FFT math question

[needtolearnfast]

When I run the Labview Power Spectrum Measurement.vi, I know on the X axis the peak will appear at the frequency of the signal, but what are the units they are using on the Y azis to determine how high the peak is going? Thanks.

[LV_Pro]

I'm not certain, from my fairly short examination of the vi you are using, but in the signal analysis palette the power spectrum vi output is described as " If X represents actual measurements in volts, the VI expresses the normalized units of the output sequence Power Spectrum in watts on a 1- basis" so my guess is that the one you are using is probably the same.


If I get a few more minutes later I'll try and look deeper.



Putnam Monroe

[needtolearnfast]

I'm still not sure I get what it's doing. If I pass in an Amp of 10 V at 200 htz I get a peak at 200 htz which goes up to 50 on the Y scale. I dont really understand how this Y value is computed, anyone know? Thanks.

[LV_Pro]

From the Auto Power Spectrum help "Power Spectrum is the single-sided power spectrum in volts rms squared if the input signal is in volts".
If you are putting in a sine wave of 10V the rms value will be approximately 7.07V. This value squared gives you 50.


Putnam Monroe

[needtolearnfast]

Thanks, I was doing some incorrect math on my RMS values.