[USRP-users] How good can USRPs with internal GPSDOs being synchronized?

Marcus D. Leech mleech at ripnet.com
Thu Sep 5 08:45:36 EDT 2013

On 09/05/2013 03:10 AM, Cheng Chi wrote:
> Hi Matt,
> The time difference is the TDOA we estimated by cross-correlating the 
> two signals. For our setup, we expected that the TDOA would be less 
> than 100ns, but constantly got a TDOA between +0.2 and -0.2 us.
> So the code to align LOs in the frontend would have no effect for 
> DBSRX2. Do you have any suggestions on how to deal with this phase 
> difference? We use single tone signal to measure the phase difference 
> between the two signals, it seems that the phase difference is not 
> constant.
> Best regards,
> Cheng Chi
The phase difference will be different every time the devices are tuned 
or reset.  That's due to the nature of Frac-N synthesizers, even when they
   have a common reference clock (or, in your case, a common-ish 
reference clock since you're using two different GPSDO units).

The SBX card provides a phase-reset feature that can be used in 
conjunction with timed commands to arrange for the synthesizer 
phase-offset to be
   common between two or more units.  But the DBSRX2 doesn't have this 

The drift characteristics of your two GPSDO will be slightly different, 
and they'll have different "fine structure" in their phase noise. The 
only way to do
   what you're trying to do in that situation is to take frequent 
measurements and adjust accordingly.

Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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