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Altera_Forum
Honored Contributor I
946 Views

FFT_verification

Hi ! 

 

When I run FFT and IFFT tool in megawizard, the results are not logical. For example firstly I run the FFT tool with the megawizard tb (input: natural order, output : digit reversed order) then I use the FFT output txt files for the input files of IFFT ( input : digit reversed order output: natural order) the result of IFFT dont match with the input of FFT . 

What could be the problem ? 

 

P.S = I use single precision floating point representation for the 16-FFT tool. 

 

 

Thanks all
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4 Replies
Altera_Forum
Honored Contributor I
39 Views

Have you tried checking what the input file looks like when you generate a testbench for the IFFT, there's no guarantee the output file of the FFT matches the input format for the IFFT.

Altera_Forum
Honored Contributor I
39 Views

I used the output of FFT for the input of IFFT.I mean I modified the tb for IFFT. 

 

By the way I found the solution ;) When you check in binary for floating point it is hard to find what is wrong. 

 

When I converted to decimals in Matlab I saw that the output multiplied with the FFT size. 

For example if you use 8-IFFT the outputs are 8 time greater than the outputs had used to be. You should normalize them by IFFT size ;) 

 

Thanks anyway for your reply
Altera_Forum
Honored Contributor I
39 Views

Hi ferdiskaras, 

 

I guess I have the same problem you went through. I have implemented a VARIABLE STREAMING (N=8), SINGLE-FLOAT-POINT FFT/IFFT. These are my results: 

 

input (natural order): (8 0 0 0 8 0 0 0)  

FFT  

output (Digit-reverse order): [16 16 0 0 16 16 0 0] /* the same as [16 0 16 0 16 0 16 0 ] Natural order */ 

 

input(Digit-reverse order): [16 16 0 0 16 16 0 0]  

IFFT 

output (natural order): (64 0 0 0 64 0 0 0) 

 

How did you normalize it? Where did you make de division by N? In your hardware or in your software? Is there a way to set FFT core to output data already normalized? 

 

Thanks
Altera_Forum
Honored Contributor I
39 Views

If change the "I/O Output Order" of FFT and "I/O input order" of IFFT to "digital reverse Order", then the input of FFT should be equal to output of iFFT. I'm able to get the correct result.

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