Unit of Power Spectrum y-axis

[wisjaf12]

Hello

I am acquiring acceleration from an IEPE accelerometer. Units of the y-axis of the time domain signal are in (g). The time domain signal is then fed into the Spectral Measurements Express VI to get the power spectrum of the signal. In the Spectral Measurements Express VI, I set the results to Linear and the window to (Hanning). My very basic question is: what will be the Units of the y-axis of the power spectrum with linear option and how can convert them back to the unit (g). If you provide an example calculation that would be very appreicated. A shot of my simple code is attached.

Regards

[Door-C]

Hi wisjaf12,

Here is a link to a page that describes what the power spectrum of a signal is: http://digital.ni.com/public.nsf/allkb/696CB6C5C815B64B862574E20058ACE3?OpenDocument

According to the article the power spectrum is just the average of the FFT which in the frequency domain means the square of the FFT's magnitude. The units of the magnitude of an FFT correspond to the units of the time domain signal. If the signal that is passed into your Spectral Measurements Express VI is in the units of g-force then the power spectrum would have the units of g-force^2.

Regards,

[wisjaf12]

Door-C

I would like to thank you for your interest in my question. However, when I take the square root of the power value it does not give me the exact value of the amplitude of the time domain. Based on your post, the unit of the power is g^2. So when I take the square root of that value it should give me back the amplitude value of the time domain (g). Do you think this discrepancy is caused by windowing?

[dsb@NI]

I suspect you had rms instead of peak units, and that is why you didn't see the amplitude (implies peak units) you expected in the spectrum.

[dsb@NI · Accepted solution]

Spectrum Unit Conversion VI can be found in the LabVIEW palettes: Functions>>Signal Processing>>Spectral Analysis

[wisjaf12]

Doug

I really really apprecuate your effort which was very helpful. Can you please attach this VI for me because I just need to make sure of something.

Best regards

[dsb@NI]

The block diagram screenshot above is a VI snippet. Save the *.png to your computer and then drag the file onto a block diagram in LV 2012.

[wisjaf12]

Doug

I am using LV 2011. If you could attach the VI in LV 2011 file, I would appreciate it. Many thank for your help.

Best Regards

[dsb@NI]

Saved for LV 2011; sorry for delay.

[wisjaf12]

Thank you So much Doug. I appreciate your help.