Re: [Discuss-gnuradio] am_rcv.py

[Eric Hill Matlis]

Matt-

thanks for getting back.  I've attached the code as it currently stands. 
What parts are out of date?  The important parts (formatted slightly to 
fit on the screen) are:
**************************************************************************
        adc_rate = self.u.adc_rate()                # 64 MS/s
        usrp_decim = 250
        self.u.set_decim_rate(usrp_decim)
        usrp_rate = adc_rate / usrp_decim           # 256 kS/s
        chanfilt_decim = 16
        demod_rate = usrp_rate / chanfilt_decim     # 16 kHz
        audio_decimation = 1
        audio_rate = demod_rate / audio_decimation  # 16 kHz

        chan_filt_coeffs = gr.firdes.low_pass (1,           # gain
                                            usrp_rate,   # sampling rate
                                            9e3,        # passband cutoff
                                            10e3,       # stopband cutoff
                                            gr.firdes.WIN_HANN)
        self.ddc =  \
gr.freq_xlating_fir_filter_ccf(chanfilt_decim,chan_filt_coeffs,0,usrp_rate)

        self.magblock = gr.complex_to_mag()
        audio_coeffs = gr.firdes.low_pass (1.0,            # gain
                                           usrp_rate,      # sampling rate
                                           9e3,
                                           4e3,
                                           gr.firdes.WIN_HANN)
        audio_filter = gr.fir_filter_fff (audio_decimation, audio_coeffs)
*****************************************************************************

any comments?  Particularly on the filter coefficients.  I'm not sure yet 
how to select these.  My requirements at this stage are to demodulate 
sidebands with frequencies of up to say 8 kHz riding on a 2 or 3 MHz 
carrier.  (using the LFRX DC-50MHz daughterboard).  Currently I don't care 
about the audio port as a sink; I simply want to display the fft's on the 
screen.  Eventually I may want to capture to file.

thanks!

p.s.- do you have pictures of the enclosure?  I just built one using an 
extruded aluminum case from Box Enclosures, Inc that has very similar 
dimensions.  It's a very handsome box, only I couldn't get the TVRX module 
to fit; the USRP would only go in one way.  If I rotated 90 (which would 
have allowed the TVRX to fit) then it wouldn't work because the 
motherboard was about 0.03" too big.  There are pics of it at 
www.nd.edu/~ematlis/z.gnuradio/z.enclosure_pics

************************************
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

On Thu, 2 Nov 2006, Matt Ettus wrote:

> Have you solved this yet?  Is it possible that you are decimating too much?
>
> The am_rcv code is very old. I would start fresh if I were you.
>
> Matt
>
> Eric Hill Matlis wrote:
> > 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

am_rcv_plasma.py
Description: Text document