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Thread Subject:
fft analysis

Subject: fft analysis

From: not

Date: 25 Mar, 2013 12:32:10

Message: 1 of 2

hii
 i have taken voltage signal and have a matrix vector. And i wand to calculate the frequency from the voltage versus time signal. When i did fft transformation by writing y=fft(v); where v is voltage matrix, but i am getting complex value. Can u tell me how i can get frequency from these operation what is physical meaning of it.

Subject: fft analysis

From: Nasser M. Abbasi

Date: 25 Mar, 2013 13:10:59

Message: 2 of 2

On 3/25/2013 7:32 AM, not wrote:
> hii
> i have taken voltage signal and have a matrix vector. And i wand
>to calculate the frequency from the voltage versus time signal. When i did
>fft transformation by writing y=fft(v); where v is voltage matrix, but
>i am getting complex value. Can u tell me how i can get frequency from
>these operation what is physical meaning of it.
>

voltage matrix? Why not do one signal at a time? i.e. take
the fft of each signal. why complicate it and process
many at one time?

any way...

fft just finds the Fourier coefficients (c's) basically.
(the complex form of them). one for each harmonic.

If the data is real, these c's will be complex, but they
come in conjugate pairs. (they have to, because
the signal is real, hence its Fourier approximation
must also be real, so the Fourier coeff. must be real
numbers).

all what fft tells you, is the amplitude and phase of each
harmonic in your real signal (the sampled version of your signal).

For a complex number F, F+F* where F* is it
conjugate, you get real number:

(1+2i) + (1-2i) = 2

So, you get twice the real part of each complex number.

But you just want to know what to do with fft, right?
why not just look up some examples, here is some

http://www.mathworks.com/help/matlab/ref/fft.html
http://www.mathworks.com/help/matlab/math/fast-fourier-transform-fft.html

many examples how to use the result of fft and look at
the frequency content of a signal using fft.

--Nasser

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