIEW 2017 Full
  • LabVIEW Sound and Vibration Toolkit

Issue Details

I have recorded time domain data from my instrument and I would like to find out its frequency spectrum. I have used the FFT Power Spectrum and PSD VI for doing the same but the results are not as I expected. What is the expected output of the FFT Power spectrum and PSD VI ?

Solution

The Power Spectrum / PSD VI returns the averaged power spectrum or power spectral density and the frequency scale, according to export mode. Following are some more details :
  • The magnitude output gives the magnitude of the averaged power spectrum or power spectral density.
  • If the input signal is in volts (V), magnitude has units of volts-rms squared (Vrms^2) for power spectrum and volts-rms squared per hertz (Vrms^2/Hz) for power spectral density.
  • If the input signal is not in volts, magnitude has units of the input signal unit-rms squared for power spectrum, and input signal unit-rms squared per hertz for power spectral density.
  • If dB On is TRUE and the input signal is in volts, magnitude has units of dBV for power spectrum and dBV/Hz for power spectral density.
More details about the same can be found on its help page.

Your requirements may include manipulating FFT and power spectrum results to extract useful frequency information. This customization is better achieved by writing some additional code in conjunction with the FFT VI which is a part of the analysis VIs on the signal processing palette. More information on FFT properties, how to interpret and display FFT results, and how to manipulate them can be found here.

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