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

[John Wilson]

Hey guys,

 

I've got a quick question on a project I'm currently working on which is scanning a range of frequencies using GNU radio and the USRP-N210 with the SBX daughterboard.

 

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.

 

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

 

Cheers,

 

John