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10,510 Views
Registered: ‎09-03-2009

FFT Simulation Results Problem: Incorrect Imaginary Values

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I am usinf FFT IP core V6.0. When I took the FFT of following data  points

 

1.1     2.2      3.3     4.4    1.1     2.2     3.3       4.4  

 

  in floating point format using the transform length of eight, i get the following Result

 

22     0       -4.4000 - 4.4000i      0          -4.4000           0            -4.4000 + 4.4000i        0  

 

whereas MATLAB gives the following Result

 

22     0       -4.4000 + 4.4000i      0          -4.4000           0            -4.4000 - 4.4000i        0  

 

 

The problem is that the signs of imaginary part is inverted. I have check it in both fixed point and floating formats, but problem remains the same, the signs of the imaginary parts are inverted, when we compare the results of MATLAB with that of FFT core.

 

Regards

Yaasir

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1 Solution

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11,975 Views
Registered: ‎09-03-2009

Re: FFT Simulation Results Problem: Incorrect Imaginary Values

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thanks everyone, got it right

actually there was some mistake with my initializations

 

thanks again

 

Regards

Yasir

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9 Replies
Xilinx Employee
Xilinx Employee
10,488 Views
Registered: ‎11-28-2007

Re: FFT Simulation Results Problem: Incorrect Imaginary Values

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Can you attach your model or testbench?

 

Cheers,

Jim

Cheers,
Jim
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Xilinx Employee
Xilinx Employee
10,437 Views
Registered: ‎01-09-2008

Re: FFT Simulation Results Problem: Incorrect Imaginary Values

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Hi,

 

I think you've done an IFFT instead of an FFT.

Check the 'fwd_inv' input; it must be set to 0 for  inverse transform and 1 forforward transform

 

Olivier

==================================
Olivier Trémois
XILINX EMEA DSP Specialist
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Explorer
Explorer
10,399 Views
Registered: ‎10-01-2007

Re: FFT Simulation Results Problem: Incorrect Imaginary Values

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Hello.

FFT datasheet includes waveforms how initialization should be done. This is the first step. Second, you need to add 3 clocks delay between every input. How do you read output?

Best Regards,
Vytautas
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11,976 Views
Registered: ‎09-03-2009

Re: FFT Simulation Results Problem: Incorrect Imaginary Values

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thanks everyone, got it right

actually there was some mistake with my initializations

 

thanks again

 

Regards

Yasir

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Newbie mofa77
Newbie
8,292 Views
Registered: ‎10-17-2011

Re: FFT Simulation Results Problem: Incorrect Imaginary Values

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i use the fft ipcore to compute fft and ifft 

but both the result are fft data

i have set the fwd_inv both 0 and 1

the result is fft result

can you tell me some reason

thank you 

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Explorer
Explorer
8,273 Views
Registered: ‎01-30-2011

Re: FFT Simulation Results Problem: Incorrect Imaginary Values

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@ vytautas sir, i have set the clk frq 250 Mhz. .then how do i insert 3 clk cycle delay in testbenvh. plz help.
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Observer vinodlk123
Observer
8,220 Views
Registered: ‎01-27-2012

Re: FFT Simulation Results Problem

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hello sir,

when im simulating fft core  7.1, im getting unexpected results....

i am not able to understand where i'v done wrong ...

i'v added some simulation results...

if u can plz provide the correct code or help me to get correct output...

thank you...

 

 

input-8 FFT.JPG

 

output- 8 FFT.JPG

 

 

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Xilinx Employee
Xilinx Employee
8,196 Views
Registered: ‎08-02-2011

Re: FFT Simulation Results Problem

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3,932 Views
Registered: ‎04-27-2015

Re: FFT Simulation Results Problem: Incorrect Imaginary Values

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I got the same problem

I wanna know what is the mistake with your initializations

thank you

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