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

[Josh Blum]

> 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 =
samp_rate

> 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.


-josh