Brais, I have two USRP here in front of me: B210, B200mini When I run a sample program (C++) which specifies an external reference, the B200mini shows a variable frequency offset. It seems to lock (with high jitter) but then it "jumps" and I would say the offset changes by +-10 Hz for a few seconds, then it stabilizes (and repeats this behavior, forever).
When I run the same exact program with my B210 the phase stays visibly locked and I see no problems with the frequency jumping around. Therefore (I hesitantly say) I believe there is something undesirable going on with the B200mini or something in UHD/firmware. My test setup is thus: 10MHz rubidium reference --> N5182B Vector Signal Generator (BPSK mode, 10kHz symbol rate) --> Ettus B200mini --or-- B210 (same 10 MHz reference attached) Sample code is C++, derived from usrp_recv_to_udp [INFO] [UHD] Win32; Microsoft Visual C++ version 14.1; Boost_106800; UHD_3.13.0.3-0-unknown External reference is specified (and an exception is raised if 10 MHz is not present). My code is filtering and downsampling to baseband. I am viewing the data in a constellation diagram which allows me to see the phase lock or baseband rotation of the BPSK points. I am happy to provide more information to solve this issue. -- Martin Klingensmith On Wed, Jan 2, 2019 at 9:56 AM Brais Ares via USRP-users <[email protected]> wrote: > > Hello, > > We've just bought an AXIOM90 OCXO [1] (actual configuration: 5 V, ±50 ppb, > +7.7 dBm) and we are having trouble configuring it as an external clock > reference on a B200mini. > > All we do in code is set the clock source as external: > > usrp->set_clock_source("external"); > > And loop until it gets locked by checking: > > uhd::sensor_value_t ref_locked = usrp->get_mboard_sensor("ref_locked", 0); > > Once it's locked we capture 60 seconds of a RF sinusoid waveform centered at > the central frequency (100 MHz) and plot its spectrum using Matlab. We would > expect: > > Frequency drift - Internal oscillator [2] @ ±2 ppm = ±200 Hz > > Captured tone shows a freq. offset of +126 Hz, makes sense (see image and > image zoomed in) > > Frequency drift - External oscillator (Agilent E4438C Signal generator, > output power: +10 dBm) > > Captured tone shows a freq. offset of -3 Hz, makes sense, but the tone is > heavily distorted (see image and image zomeed in). > > Frequency drift - External oscillator (AXIOM90) @ ±50 ppb (0.05 ppm) = > ±5.4 Hz > > Freq. offset of +127 Hz, which does not make sense, and the tone is heavily > distorted as well (see image and image zomeed in). > > We've been fighting this for a few days. Any point of view is welcome here. > > Regards and happy new year :) > Brais. > > [1] - http://www.axtal.com/cms/docs/doc86239.pdf > [2] - https://www.ettus.com/content/files/USRP_B200mini_Data_Sheet.pdf > > _______________________________________________ > USRP-users mailing list > [email protected] > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com -- Martin K. _______________________________________________ USRP-users mailing list [email protected] http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
