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Altera_Forum
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16 years ago

Altera FFT - output scaling. How to do this correctly?

Hi, I do not get correct scaled results if I do everything the FFT user guide recommends. I have instantiated a 1024 points FFT, with single output and burst architecture. The data width...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
16 years ago

Ignore my above remarks since your fft output is 16 bits.

I understand from your equations that you are converting the 16 bit output into 32 bit bus then doing the two stage shift(by exponent then by 9). Your work-around of 9 bits shift then taking 16 LSBs doesn't make sense to me.

To avoid calculation problems of software Let us go back to your simple example:

You said for your input sine of 30146 amplitude you get fft output as:

Real: 3384 (0x0D38)

Imaginary: -14391 (0xC7C9)

fair enough. what is your exponent value in above example?

It should be -1 so that output scales to 2^ -(-1) = 2

otherwise there is something wrong.