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Re: [Discuss-gnuradio] am_rcv.py


From: Eric Hill Matlis
Subject: Re: [Discuss-gnuradio] am_rcv.py
Date: Fri, 27 Oct 2006 11:27:30 -0400 (EDT)

I do have 5 kHz sidebands, and this is verified by the location of the peaks in the first two fft graphs. It's just the third graph which showed an aliased signal. Could the filter taps be incorrect?

eric

On Thu, 26 Oct 2006, Matt Ettus wrote:

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






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