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Altera_Forum's avatar
Altera_Forum
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16 years ago

Ofmd modulator: IFFT

Hi All.

I am realizing an Ofmd modulator.

I have a problem with zero padding of the IFFT. :confused:

I decide to use the FFT MegaCore Function v9.0.

I have 1536 samples and I choose a 2048 samples IFFT.

Now, how I have to do zero padding?

I think it depends from

1) the input data mode that I choose in FFT MegaCore (natural, bit reversed, DC centred)

2) my samples order. They are DC-centered because they corresponding to carriers from (-768 to 768) excluding 0, that is DC carrier.

In that case where have I to put zeros?

Moreover from FFT MegaCore User Guide: what does it mean that the signal is in "natural order", "bit-reversed" or "DC centred?

And if I pass to the IFFT Ip core a signal DC-centred and the output is in natural order, have I to re-order samples in DC-centred order before transmission?

I hope someone can explain me, sorry if I was a bit confusing.

3 Replies

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

    To keep the shape of the spectrum, u should pad ONE zero to the front to keep DC centred, and 2048-1536-1 zeros to the center of the 1536 samples.

    Scale factor should be considered.
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    16 years ago

    Thank you hawke. I don't undestand two things. 1) Why have I to put a zero in front of my vector? 2) And why "Scale factor should be considered"? Thank you again.