Re: [Discuss-gnuradio] am_rcv.py

[Matt Ettus]

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