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11,193 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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12,658 Views
Registered: ‎09-03-2009

thanks everyone, got it right

actually there was some mistake with my initializations

 

thanks again

 

Regards

Yasir

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9 Replies
ywu
Xilinx Employee
Xilinx Employee
11,171 Views
Registered: ‎11-28-2007

Can you attach your model or testbench?

 

Cheers,

Jim

Cheers,
Jim
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oliviert
Xilinx Employee
Xilinx Employee
11,120 Views
Registered: ‎01-09-2008

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 SW Marketing AI Engine Tools
Don't forget to reply, give kudos, and accept as solution.
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vytautas
Explorer
Explorer
11,082 Views
Registered: ‎10-01-2007

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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12,659 Views
Registered: ‎09-03-2009

thanks everyone, got it right

actually there was some mistake with my initializations

 

thanks again

 

Regards

Yasir

View solution in original post

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mofa77
Newbie
Newbie
8,975 Views
Registered: ‎10-17-2011

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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technovlsi
Explorer
Explorer
8,956 Views
Registered: ‎01-30-2011
@ 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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vinodlk123
Observer
Observer
8,903 Views
Registered: ‎01-27-2012

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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bwiec
Xilinx Employee
Xilinx Employee
8,879 Views
Registered: ‎08-02-2011
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4,615 Views
Registered: ‎04-27-2015

I got the same problem

I wanna know what is the mistake with your initializations

thank you

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