Re: [Discuss-gnuradio] USRP2 Spectrum Sensing Help

[Marcus D. Leech]

On 18/03/2011 1:10 PM, devin kelly wrote:
> Hello,
>
> I've created a spectrum sensor with the USRP2, my python script uses 
> the same idea as usrp_spectrum_sense.py found in gnuradio.  That is, 
> it tunes, takes an FFT, records, retunes, etc.
>
> I have two problems with my data though.  In the file attached is some 
> TV spectrum (left half) and noise (right half).
>
> My first question is this: why isn't the spectrum for the TV signal 
> flat, it seems to bob up and down.  Note that each segment is from a 
> different FFT, that is each FFT produces that oval shape.  At first I 
> thought this had something to do with the window I was using but I've 
> tried hamming, blackman-harris, and rectangular windows and they all 
> have this effect.
The CIC decimator in the FPGA necessarily effects a bandpass filtering 
effect, whose "skirts" are not infinitely steep, the shape of that 
bandpass response is roughly a "rounded hump".  If you take multiple 
"snapshots" of spectrum, over multiple tunings, you'll get a bumpy 
effect because the pass-band isn't flat.  One way to "fix" this for 
making nicer-looking displays is to invert the "hump" effect in the FFT 
magnitudes, prior to "stitching".