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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:1.9.2.21) 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 =
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