On Fri, Mar 18, 2011 at 4:29 PM, Moeller <moelle...@gmx.de> wrote: > On 18.03.2011 18:10, devin kelly wrote: > > 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. > > Why should it be flat? On the transmission side OFDM is quite flat. > Typically (NLOS conditions) you will receive multiple paths and they > interfere. > The "frequency selective fading" effect is very strong for wide-band > signals like DVB-T. > NB signals like GSM assume the "flat fading" model, by contrast. > > I suppose flat is the wrong word. If you look at the image I provided in the first email you'll find that all the FFT outputs have this hump shape to them.
> > 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. > > Can you switch off the AGC? AGC can destroy the whole spectrum measurement. > Single spikes could be spurious mixing products from the receiver itself. > I've only started using the AGC block very recently. I started using because I wanted some way to control the AGC. This makes me think of the question, is the AGC on by default? That is, if I don't have an AGC block will the AGC be doing anything? Also, in the first two or three ICs in the WBX there is a digitally controlled attenuator and an LNA, those are part of the AGC correct? > A global filtering shape is expected from the analog RF filters. > Of course, on the borders of FFT blocks you have incorrect transitions, > depending on the shape within the reception bandwidth. > I assume this could be compensated by scaling the FFT values. > This is going to be the next thing I try. > To get the spectrum more smooth, you have to capture longer signals > within the blocks of measurement bandwidth, for averaging the PSD estimate. > > > Any help with either of these questions would be appreciated, > > Devin > > Do you want to publish the program? I think it would be very interesting > for the examples section of Gnuradio. Surely, you will get support from > the Gnuradio community to improve the sensing function. > > Btw, did anybody try to write a spectrum sensor with the Funcube USB > dongle? > This would be a low-cost alternative. The measurement bandwidth is not > high, > but it can scan a broad range of the spectrum. > Possibly, after switching off AGC, compensating within the meas. bandwidth > and compensating the global filter shape, the result could be quite useful. > (if there are not too many spurious frequency components within the > receiver itself) > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > Thanks Again, Devin
_______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
