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- Ray Andraka

July 8, 2003, 8:15 pm

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:

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:

--

--Ray Andraka, P.E.

President, the Andraka Consulting Group, Inc.

--Ray Andraka, P.E.

President, the Andraka Consulting Group, Inc.

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