[USRP-users] LO phase alignment

Josh Blum josh at ettus.com
Fri Jun 22 13:40:35 EDT 2012



On 06/22/2012 09:26 AM, Christophe ALEXANDRE wrote:
> so the only solution is to share
> the same LO across multiple SBX ?
> 
> is there any "simple" way to do this ?
> 

Maybe I misunderstood the question and answered incorrectly.

If you tune all N channels at the same time,
they will have the same phase.

-josh

> 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 
> 
> 




More information about the USRP-users mailing list