[USRP-users] LO phase alignment

Christophe ALEXANDRE christophe.alexandre at cnam.fr
Fri Jun 22 12:26:09 EDT 2012


so the only solution is to share
the same LO across multiple SBX ?

is there any "simple" way to do this ?

 regards.


 Christophe ALEXANDRE
 Conservatoire National des Arts et Métiers (CNAM)
 Laboratoire CEDRIC-LAETITIA
 Département EASY
 Accès 17-1-32, Case 2D2P10
 292 rue Saint Martin
 75141 PARIS CEDEX 03
 FRANCE
 email : christophe.alexandre at cnam.fr
 tel. 0140272699
 mob. 0651087311
 fax. 0140272994


----- Original Message ----- 
From: "Josh Blum" <josh at ettus.com>
To: "Christophe ALEXANDRE" <christophe.alexandre at cnam.fr>
Cc: <usrp-users at lists.ettus.com>
Sent: Friday, June 22, 2012 6:13 PM
Subject: Re: [USRP-users] LO phase alignment


>
>
> 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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