[USRP-users] Phase Synchronization of two usrp b210's

Marcus D. Leech patchvonbraun at gmail.com
Mon Feb 11 18:55:57 EST 2019


On 02/11/2019 06:34 PM, Aravind Dhulipalla via USRP-users wrote:
> Hi Marcus,
>
> Yes, I am following the link and using an external 10MHz and 1pps 
> reference signals. The way I am trying to synchronize is to have two 
> usrp's transmit different sine waves one with constant phase and the 
> other with a varying phase from 0 to 360degrees. I am calculating the 
> sum of amplitudes of the received signals and the phase at which the 
> amplitude sum is maximum will be the phase offset. By following this 
> approach I am getting a phase offset between 0 to 3 degrees. But when 
> I issue the next stream command with sine wave of calculated phase 
> offset I am seeing a huge phase difference between them anywhere 
> between 0 to 180 degrees for each run. I am not sure if my approach is 
> a correct one.
>
> Thanks
> Aravind.
I cannot quite parse what you're doing.

Are you saying the *mutual phase* offset changes between runs, or that 
the phase measured *between runs* changes?

I would totally expect some phase-offset unpredictability between two 
B210s.  They aren't designed to have a predictable phase in that sense, 
because
   of the way the synthesizers work.  They have no phase-resynch, and 
since a fractional-N PLL synthesizer "naturally" doesn't lock at any 
particular phase-angle relative to the reference, no two PLL 
synthesizers, even if seeing EXACTLY THE SAME REFERENCE FREQUENCY, can 
be guaranteed to have a predictable-and-low phase-offset after tuning, 
even if tuned at "exactly" the same time.


>
> On Sun, Feb 10, 2019 at 12:59 AM Marcus Müller 
> <marcus.mueller at ettus.com <mailto:marcus.mueller at ettus.com>> wrote:
>
>     Hi Aravind,
>
>     first of all, have you seen
>     http://files.ettus.com/manual/page_sync.html ?
>
>     You'll find in the last paragraph that after each tuning, your phase
>     relationship between different B210s will be random, but constant for
>     the run (aside from minor PLL drift).
>
>     So, you need to tune, and then calibrate; how that can be done depends
>     on the system you're building.
>
>     Best regards,
>     Marcus
>
>     On Fri, 2019-02-08 at 17:03 -0800, Aravind Dhulipalla via USRP-users
>     wrote:
>     > Hi,
>     >
>     > Has anyone achieved phase synchronizing between two usrp b210's
>     using
>     > external reference and pps? If yes I would appreciate any help
>     that I
>     > can get.
>     >
>     > Thanks
>     > Aravind.
>     >
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