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Little problem with IFFT core v :9.0

Altera_Forum
Honored Contributor II
‎09-02-2009 05:32 PM
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I have 8192 bins (4095 .. 0 .. 4095) the last 4095 is a miror from first 4095 bins 

 

Sampling Frequency : 100MHz 

Bandwith : 50MHz 

Resolution : 50MHz / 4096 : 12.207 KHz 

 

For a test, I put one bin to 8000 (No :82 and also the miror) and the output is square wave !!!!!! 

 

The frequency is good but the wave is not good !!!! 

 

Is there anyone who has an idea?? 

 

Thank 

 

Jean
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Altera_Forum
Honored Contributor II
‎09-04-2009 09:18 AM
488 Views

Is your altera ifft/fft dc centred. My understanding it is not. So you should arrange your vector indexed from 0 ~ 8191 mirrored in the middle with 0 index being dc. 

 

your vector assumption: 

 

-50....|..............dc..............|....+50 

 

ifft assumption: 

 

dc....|..............+50..............|....dc 

 

so still symmetrical but the difference is in frequency value. You should get: 

 

100*82/8192 = 1MHz
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Altera_Forum
Honored Contributor II
‎09-04-2009 11:00 AM
488 Views

Thank you Kaz.

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