[Discuss-gnuradio] am_rcv.py

[Eric Hill Matlis]

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