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Re: [Discuss-gnuradio] FFT of noise assumes a gaussian shape at certain

From: Josh Blum
Subject: Re: [Discuss-gnuradio] FFT of noise assumes a gaussian shape at certain sample frequencies
Date: Fri, 16 Sep 2011 09:15:03 -0400
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110831 Lightning/1.0b2 Thunderbird/3.1.13

> Basically when I take the FFT of the wgn receiver noise, at certain sample
> rates (e.g. 5Msps) the spectrum output is roughly flat, while at other rates
> (e.g. ~6 Msps), the spectrum takes on a decidedly Gaussian look, where the
> magnitude of the first and last bins of the FFT are 5-10 dB below the centre
> bins of the FFT. Does anyone know why this is? I've confirmed this behaviour
> with the fft_sink2 block.

You may be seeing the filter profile of the half-band filters in the
down converter (or lack there of). When you have odd decimations, there
are no half-band filters in the down conversion chain. 100e6/decim =

> Also, in an unrelated question, is 25Msps the maximum bandwidth of the
> host->USRP interface for the N210 without having to modify the firmware?

Matt is adding support for smaller samples, but 25 is near link
saturation at 32 bits a sample.


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