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Re: fourier transform

  • To: mathgroup at
  • Subject: [mg61237] Re: [mg61203] fourier transform
  • From: Pratik Desai <pdesai1 at>
  • Date: Thu, 13 Oct 2005 01:39:45 -0400 (EDT)
  • References: <>
  • Sender: owner-wri-mathgroup at

Masrur Hossain wrote:

>Hi folks
>           I have a list of pairs of numbers like
>  {{1.2, 3.4}, {2.2, 3.6}............}
>The first part is time and second part is amplitude.
>when I do the fourier transform by using
>it gives me list of complex numbers but I expect real numbers
>(i.e . amplitude versus frequency)
It should give you complex numbers, that is the whole idea of Fourier 
Transform (  I think  :-\ )
You can find the amplitude(frequency domain Frequency Response Function) 
simply by taking the absolute value and plotting it with respect to the 
number of points, and obviously the  phase is given  by taking Arg. I 
may be wrong about this but I don't think the way you have constructed 
your input list will work, the best way is to do is to construct your 
list with only amplitudes with a suitable time step. To gurantee good 
results the number of  points  should be something like  2^n ;-)       
.....256, 512, 1024 etc.

>Any help appreciated.
Hope this helps

Pratik  .

Pratik Desai
Graduate Student
Department of Mechanical Engineering
Phone: 410 455 8134

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