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Re: FFT phase? [message #82909 is a reply to message #82902]

Fri, 25 January 2013 05:18

Yngvar Larsen
Messages: 134
Registered: January 2010

Senior Member

On Friday, 25 January 2013 07:49:00 UTC+1, xqin...@gmail.com wrote:
> Thanks. I have known the reason. What I use is like x= 2*!dpi*2*dindgen(25)/16, so the amplitude and phase are not so accurate.

As a rule of thumb for harmonic analysis, you need at least 10 cycles in your data to get a reliable phase/magnitude estimate. Your example using around 1.5 cycles is probably the worst case scenario.

OR, if you know the frequency of your signal exactly a priori, you have to truncate your data to a periodic signal, i.e. an integer number of cycles, in order to extract A and B reliably. This is what Craig did in his reply, using only a single cycle.

--
Yngvar

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[Message index]

		Re: FFT phase? By: Craig Markwardt on Fri, 25 January 2013 13:48
		Re: FFT phase? By: Yngvar Larsen on Fri, 25 January 2013 05:18
		Re: FFT phase? By: xqinshan on Thu, 24 January 2013 22:49
		Re: FFT phase? By: Craig Markwardt on Thu, 24 January 2013 18:11

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