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

[Khalid Jamil]

Hi,

I was building an 8-channel system using N210s. I got a 10MHz/1PPS OCXO clock from jackson labs and applied it across all channels. The result is that all channels got phase-locked but still with a random phase offset between channel-to-channel. The bad thing is that this phase offset is random every time you tune or re-tune the system. The reason being, what I understood, that we are not distributing the same LO across all N210s but asking their freq synthesizers to PLL to a 10MHz freq reference. This is not the best way to achieve a phase coherent system anyway. The best thing would'a been to daisy-chain a single LO across all usrps but that's not the case here.

 So, now I am trying is to build my own calibration mechanism by injecting a known phase at each channel before acquiring the actual signals without re-tuning the system. I hope it will do the job. At least the first attempt was encouraging. HIH.

Khalid.

On Wed, Jun 22, 2011 at 5:38 PM, Scott Johnston <address@hidden> wrote:

Hi Khalid,

Did you ever get this straightened out? I am trying to do the same thing. Did you ever get a work out the phase ambiguities?

Thanks for any info

Scott

Khalid Jamil wrote:

Please correct me if I am wrong. I think that this random channel-channel phase offset at each acquisition depends on when each usrp 100MHz LO locks to 10MHz external frequency reference by PLL. How PPS will help in that case? Unless PPS is locked somehow to 10MHz clock?

Is there a good example to follow, may to start with 2 or 4 channels?

So grateful for your kind help.

Khalid

On Tue, Apr 26, 2011 at 8:23 PM, Josh Blum <address@hidden <mailto:address@hidden>> wrote:



   On 04/26/2011 09:36 AM, Khalid Jamil wrote:
   > The GRC file is attached. Here i am showing it scope sink with six
   > channels.
   >

   The trouble is that you need to synchronize the time across all N
   devices, and tell them to start streaming at a specified time.

   So, if you want to synchronize N devices in GRC, every device needs to
   all go into the same "uhd usrp" block; and specify the "sync unknown
   pps" and "external reference" options.

   Outside of GRC, if you wanted to use multiple device handles, you can
   accomplish this by catching the pps edge and calling set_time_next_pps
   for each handle.

   -Josh

--
Scott Johnston
MIT Lincoln Laboratory
244 Wood Street, Lexington, MA 02420-9108
(781) 981-8196
address@hidden