FFT spectrum (Mag-Phase) VI - averaging: peak hold

[m.wei]

Hi.     I don't think this specific question has been asked before in terms of the actual outputs of the FFT Spectrum (Mag-Phase)VI.

I was using the Sound and Vibration Toolkit version of the FFT Spectrum, but since not everybody has it, I then substituted the regular LabVIEW2014 Developer version of the FFT implementation in the attached VI example.

Basically, it appears that the "Peak hold" input parameter of the VI doesn't actually generate a peak/maximum value for the FFT but is merely the RMS.     I've checked this with the express VI Spectral Measurements VI and when you convert that express VI to a SubVI and look at the block diagram the Peak hold FFT actually multiplies back the RMS (attached "expres VI peak hold.png").    Whereas the express VI for RMS doesn't have the re-multiplication of the 1/RMS factor.

I'm wondering if this is an arbitrary solution for the Peak hold averaging parameter or am I just simply missing something from the start with the FFT Spectrum VIs?

Also, Sections 4-35 and 4-36 of the "LabVIEW analysis concepts" pdf has a good explanation of how this is implemented, but I've not tested it out from a "true step-by-step FFT analysis" VI to see what the actual numbers are, but is this something to be aware of for FFT analysis within LabVIEW in the future?

http://www.ni.com/pdf/manuals/370192c.pdf

[AGJ_CR]

Hi m.wei,

According to the information provided here: https://www.ni.com/docs/en-US/bundle/labview-api-ref/page/vi-lib/measure/maspectr-llb/fft-spectrum-m... the FFT Spectrum VI will always return the magnitude of the averaged FFT spectrum in units of RMS hence it’s not something that we can actually control. The ‘Averaging parameters’ input of this VI is intended to configure the method for the averaging process in order to increase the accuracy. This is why the Express Spectral Measurements VI multiplies back the inverse RMS when you configure it to calculate the peak magnitude of the spectrum and doesn’t modify the signal when is configured to calculate the RMS value.

Hope I’ve been able to understand your question and that this reply can clarify your doubt.

Best Regards.