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Re: [Discuss-gnuradio] FFT bin ordering


From: CEL
Subject: Re: [Discuss-gnuradio] FFT bin ordering
Date: Mon, 12 Mar 2018 12:05:11 +0000

Hi Dirk,

On Sat, 2018-03-10 at 22:22 +0000, Dirk Gorissen wrote:
…snippety snip…
> My assumption was that the center frequency is at the middle bin
> (Shift = True) and that if I want +/- 500Hz (for example) I can just
> take the bins (middle bin index - 500) -> (middle bin index + 500)

Think about how wide your bins are! An FFT transforms "input vector
length" =: N samples from time domain into frequency domain. Thus,
every bin represents Nyquis bandwidth / N of bandwidth. Since Nyquist
bandwidth is sampling rate, you get f_sample/N*1000 of bandwidth.

> The python block should be trivial in that it just snips out a subset
> of the FFT bins (around the center frequency).

Now, to act a bit like Clippy:

> It looks like you're building a decimating low pass filter. Do you
want help with that? 

Generally, cutting out spectrum from the DFT of your signal *might*
lead to a distorted view at your signal; see [1].
It might be OK (correct, even), however, for specific purposes. What's
the bigger thing you're planning to do?

Best regards,
Marcus

[1] https://dsp.stackexchange.com/questions/6220/why-is-it-a-bad-idea-t
o-filter-by-zeroing-out-fft-bins

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