Andreas, Just a guess: Are both of your X310's using a common 10 MHz reference? If not, this is likely just the difference between the LO's on the two devices. Even tuned to the same center frequency, there will be a slight difference in the LO's. If you feed both X310's a common 10 MHz reference, and tell the X310's to use an external reference, this should go away.
-Daniel On Thu, Dec 13, 2018 at 9:39 AM Enz Andreas via USRP-users < usrp-users@lists.ettus.com> wrote: > Hi all, > > I'm new to the world of USRP and DSP and radio hardware in general. The > problem I have is probably something > very fundamental I missed, but after trying a lot of stuff on my own I > decided to ask here. > Attached pictures hopefully explain what I'm trying to describe in the > text below. > > I'm sending a QPSK pulse train (see pictures) over X310 hardware to > another X310 > connected by a wire (yes, attenuated) and don't understand why my signal > seems > to slowly oscillate. > For a long time I thought this was expected and frequency plus clock offset > correction can get rid of it. But now I think it might be something else, > because the received > constellation keeps rotating (compare last stage of rcvd.png and > rcvd2.png) plus we looked at the > transmitted signal with an oscilloscope and it seems the slow oscillation > is even on the wire, > which would mean the effect does originate from the transmitter. > > > If needed I can provide more information, but I decided to keep this short > because most is probably > unnecessary detail to something obvious I missed. In short I did: > - look at transmitted signal on oscilloscope. Effect seemingly also on the > wire. > - use different devices B210, N210. Same effect. > - physical band pass filter (3.5GHz - 4GHz) on the wire > - try different parameters (also tried with costas loop and cma equalizer) > - try with BPSK, same problem > - try it with a random source instead of vector. Same problem but harder > to see. > - do the PSK demod tutorial (simulated channel, that worked fine) > > > What am I looking at here and how can I try to fix it? > Is this expected behavior and I just did not get the receiver right? Any > pointers? > Could this oscillation originate from the transmitter alone? > > Hopefully this is understandable and mostly relevant information. > Please ask if I should clarify something. > > Thanks, > Andreas > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >
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