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?
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.
3) What sets the limits on the axes of these graphs? Is it possible to
modify them?
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.
Thanks in advance,
eric
************************************
Eric H. Matlis, Ph.D.
Aerospace & Mechanical Engineering Dept.
120 Hessert Center for Aerospace Research
University of Notre Dame
Notre Dame, IN 46556-5684
Phone: (574) 631-6054
Fax: (574) 631-8355
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