On 01/14/2011 02:56 PM, emat...@yahoo.com wrote: > Martin, thanks for responding. See my follow ups below: > --- On Thu, 1/13/11, Marcus D. Leech <mle...@ripnet.com> wrote: > > >> From: Marcus D. Leech <mle...@ripnet.com> >> Subject: Re: [Discuss-gnuradio] grc amplitude demodulation questions >> To: discuss-gnuradio@gnu.org >> Date: Thursday, January 13, 2011, 4:26 PM >> >>> Hello- >>> >>> I am using GRC 3.3 to create a flow graph using the >>> >> USRP2 and a LFRX to do amplitude demodulation. I have >> a few questions. >> >>> 1) What are good values for "Audio Pass" and "Audio >>> >> Stop" within the "AM Demod" block? >> >>> 2) I observe a DC offset at the output of the >>> >> demodulation. Is this expected, and what is the best >> way to remove it? I am currently using a high-pass >> filter. My incoming signal is a 27 kHz sine modulated >> at 1 kHz. >> There will always be a certain amount of DC offset in an AM >> demodulator, since it's just a power detector, and since the >> RF chain always >> produces at least *some* noise, the detected version >> of that noise results in a small residual DC offset. You can >> null it out with an >> an "ADD" block. >> > I've tried an ADD block, but I noticed that if my decimation is not a power > of 2, the offset changes. > Decimation filters will have slightly different output magnitude, depending on parameters. You'll always have to re-null after changing decimation. Fact of life.
> > > 2nd FFT: self.wxgui_fftsink2_1 = fftsink2.fft_sink_f( > self.notebook_0.GetPage(3).GetWin(), > baseband_freq=0, > y_per_div=20, > y_divs=10, > ref_level=10, > ref_scale=2.0, > sample_rate=100e6/usrp2_decim/sw_decim, > fft_size=1024, > fft_rate=10, > average=False, > avg_alpha=None, > title="Demodulated FFT Plot", > peak_hold=False, > > > > My first thought is evaluation ordering in the 100e6/usrp2_decim/sw_decim. What happens if you re-express it as: (100e6/usrp2_decim)/sw_decim? -- Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
