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

[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

[NInjaneer_0]

Hello m.wei, 

I'm not entirely sure if I understand your question. I was unable to find the RMS - DDT or Peak - DDT portions of code from your pictures in the example vi attached nor when I converted the Spectral Measurements Express VI to a SubVI. 

Which version of LabVIEW are you using?