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Re: FFT help [message #41620 is a reply to message #41618]

Fri, 05 November 2004 06:23

Norbert Hahn
Messages: 46
Registered: May 2003

Member

"IDLmastertobe" <shi_lee@hotmail.com> wrote:

> Hey, I am experiencing problem using fft function. Since FFT function
> provides the fourier transform of a certain input, for example,
> 8*cos(!PI*x/6),

First of all, this is a function. Please supply the min and max
values of x.

> I would expect an amplitude of 4 at 2 shifted frequencies,
> namely -!PI/6 and PI/6.

This should occur for an infinite number of values.

> However after I recieved data from the FFT, I
> recieved some amplitudes close to 2 at some random locations, can anyone
> explain to me why it happened? Thanks for your time.

I tried to check with the following code:

x=findgen(1000)/10
y = 8*cos(x)
plot, y
z = fft(y)
plot, abs(z)
print, max(abs(z))
; 3.32737

I get a distribution with peaks at x=1 and x=998 and a maximum of 3.32737.
Next I used 10,000 numbers and looked at
plot, [abs(z[0:20]),abs(z[9979:9999])]

Two peaks with 3.94421 as maximum values. So it approaches 4 as you expected.

HTH
Norbert

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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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