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 <scott.johns...@ll.mit.edu>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 <j...@ettus.com <mailto: >> j...@ettus.com>> 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 > scott.johns...@ll.mit.edu > >
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