Thanks for the clarification. We are using GPSDO to provide 10 MHz/PPS to the USRPs. Jackson Labs OCXO for the X310 and TCXO for the B210. For E310 we just use the GPS antenna which I guess is connected to a ublox module.
Best, John On Thu, Feb 3, 2022 at 11:42 PM Marcus D. Leech <patchvonbr...@gmail.com> wrote: > On 2022-02-03 15:35, John Hodgins wrote: > > Thanks for the prompt response Marcus. Should the discrepancies express > themselves as a constant offset in cross-correlation or can the delay > measurements also oscillate in a volatile fashion? For B210-E310 we see a > more constant looking offset whereas B210-X310 is just wild. > > Don't forget that the master-clock is quite different between B210 and > X310--I assume that you're sharing a common (10MHz) reference? > > > Also I remembered this discussion > <https://lists.ettus.com/empathy/thread/2H5WCI2MEWWJKXHXGPMK5UWQXETQ4JRW?hash=L7PPGLOU2FJDCA4ELC2TXJZW75WJQX4C#L7PPGLOU2FJDCA4ELC2TXJZW75WJQX4C> > from sometime ago where rising vs falling edges were used for different > USRPs. > > Yes, I think the X310 uses falling-edge, which will make it not-align with > any of the other units that use rising-edge. I think R&D > is revisiting this decision. > > > > Given your answer, can we say that for timed commands it is not a good > idea to mix and match different USRP series? > > Absolutely. Realistically, it has never been a design goal to provide > precise alignment among different types of USRPs. > > > Best, > John > > On Thu, Feb 3, 2022 at 11:27 PM Marcus D. Leech <patchvonbr...@gmail.com> > wrote: > >> On 2022-02-03 15:17, John Hodgins wrote: >> > Hi there, >> > >> > We have a simple set up where we collect iq snippets from 3 usrp's at >> > the same rx-times via timed commands. We introduce delays between the >> > usrp's via a reference delay box and try to recover the introduced >> > delays via cross-correlation. >> > >> > Things work great when all the rx's belong to the same USRP family. >> > For instance we have been able to recover the correct delay for all >> > B210 (with GPSDO/TCXO) or all E310 (with GPS antenna) scenarios. >> > However, when we mix different USRP's, the delays computed via >> > cross-correlation no longer make sense. >> > >> > For instance, when we use 1 B210 and 2 E310's the delays between B210 >> > and E310's just oscillate wildly but the relative difference between >> > E310 remains relatively stable. We tested 2 B210's with a single X310 >> > (with GPSDO/OCXO) as well and we see the same problem as well. The >> > interesting thing is though visualizing the spectrum of collected >> > snippets from different USRPs show pretty much the spectrum of the >> > known applied signal. >> > >> > Is there some fundamental discrepancy between different USRP families? >> > Or maybe UHD somehow interprets samples differently for different >> > USRP's (flipping i and.q, different rx-time, pps implementations, >> > some precision issues maybe)? >> > >> > In any case I would be grateful if anyone can shed some light onto >> > this behavior. >> > >> > Best, >> > John >> > >> > _______________________________________________ >> > USRP-users mailing list -- usrp-users@lists.ettus.com >> > To unsubscribe send an email to usrp-users-le...@lists.ettus.com >> The group delays between USRP families will be different due to >> different FPGA implementations, the exact "placement" of time-of-day >> registers, etc. >> >> It would be very difficult to guarantee phase-coherence across different >> USRP device types, because the implementations are different--not just >> the FPGA bits and >> pieces (DUCs, DDCs, etc), but also the analog stages will be different. >> >> _______________________________________________ >> USRP-users mailing list -- usrp-users@lists.ettus.com >> To unsubscribe send an email to usrp-users-le...@lists.ettus.com >> > >
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