Re: [Discuss-gnuradio] A medley of questions

[Eric Blossom]

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 <address@hidden> 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