Sorry for not specifying, but I was talking about two N310 or N320 devices. Do you know how it is in that case?
Am Fr., 19. Juli 2019 um 16:58 Uhr schrieb Marcus D. Leech via USRP-users < usrp-users@lists.ettus.com>: > On 07/19/2019 05:24 AM, Sammy Welschen via USRP-users wrote: > > Thanks for your reply. > > I am a bit confused now. Since the LO for this stage is derived from the > sample clock, wouldn't I be in the same situation as if I only shared 10 > MHz reference and PPS signals? > Quote from https://files.ettus.com/manual/page_usrp_n3xx.html: > > My understanding was that we were talking about the N320, and only a > single unit? > > I need to confirm, but I think the upconverter LO on the N320 is a single > (clock-derived) oscillator, shared among the channels in the unit. In that > case, the > phase relations will be static across powerups, assuming that you share > the other LOs. > > In the case of multiple N320s, I don't yet have an answer, since it > depends critically on how that upconverter LO is produced, and how the > external-reference > PLL works. > > > > ---------------------- > > Reasons to use an external LO include: > Improving phase alignment: The N310 itself has no way of aligning phase > between channels, and phase will be random between runs. By applying an > external LO, the phase ambiguity is reduced to 180 degrees, produced by a > by-2 divider in the AD9371 transceiver IC. > Improving phase noise: The quality of the onboard LO depends on the > external reference clock, among other things. By providing a custom LO > signal, it is possible to more accurately tune, assuming the externally > generated LO signal is coming from a high-quality oscillator. > ---------------------- > > I would still have a certain fixed phase relation between channels, but > with more fluctuations and a phase difference that changes on every turn on > of the devices (i.e. the same situation as when sharing 10 Mhz and PPS > signals). See for example the plots on pages 24-25 of > https://forums.ni.com/ni/attachments/ni/grp-1008/110/1/Fundamentals%20of%20Phase-Coherent%20RF%20Measurements.pdf > for an illustration of what I mean. > > Am Do., 18. Juli 2019 um 19:25 Uhr schrieb Marcus D Leech via USRP-users < > usrp-users@lists.ettus.com>: > >> >> >> >> I have just been corrected by one of my colleagues at Ettus. >>> >>> While there is an up conversion stage for frequencies below 450Mhz, the >>> LO for that stage is fixed frequency, and derived from the sample clock and >>> coherent across channels. >>> >>> So there should be no random phase offset among channels even below >>> 450Mhz when LO sharing. >>> >>> This is correct as far as I know. Although I don’t have an N320 in my >>> lab, it uses an up conversion scheme for lower frequencies. That scheme >>> does not participate in the LO sharing. >>> >>> Sent from my iPhone >>> >>> >>> On Jul 18, 2019, at 11:17 AM, Sammy Welschen via USRP-users < >>> usrp-users@lists.ettus.com> wrote: >>> >>> I have to acquire phase coherent data on multiple channels using USRP >>> N310/320 devices. >>> >>> >>> >>> Am I correct in assuming that for frequencies below 450 MHz, there is no >>> way to remove random phase differences between channels due to the >>> additional frequency shift involved (shown for example in the block >>> diagram >>> http://www.ettus.com/wp-content/uploads/2019/03/N320BlockDiagram.png >>> <https://urldefense.com/v3/__http:/www.ettus.com/wp-content/uploads/2019/03/N320BlockDiagram.png__;%21fqWJcnlTkjM%219UAjRx1UqNGGgw_MyGxeNp4dcd08afk3IjpbsIlbrDYEk9H2AfliCLM52OcRdC4$>)? >>> As I understand it, by using the same LO signal for all channels I could >>> remove the differences for frequencies above 450 MHz, but this is of no use >>> for frequencies below 450 MHz due to this the additional stage. >>> >>> _______________________________________________ >>> USRP-users mailing list >>> USRP-users@lists.ettus.com >>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >>> <https://urldefense.com/v3/__http:/lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com__;%21fqWJcnlTkjM%219UAjRx1UqNGGgw_MyGxeNp4dcd08afk3IjpbsIlbrDYEk9H2AfliCLM5ZdVPAfg$> >>> >>> _______________________________________________ >>> USRP-users mailing list >>> USRP-users@lists.ettus.com >>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >>> <https://urldefense.com/v3/__http:/lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com__;%21fqWJcnlTkjM%219UAjRx1UqNGGgw_MyGxeNp4dcd08afk3IjpbsIlbrDYEk9H2AfliCLM5ZdVPAfg$> >>> >>> _______________________________________________ >> USRP-users mailing list >> USRP-users@lists.ettus.com >> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >> > > > _______________________________________________ > USRP-users mailing > listUSRP-users@lists.ettus.comhttp://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >
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