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Re: [Discuss-gnuradio] FFT bin ordering


From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] FFT bin ordering
Date: Sat, 10 Mar 2018 17:48:20 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 03/10/2018 05:22 PM, Dirk Gorissen wrote:
Hi All,

This should be a simple question. I have the following:

sdr -> decimation/filters -> stream to vector -> FFT -> complex to
mag2 -> log10 -> python block - > ...

The FFT block is configured as:
type: complex
Direction: Forward
Shift: yes
length: equal to input vector length

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

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)

However that does not seem correct. Perhaps there is a bug somewhere
else but just wanted to check Im doing the right thing.

Cheers
Dirk


Your assumption would only be correct if your FFT resolution was 1Hz/bin.





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