On Fri, Mar 18, 2011 at 10:10 AM, devin kelly <dwwke...@gmail.com> 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.
Could you post a flowgraph or tell us what frequencies you are tuning sweeping across, what step size, any averaging you use, sample rate or decimation, 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 TVRX has a 6MHz bandpass filter, which will add shape to the spectrum you sense and even if you pick a smaller than 6Msps sample rate, you will still have several effects to consider: * Aliasing of energy inside the 6MHz bandpass * Probably 75% of the 6MHz is the flat region * TVRX has finite tuning steps, so you may be causing the USRP2 DDC to push your IF against the edge of the TVRX bandpass > My second question is why isn't the transition between TV spectrum and noise > more smooth? It looks like it could be smooth but the gain seems to jump for > an FFT right after 566 MHz and then settle down again. I thought this might > be the AGC but I've tried different attack/decay rates, different reference > levels, etc with the agc2_cc block but the problem remains. > I am not familiar with the agc2 block, but I would recommend against using AGC until you are confident you have handled the bandpass nature of TVRX properly > Any help with either of these questions would be appreciated, If you are using UHD, you might consider using the uhd::tune_result_t returned by set_center_freq() to make sure you are staying centered in the bandpass filter Jason _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio