Marcus D. Leech wrote: > On 2022-09-08 04:54, Aiden Morrison via USRP-users wrote: > > > Hello, > > > > I am attempting to do synchronized multi-site reception using USRP > > N210 radios. Our first attempt was to make use of 2x radios with ettus > > GPSDO modules installed, feeding the radios with a common PRN sequence > > and correlating the outputs to evaluate timing offset. The Ettus GPSDO > > radios say they provide +/-60ns timing accuracy, so even in the worst > > case where one of the radios goes to each side of the limit, at 10 MHz > > we would have slightly more than 1 sample of offset between them, but > > in testing we observed +0 to +3 samples of offset. > > > > To isolate the problem we repeated the test with two different N210 > > radios fed by a common external 10 MHz and PPS signal from a GNSS > > disciplined Rubidium source, and observed exactly the same +0 to +3 > > samples of shift between the captured sequences. Seeing the same error > > when using a common clock indicates the problem is very likely to be > > in our test software. > > > > In our software we follow > > https://files.ettus.com/manual/page_sync.html for synchronization: > > > > 1. We create two USRP objects, one for each radio. > > 2. We command the USRPs to use the external synchronization (radios > > report back that they are synchronized to the GPSDO in the former > > configuration) > > 3. We use a common time in the time spec commands for each > > configuration change (e.g. setting LO frequencies) > > 4. For each burst capture of 50,000 samples we set a time_spec for > > each USRP a long time in the future (1.0 seconds since the last > > capture) > > 5. Step 4 is repeated 1000 times giving us 1000 bursts for evaluating > > the synchronization > > > > The only factor I can see is that we are using WBX radio modules, and > > we are uncertain if this is somehow a factor in what we are seeing. > > > > Thanks in advance for feedback. > > > > \-Aiden > > > > ---
Hello, Aiden was talking about time offset not phase offset. WBX phase ambiguity is important information but a bit off-topic. As far as I remember GPS synchronization for N2x0, the numbers from Ettus specs are a bit optimistic. It seemed what they are putting there is standard deviation of the time difference not max/min. Then it would make sense. But I didn’t try to synchronize N2x0’s for some time (many years) so I’m not sure. Maybe something changed. Best Regards,\ Piotr Krysik
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