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Old 07-08-2003, 10:15 PM
Ray Andraka
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Default Re: scaling fixed point fft

You didn't mention the radix of each stage. Divide by 4 is too much if
it is a radix2 butterfly. If it is radix 2, the gain is at most 2, not
4. The max output from an FFT occurs when all the input energy falls
into one output bin, which implies the input is a pure sinusoid with a
frequency that is an integer multiple of the sample time. If the input
is white noise, the output signal energy is going to be spread evenly
among all the output bins, in effect dividing the input signal by N. As
you can see, this results in a fairly wide dynamic range. If you know
the nature of your input, you can scale accordingly. If not, you need
to either carry extra bits or you can use dynamic scaling such as a
block floating point scheme.

Bob wrote:

> Hello,
>
> I have constructed a 256 pt complex fft. My scaling is causing
> problems as the outputs from each stage are divided by 4 to avoid
> overflow. The input data and the twiddle factor coeffs are 16 bits
> wide (Q15). My problem is that when the data arrives at the last two
> butterfly stages of the FFT, it is non-existant, due to all the
> scaling beforehand. All inputs to these stages are zero. Thus I get
> nothing at the output.
>
> How can I work around this?
>
> Many thanks
>
> Bob


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