Dear Ettus team, I probably found a small feature-bug / documentation mismatch in UHD. I use two X310 equipped with a GPSDO. According to the manual at http://files.ettus.com/manual/page_gpsdo_x3x0.html you describe that once the GPSDO is detect upon device creation, it is used as the reference clock ref_clock, see "By default, if a GPSDO is detected at startup, the USRP will be configured to use it as a frequency and time reference.". What I discovered: this is not the case. Although the GPSDO is installed and detected, the internal clock is used by default, and not the GPSDO. My setup to prove this: The first X310 uses a 10MHz reference clock from a caesium clock and acts as a receiver. My second X310 acts as transmitter connected over attenuators to the first X310. The second X310 transmits a burst with known burst-length continuously, and the first X310 receives data continuously for about 1 minute at a sampling rate of 30MHz. The RF carrier frequency is 5.5GHz but that does not matter. Through post-processing I can easily estimate the clock frequency difference between the second X310 and the first X310. Now what I discovered was: * Second X310 configured to use the internal clock: a drift of about 2e-7 to 3e-7 results from different measurement runs. * Second X310 configured to use the GPSDO: a drift of about 2.7e-8 results, which is roughly a factor of 10 better and also fits to the GPSDO specification in the free running case as far as I remember (no GPS lock). * Second X310 configured without a specific clock given, such that UHD chooses the correct clock by default: there I discovered a drift of about 2e-7 to 3-e7 again. The same drift which results from the internal clock. Hence, by default, the GPSDO is not selected as reference clock. Please check in your setup whether my discovery is correct. I use the latest UHD release. Both X310 devices where running for about 1 hour to warm up before taking any measurements. Regards, Emanuel

Hi Emanuel, I think you're right; we do no longer set everything automatically to the GPSDO as soon as we find one on initialization. You can check what is currently selected by querying the get_time_source and get_clock_source calls. We'll fix documentation pretty soon, I'm confident. Right now, you probably want to know how to set things right: http://lists.ettus.com/pipermail/usrp-users_lists.ettus.com/2015-November/016925.html // has the recommended method. Best regards, Marcus On 26.11.2015 15:20, Emanuel via USRP-users wrote: > > Dear Ettus team, > > > > I probably found a small feature-bug / documentation mismatch in UHD. > I use two X310 equipped with a GPSDO. According to the manual at > http://files.ettus.com/manual/page_gpsdo_x3x0.html you describe that > once the GPSDO is detect upon device creation, it is used as the > reference clock ref_clock, see */“By default, if a GPSDO is detected > at startup, the USRP will be configured to use it as a frequency and > time reference.”/*. What I discovered: this is not the case. Although > the GPSDO is installed and detected, the internal clock is used by > default, and not the GPSDO. > > > > My setup to prove this: > > The first X310 uses a 10MHz reference clock from a caesium clock and > acts as a receiver. My second X310 acts as transmitter connected over > attenuators to the first X310. The second X310 transmits a burst with > known burst-length continuously, and the first X310 receives data > continuously for about 1 minute at a sampling rate of 30MHz. The RF > carrier frequency is 5.5GHz but that does not matter. Through > post-processing I can easily estimate the clock frequency difference > between the second X310 and the first X310. > > Now what I discovered was: > > · Second X310 configured to use the internal clock: a drift of > about 2e-7 to 3e-7 results from different measurement runs. > > · Second X310 configured to use the GPSDO: a drift of about > 2.7e-8 results, which is roughly a factor of 10 better and also fits > to the GPSDO specification in the free running case as far as I > remember (no GPS lock). > > · Second X310 configured without a specific clock given, such > that UHD chooses the correct clock by default: there I discovered a > drift of about 2e-7 to 3-e7 again. The same drift which results from > the internal clock. Hence, by default, the GPSDO is not selected as > reference clock. > > > > Please check in your setup whether my discovery is correct. I use the > latest UHD release. Both X310 devices where running for about 1 hour > to warm up before taking any measurements. > > > > Regards, > > Emanuel > > > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com