Eric Hill Matlis wrote:
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
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!