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From: "John Costello" <JohnCostello@ChampAero.com>
Newsgroups: comp.soft-sys.matlab
Subject: FFT - Output to RMS Equivalents
Date: Fri, 2 May 2008 13:23:03 +0000 (UTC)
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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