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

From: Jonathan Friedman
Subject: Re: [Discuss-gnuradio] unknown phase-relation between rx and tx
Date: Sun, 22 Jun 2008 03:43:01 -0700

Unless I misunderstand what you are doing, what you are describing is
the coherence error of the system. The long-answer made short is "no".
You have to calibrate it out or otherwise live with it. The TX and RX
paths of the USRP are derived from separate hardware local oscillators
so there will always be an unknown phase error between them. What's
worse is that it will vary slowly and dynamically as temperature and
voltage change during system operation.


On Sun, Jun 22, 2008 at 2:42 AM, wolfgang buesser
<address@hidden> 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
> --
> View this message in context: 
> http://www.nabble.com/unknown-phase-relation-between-rx-and-tx-tp18052839p18052839.html
> Sent from the GnuRadio mailing list archive at Nabble.com.
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