Eric Hill Matlis wrote: > Hello- > > I am attempting to demodulate an A.M. modulated signal using > am_rcv.py, the USRP, an LFRX daughterboard, and the latest svn > gnuradio distribution. At present the signal is being generated by a > function generator (Stanford Research Systems Model DS345) with a 2 > MHz carrier and a 5 kHz modulation. I have some questions about the > demodulation being performed by the am_rcv.py program found in the > examples directory for the usrp. > > 1) The first graph is labeled as "Pre-Demodulation", yet it is > actually not representative of the original waveform as it has been > downshifted to the offset frequency of 30 kHz (as I recall, there is > an issue with the USRP that requires this 30 kHz offset, but I don't > know the particulars about that, perhaps somebody can explain?). I > would like to plot the original waveform. Is there a way of showing > the fft of the original waveform with the carrier at 2 MHz and > sidebands at 1.995 and 2.005 MHz?
We don't need that 30 kHz offset anymore. To show the original waveform, just follow the code in usrp_fft.py > > 2) While the second "Post Demodulation" graph correctly shows the > sidebands at +/- 5 kHz, the decimation filters are aliasing the > sideband in the "Post Filter" graph to 2.5 kHz. I have to reduce the > "audio_decimation" from 2 to 1 to prevent this aliasing. Why would > this be happening? The original final decimation of 2 produces an > effective sampling rate of 32 kHz, which should be sufficient to > resolve the sideband at 5 kHz. Sounds odd. Are you sure you have 5 kHz sidebands? > 3) What sets the limits on the axes of these graphs? Is it possible > to modify them? > The x-axis will match the sample rates. The Y-axis can be modified by right-clicking. > I am hopefully going to be using the USRP and gnuradio to demonstrate > a sensor I am developing that produces AM modulated waveforms at the > Division of Fluid Dynamics APS conference in November, so I would like > to fine-tune this application as much as possible. There will be a > booth for the commercial vendors at the meeting, and it would be cool > if I could use Gnuradio at our display in this conference. That would be great! Matt _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
