[USRP-users] 10 MHz reference signal on B210

Dario Fertonani dario.fertonani at gmail.com
Tue Oct 3 14:05:17 EDT 2017


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 <kk6ygb at 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 at 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 at lists.ettus.com
>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>
>>
>
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