[USRP-users] LO phase alignment

Christophe ALEXANDRE christophe.alexandre at cnam.fr
Thu Jun 21 10:27:33 EDT 2012


Hi all,

i want to use 2 x (N210 + SBX) for a laser telemetry application.
i need to synchronize clocks, align sample times and also have precise knowledge
of LO phase difference between both N210s. 

At the moment, using a mimo cable, i use the following
method : i generate a 1.00005 GHz and inject it on both N210s
with a center frequency equal to 1 GHz. 

I get 2 exact 50 kHz complex signals with a constant phase difference. Each time
i start a measure, i get an other phase difference, still constant.

if i change the frequency from 1 GHz to 1.1 GHz then go back 
to 1 GHz, the phase difference has changed.

I can't use this for a telemetry application.


i have read this application note : 

http://www.ettus.com/content/files/kb/Selecting_an_RF_Daughterboard.pdf

and find this : 

Do I plan to implement beamforming or direction finding (DF) capability? 
While all daughterboards can be used in a MIMO configuration, this does not necessarily imply there is phase alignment between the RF chains of one or more daughterboards. The BasicRx, BasicTX, LFRX and LFTX are exceptions to this, since they do not include local oscillators that contribute to phase ambiguity between channels. 
The Ettus Research SBX daughterboard utilizes an RF PLL that includes a resynchronization feature, which can be used to align the LOs across multiple SBXs, and multiple USRP hardware devices. Using the 
UHD (USRP Hardware DriverT), timed SPI commands can drive this re-synchronization feature. At the time of this writing, this feature is not supported in the mainline UHD. However, this feature is planned for release in 2012. 


I can't work without phase alignment. So my question is : 

    when could we get this feature in UHD ?
 
 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    
 
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