[USRP-users] LO phase alignment

Josh Blum josh at ettus.com
Fri Jun 22 12:13:20 EDT 2012



On 06/22/2012 01:58 AM, Christophe ALEXANDRE wrote:
> Hi Josh,
> 
> i've read the link :
> 
> Using timed commands, multiple frontends can be tuned at a specific
> time. This timed-tuning ensures that the phase offsets between VCO/PLL
> chains will remain constant after each re-tune. See notes below:
>  a.. There is a random phase offset between any two frontends
>  b.. This phase offset is different for different LO frequencies
>  c.. This phase offset remains constant after retuning
>    a.. Due to divider, WBX phase offset will be randomly +/- 180 deg
> after re-tune
>  d.. This phase offset will drift over time due to thermal and other
> characteristics
>  e.. Periodic calibration will be necessary for phase-coherent applications
> I still don't understand the principle you use. is it a VCO special
> property ?
> 

Its a simple principal. When you tune the SBX, the last thing that
happens is that the accumulators reset to zero.

So if you time the event of tuning across multiple devices, the
accumulators will all reset at the same time. Ergo, the phases of the
LOs march in lock step.

> some questions still remain :
> 
> Q1) if i change LO freq from f1 to f2 then back to f1, should i get the
> same phase offset on f1 ?
> 
> Q2) if i turn off the N210s then on, should i get the same offset on the
> same freq ?
> 
> Q3) if the Q1, Q2 answers are yes, what kind of phase offset precision
> should i expect ?
> 

Essentially no. There is no absolute phase reference to come back to. A
group of LOs simply become phase aligned upon tuning.

-josh




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