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From: email@example.com (Adam Seychell)
Subject: Re: fft in hc11
Date: 9 Oct 2002 04:07:44 -0700
NNTP-Posting-Date: 9 Oct 2002 11:07:44 GMT
firstname.lastname@example.org (Albert Leng) wrote in message news:...
> I'm new to hc11 and have to do a project in hc11 performing fft to
> calculate the total harmonic distortion present in the signal.
> I have looked through the operation of FFT which consists of
> 1) Bit Reversal Data Sorting
> 2) Decompose an N point time domain signal into N signals each containing
> a single point
> 3) Find the spectrum of each of the N point signals
> 4) Synthesize the N frequency spectra into a single frequency spectrum
> After studying through the code "fft routine" from motorola, i really
> have no idea what the code is about. Can you please give me some ideas on
> how the code relates to the above 4 steps and how i can set the A/D for
> the inputs? Also, how can i calculate the average of the each frequency
> amplitude presented?
I'm not sure if FFT is the best way to calculate distortion. Normally
distortion is measured by first passing the signal through a notch
filter so the fundimental is removed. Distortion is simply the ratio
of the RMS level of the filter output divided by the RMS level of the
unfiltered signal. Simple 2nd order IIR notch filter is described in
many DSP books and is easy to impliment. To get RMS levels requires
squaring the signal followed by low pass filtering and finally square
root the value.
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