Re: [Discuss-gnuradio] Single-tone test using USRP2 with RFX2400 and XCVR2450

[Eric Blossom]

On Fri, Aug 28, 2009 at 02:05:02PM -0300, Igor Almeida wrote:
> On Fri, Aug 28, 2009 at 12:56 PM, Douglas
> Geiger<address@hidden> wrote:
> > That sounds like it could be a frequency offset between TX and RX. You
> > say you aren't synchronized (which I interpret to mean you don't lock
> > the clocks of the TX and RX together somehow) - do you have a costas
> > or similar carrier offset recovery block/process on your receiver?
> 
> Correct, the USRPs are physically disconnected.
> Right now, the graphs are:
> 
> sig_source_f -> float_to_complex -> head -> usrp.sink_c

If you're trying to send a sine wave, this will work much better:
(Generates a complex sinusoid == exp(jwt))

  sig_source_c -> head -> usrp.sink_c

Or you could use a hilbert transform to generate the quadrature
component.  float_to_complex will be setting Q to zero the way you're
using it.



> usrp.source_c -> file_sink (on the other PC)
> 
> TX usrp sends the first 1000 cycles of a 2MHz sine wave (sig_source
> with 4MS/s), then the "training graph" is disconnected and another one
> with the QAM modulator kicks in. Both scripts tune to 2.5G.
> The strange thing is no matter what frequency I set the sig_source to
> (already tried 1.5k, 2k, 1M and 2MHz, sampling frequency 4MS/s), the
> fft on matlab always shows a single impulse at aproximately -3kHz. In
> the time domain, the signal is sinusoidal on both real and imaginary
> parts, when I expect it to be only real since the carrier has (should
> have) been removed.
> 
> I believe I am missing some trivial concept here.

See above.

Eric