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Altera_Forum's avatar
Altera_Forum
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16 years ago

Little problem with IFFT core v :9.0

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

2 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    16 years ago

    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