FFT - Output to RMS Equivalents

John Costello — Fri, 02 May 2008 13:22:02 -0400

I need some help converting the Matlab FFT output into
equivalent RMS levels for signal analysis.

I have numerical data from an oscilloscope that has three
primary frequencies: these are a DC component and two
separate frequencies. I can calculate the RMS of the
composite signal with a bit of integration and the use of
the RMS defintion.

The FFT routine identifies the equivalent power at the
discrete frequencies with the Matlab FFT algorithm. I am
plotting the frequency vs. the FFT * the conjugate of the
FFT to obtain the power signal. For the n point data
acquisition, I am doing the following to obtain the power:

Y = fft(v,n);

Pyy = Y.*conj(Y)/n ;

Does someone out there know how to extract the equivalent
RMS components of the other frequency content?

best regards

J. Costello

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FFT - Output to RMS Equivalents