I'd like to assume that *OctoClock-G*'s 10 MHz output is good to use as 10 MHz input on B210 (and other) USRP boards, given who the vendor of OctoClock-G is. If this is a bad assumption, please correct me. if this is a good assumption, then I'll use the OctoClock-G specs <https://www.ettus.com/content/files/Octoclock_Spec_Sheet.pdf> to infer what a good 10 MHz input for USRP board looks like. Here's what said specs say about the 10 MHz signal produced: * 1.4 Vpp, * square wave, * 50 ohm impedance. I'll assume a good 10 MHz source has to match those specs. Comments?
Thanks, Dario On Tue, Oct 3, 2017 at 10:49 AM, Vladimir Rytikov <kk6...@gmail.com> wrote: > Dario, > > 10 MHz signal input is meat to be freq reference. If a radio could tell > that 10 MHz is not 10 MHz - there was no need to feed it externally. USRP > boards seem have +/1ppm freq stability. 1ppm means when you tell radio to > tune to 100 MHz - it might end up giving you 100 MHz + 100 Hz or might end > up at 100MHz - 100Hz or actually anywhere within that range and even jump > around depend on temperature for example. When you tell radio go to 2 GHz > it might go to 2 GHz +/- 2 kHz. it is not a big deal for some applications > while for other applications it is a problem. > > there is some mentions of the spec for 10 MHz input: > https://kb.ettus.com/B200/B210/B200mini/B205mini > > based on AD4001 spec - it says it is CMOS square wave: > http://www.analog.com/media/en/technical-documentation/data-sheets/ > ADF4001.pdf. and reference signals goes almost directly to AD4001. > Power supply in B200 seems 3.3V. 2.9 V for min of logic 1. > and 0.4 V for max of logic 0. which makes me wonder what the USRP spec > means by mentioning higher voltages. In any case they seem have protective > diod which will clip the signal and probably add extra distortions to your > reference freq. > > > PPL on the radio will be happy to lock to whatever it gets and it will > treat it as 10 MHz. somehow could trigger however you are really far off - > like 20 MHz instead of 10 MHz. You can probably can build a counter (in > FPGA) which will run for a week off ref freq and off on board freq and see > of you reference is doing worse than 1ppm or so. > > Here is the video which tells you why devices have 10 MHz reference input > - that is probably should clear some of your questions: > https://www.youtube.com/watch?v=I55uLRRvLCU > > > there is another spec for clock - it's jitter. that one seems way more > important. jitter hurts you in a way that you are not just off freq, but > your freq is constantly jumping around in some kind of random-ish pattern. > I see the USRP spec for it - however I can't really comment on that. > > > Thanks, > Vladimir > > > On Tue, Oct 3, 2017 at 8:44 AM, Dario Fertonani via USRP-users < > usrp-users@lists.ettus.com> wrote: > >> I'm testing the behavior of B210-based systems, comparing the performance >> with "internal" and "external" (10 MHz) clock source. Expect for the >> following "is the 10 MHz input actually present" check running when the app >> starts, the two branches share the same code. >> >> rfBoardPtr->set_clock_source( "external" ); >> sleep( 2 );//give board time to lock >> if ( rfBoardPtr->get_mboard_sensor( "ref_locked" ).to_bool( ) == false ) >> { >> throw std::runtime_error( "Unable to find a valid 10 MHz reference >> signal. Please check that the signal source is properly plugged in." ); >> } >> rfBoardPtr->set_time_unknown_pps( 0.0 ); >> >> >> Besides that check, is there a way of measuring the quality of the signal >> via (UHD) software API, ideally in a more granular way? The check above >> "passes" even when the input signal is poor, which I see by validating >> through external instruments the quality of the radio signal emitted by the >> board. Ideally, I'd want an API that tells me about such problems before I >> actually check the radio output. To be clear, these are relatively-minor >> radio issues, but are sufficient to reduce the DL peak rate of my LTE >> system from 150 Mbps to 50-100 Mbps with respect to a fully-functional >> board (either fed by "internal" clock source, or by a proper 10 MHz >> source). The quality of the radio output varies noticeably (at least when >> measured with advanced full-stack metrics) when I change the amplitude of >> the 10 MHz reference, which is surprising since said changes are within the >> recommended range of the 10 MHz reference. Could someone please confirm >> said specs, in terms of (peak-to-peak) amplitude and waveform (square, >> sine, ...)? >> >> Thanks, >> Dario >> >> _______________________________________________ >> USRP-users mailing list >> USRP-users@lists.ettus.com >> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >> >> >
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