Output Units of the Power Spectrum VI

Updated Jan 17, 2019

Reported In

Software

  • LabVIEW

Issue Details

What are the output units of the Power Spectrum VI, and how are they calculated?

Solution

The output units of the Power Spectrum VI are in Volts RMS squared (Vrms^2), but this power is halved between the positive and negative frequency components. If the input signal unit is not in Volts, the output spectrum unit will be RMS-squared of the input signal unit.

So, if the peak amplitude (Vpk) of the input sinusoidal signal is 1 Vpk, its RMS value is:

Vrms = 1/sqrt(2) = 0.7071067, 

so Vrms^2 = (0.7071067)^2 = 0.5 

But because the Power Spectrum computes the "double-sided" FFT power spectrum, the resulting expected value is half of the Vrms^2 value. For the example above, we would expect a power spectrum value of 0.5/2 = 0.25 Vrms^2 at the array index corresponding to the frequency of the sinusoidal signal.
 

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