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".
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