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Altera_Forum's avatar
Altera_Forum
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12 years ago

why fft core's output is conjugate

Dear all,

When I using quartu12.0's fft megacore IP, I found such problems:

chose variable streaming and natural order for both input and output order, set ifft, make sink real = impulse, sink_imag =0, 1024 point, the ifft output from the simulation shows that the (source_real + 1j*soure_imag) is the conjugate of the ideal result from matlab. Please help me. Thank you very much.

And the result is caputured as follows:

https://www.alteraforum.com/forum/attachment.php?attachmentid=6874

1 Reply

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

    It could be your understanding of ofrequency sense is wrong between matlab model and ifft core.

    You got cos waveform on real ifft output and -sin waveform on imag ifft output.

    This means your so called impulse is one nonzero sample at a negative frequency location i.e. after midpoint of 1024 (well further from sample 512) .

    So in which sample did you put that impulse?