Re: [Discuss-gnuradio] Transceiver Residual Carrier Offset

[Eric Blossom]

On Tue, Feb 02, 2010 at 10:12:02AM -0800, dougekim wrote:
> 
> I recently purchased two new USRP2 boards each with a XCVR2450 transceiver
> board.  In testing the two boards, I began by conducting a simple tx/rx test
> in which both boards share a COMMON 10MHz reference clock, as opposed to two
> independent references.  This test was conducted to determine if the
> measured  channel would remain relatively unchanged (time invariant) in a
> static channel environemnt. 
> 
> A single 2MHz tone was used as my baseband test signal (no BPSK or QAM).  In
> comparing the baseband tx signal to the baseband rx signal, I noticed that
> although both signals were 2MHz in frequency, they were out of phase as
> expected.  However, the phase difference was not constant over time.  Rather
> the phase difference was changing over time quite a bit.   
> 
> This occurrence may limit the MIMO capabilities of these boards since the
> residual carrier offsets must be corrected for individually for each
> transceiver.  Take a 2x2 MIMO system fore example.  Four different carrier
> offsets may need to be corrected rather than one as in the case of a fully
> coherent system. 
> 
> I'm curious as to whether other users have encountered similar problems with
> their systems as well, and if so, would be able to explain this occurrence.

Doug,

Did you call 

  u.config_mimo(WE_LOCK_TO_SMA)

in your host code?  If you didn't, the USRPs are still using their
internal free running oscillators.

Eric