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5,106 Views
Registered: ‎01-26-2009

FFT core has reversed output data

Hello,

 

I'm implementing a autocorrelation function using a fft and ifft hard core (v6.0) on a Virtex5. When starting the fft, I see at the output that the result is

reversed in the frequency domain.

 

Example: The input signal is a sinus with a frequency of 2 Hz. As output signal I expect a dirac impuls at the beginning (imaginary).
-> f(0)=0; f(1)=0; f(2)=1; f(3)=0; …; f(N-3)=0; f(N-2)=-1; f(N-1)=0; f(N)=0;   (*)

 

But I get:
-> f(0)=0; f(1)=0; f(2)=-1; f(3)=0; …; f(N-3)=0; f(N-2)=1; f(N-1)=0; f(N)=0;

 

Other input signals also get the same result, that the output is reversed.
Can you please explain me why the output order is reversed? I have no explanation!
Is it possible to reorder the output signal like (*)?

 

 

Best regards

Kris

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Visitor
Visitor
4,975 Views
Registered: ‎02-08-2009

Re: FFT core has reversed output data

While generating FFT core, there is an option of output ordering. These options are bit/digit reversal order or Natural order. So, you can choose the option among them, the default is bit/digit reversed order so that may be the resson for your problem.

Anoop

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