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mrnuke

USA
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Posted - 09/18/2006 :  2:44:02 PM  Show Profile  Edit Topic  Reply with Quote  View user's IP address  Delete Topic
Simple question:

I generated a signal that is a sin function at ~0.03281 Hz (110 cm-1).
FFT results in a peak at the expected frequency.
BUT, the inverse FFT produces a signal at twice the original frequency (~0.06562 Hz)? {confirmed with fft of results}

Thanks for any suggestions or insights on this matter
Fran
:)

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FFT - forward, amplitude, (UNCHECKED) normalized amplitude & shift results, window - rect; selected both real and img columns; default sample interval was correct (25 fs) .

IFFT - same as FFT (default sample interval was correct at 1.5625E-4)

Mike Buess

USA
3037 Posts
Posted - 09/18/2006 :  3:41:04 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Hi Fran,

That's just a matter of FFT settings and is discussed in Origin's online manual...

Help > Origin > Analysis: Advanced Math on Datasets > Fast Fourier Transform (FFT) > Troubleshooting Problems with FFT and Performing a Backward FFT

...That said, your settings (Normalize unchecked and Shift unchecked) are those recommended for reproducing a signal by IFFT. In my tests, those settings reproduce the signal exactly as show below.



Mike Buess
Origin WebRing Member

Edited by - Mike Buess on 09/18/2006 4:22:19 PM
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mrnuke

USA
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Posted - 09/18/2006 :  4:28:57 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Got it; Origin wasn't plotting the real part of the inverse.
Thanks,
Fran
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