Thanks Jason, Marcus, I forgot to mention I'm using the WBX, not the TVRX. I'm using the UHD as well.
The flowgraph uhd_single_source -> agc2 -> streamToVector -> fft_vcc -> cmplexToMagitude -> binStatistics I added the agc2 block recently, I have similar problems without it. Usually I'm interested in sweeping from 50 MHz to 2.2 GHz, but this scan was from 558 MHz to 568 MHz. The FFT size I use is usually 512 or 1024. I believe the decimation rate here is 256, but both problems persist no matter the decimation/sampling rate I select. On Fri, Mar 18, 2011 at 1:43 PM, Jason Abele <ja...@ettus.com> wrote: > 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 > > When I do a sweep from 558MHz to 568MHz with my decimation rate of 256 I get these results from tune_result_t. Note this isn't all the results, but I think you can get the idea from these. Target Inter Frequency 567277343.75 Actual Inter Frequency 567277167.277 Target DSP Frequency -128.777472496 Actual DSP Frequency -128.778629005 Target Inter Frequency 567472656.25 Actual Inter Frequency 567472527.473 Target DSP Frequency -81.0821123123 Actual DSP Frequency -81.0716301203 Target Inter Frequency 567667968.75 Actual Inter Frequency 567667887.668 Target DSP Frequency -33.3867521286 Actual DSP Frequency -33.3879143 Target Inter Frequency 567863281.25 Actual Inter Frequency 567863247.863 Target DSP Frequency -1511.94291818 Actual DSP Frequency -1511.9323507 Target Inter Frequency 568058593.75 Actual Inter Frequency 568057081.807 Target DSP Frequency -891.903235674 Actual DSP Frequency -891.904346645 Target Inter Frequency 558097656.25 Actual Inter Frequency 558096764.347 The DSP frequency is all over the place, from -30 to -1500, why is it always negative? Thanks Again, Devin
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