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Re: FFT help [message #41617 is a reply to message #41616]

Fri, 05 November 2004 07:31

Timm Weitkamp
Messages: 66
Registered: August 2002

Member

Today at 08:43 -0600, Kenneth Bowman wrote:

> [...]
>
> This function
>
> COS(!PI*x/6.0)
>
> is only a small part of a complete cosine wave and has a jump
> discontinuity at the ends. (Try plotting it.)

Mhm, but the discontinuity can be avoided if you use a grid that takes an
integer number of periods of your function, for example:

IDL> x = findgen(120)
IDL> y = 8 * cos(2*!pi*x/6)
IDL> ft = fft(y)
IDL> plot, abs(ft)

The result is pretty much what the original poster expected to get: two
peaks with values of exactly 4, at positions of 20 (=120/6) and 100 (the
mirrored peak -- see the online help for FFT for how the frequencies are
ordered in the output of FFT) on the frequency axis.

Good luck
Timm

--
Timm Weitkamp <http://people.web.psi.ch/weitkamp>

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[Message index]

		Re: FFT help By: R.G.Stockwell on Fri, 05 November 2004 10:47
		Re: FFT help By: Timm Weitkamp on Fri, 05 November 2004 07:31
		Re: FFT help By: K. Bowman on Fri, 05 November 2004 06:43
		Re: FFT help By: Norbert Hahn on Fri, 05 November 2004 06:23
		Re: FFT help By: Randall Skelton on Fri, 05 November 2004 06:21

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