On Wed, Mar 14, 2007 at 04:53:58PM -0400, Steven Clark wrote: > Eric: many thanks for your responses. My responses below: > > On 3/14/07, Eric Blossom <[EMAIL PROTECTED]> wrote: > > > >On Wed, Mar 14, 2007 at 03:45:32PM -0400, Steven Clark wrote: > >> Hi all- > >> Two very different questions for you: > >> > >> 1) As a test, I am sending a GMSK signal (created by a signal generator, > >> very low noise) at low symbol rates into the USRP and plotting the > >complex > >> baseband that reaches the PC. One would expect to see a nice tight unit > >> circle, and at low decimation rates (-d 16, etc) this is indeed the > >case. > >> However, when I increase the decimation rate, the unit circle grows > >> "thicker", which seems to indicate amplitude distortion. At -d 256 the > >> distortion is quite bad. > > > >Are you plotting points or drawing lines between points? > > > Plotting just points. They are definitely at different amplitudes. I can > link plot images if that would help.
No need. > > >Any idea what could be going on here? This shows up > >> whether using either the TVRX daughtercard or the BasicRX. Something > >going > >> on with the DDC in the FPGA? Filter ripple? > > > >Where in the processing pipeline are you grabbing the data that you're > >plotting? Before or after the clock recovery block? > > > I'm using usrp_rx_cfile.py. So hopefully I'm just looking at the raw complex > baseband coming across USB. No surprise, you're not sampling at the center of the symbols. > (incidentally, when you're using sending a > complex stream to a file sink, is the first sample always real? I assume > so...) Yes, that's right. Eric _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
