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Thread Subject:
how to use the output of fft to reconstruct part of the signal?

Subject: how to use the output of fft to reconstruct part of the signal?

From: Kit Mai

Date: 29 Feb, 2012 08:08:15

Message: 1 of 1

Hello everyone,

I am analysizing this signal: S = sin(2*pi*fd*t+23*pi/180) + 0.25*sin(2*pi*3*fd*t+68*pi/180) + 0.3*sin(2*pi*5*fd*t+16*pi/180) + 0.1*sin(2*pi*128*t+78*pi/180) + 0.15*sin(2*pi*243.2*t+94*pi/180) + 0.07*sin(2*pi*376*t);

fd=60.32Hz;
fs=5000Hz;
length of S is 1000

As you can see, in this distorted signal, there are fundamental frequency fd, 3rd, 5th harmonics frequency components, and a few interharmonic components (128, 243.2, 376Hz)

I have successfully used an algorithm to find the best length N of the sampled window for the DFT analysis, which is N=995. This N can minimize the spectrum leakage near the fundamental and harmonics frequency components, as you can see in my plot here:http://p13.freep.cn/p.aspx?u=v20_p13_photo_1202241632553300_0.png

If the original signal S can be decomposed into S=Sd+Sh+Si
where Sd is fundamental frequency signal, Sh is harmonic signals(only 3rd,5th harmonic in this case) and Si is the interharmonic components signals (128, 243.2, 376Hz components)

Now, my task is to reconstruct the Sd + Sh signal only.

When I do the FFT for the original signal S, I will get a lot of complex coefficients.

Can anyone tell me in details how to use these coefficients to reconstruct Sd+Sh only? Without the interharmonic components signals (128, 243.2, 376Hz components).

Thank you so much.
Kit

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