Re: [Discuss-gnuradio] unknown phase-relation between rx and tx

[Eric Blossom]

On Sun, Jun 22, 2008 at 02:42:03AM -0700, wolfgang buesser wrote:
> 
> Hi,
> 
> Iam using 
>    gr.sig_source_s (self.usb_freq (), gr.GR_CONST_WAVE, 0, 0, 0)
> to generate a carrier and 
>    usrp.source_c(nchan=2,decim_rate=self.interp) to resample it at the same
> frequency.
> 
> I get what I expect: 2 complex numbers which are pretty constant. But with
> every restart
> of the program the phase changes (apparently by the same amount for both
> channels). The
> amplitude does not vary much from run to run.
> 
> It seems to me, that the phase relation between the interpolator in the TX
> and the decimator in
> the RX changes with every restart.
>  
> Is there a way to "reset" the  interpolator/decimator so that I get the same
> reading for every start
> (as long as I don't change frequencies) ?
> 
> Thank's
>    Wolfgang

This is the AD9862 digital-up-converter issue.

IIRC it does come out of powerup in a constant state.  I'm not sure
that this will be useful, but if you never set the digital
upconverter, the absolute phase will remain constant.

Eric