Re: [Discuss-gnuradio] Synchronization issue with multiple USRPs

[Josh Blum]

On 05/25/2011 09:56 AM, David Evans wrote:
> 
> This has never been clear to me! 
> What about the phase ambiguity due to the fractional-N VCO's, as Marcus has 
> mentioned?
> 
> This has to be calibrated out, yes/no?, but how?, unless after a retune you 
> then 
> 
> TX a calibration sequence that you can correlate against and correct. But 
> after 
> this, then, do you really need to accurately wait for a PPS? It's just stuff 
> in 
> the buffer, with the computational overhead of alignment.
> Basically, how do you compensate for the ambiguity? (maybe it doesn't matter 
> and 
> 

Well you really need to do two things:

1) Synchronize the time registers across all the devices using the PPS.
This allows you to time-align samples. Also, the cordics across all
devices will be aligned in phase when you tell the devices to stream at
the same time.

2) Calibrate the setup. After you tune, the daughterboard LOs will have
different constant phase offsets. With boards like WBX, its actually
possible to sync the LOs using hardware, but this isnt implemented yet.
In any case, *no matter what*, you need to calibrate by passing in a
common signal and measuring your channels.

If you open/close the device handle or re-tune, you will have to re-cal.

-Josh