[time-nuts] 1 pps Accuracy in two locations

[jimlux]

On 12/4/19 8:49 AM, Magnus Danielson wrote:
> Hi,
> 
> On 2019-12-04 16:55, Tom Van Baak wrote:
>> Martyn,
>>
>>> I'm always being asked to provide equipment that can produce two 1
>>> pps outputs aligned to each other to within a few ps.
>>
>> They should look at their best 1PPS on a 'scope. You can get ns with
>> care; I doubt ps is possible. I mean, that's THz BW isn't it?
> No, you do not need THz BW for ps level timing precision, you can
> achieve it in the microwave region for sure. Stabilizing it to that will
> be a pain regardless, but doable given that the media and environmental
> conditions is good enough.



Think of this as a "phase knowledge" question.  The phase uncertainty of 
a sine wave in noise is related to the SNR.

In time terms tuncert = 2*pi*f * sqrt(1/SNR)

So you can compute the noise (probably mostly thermal noise in the 
measurement band), you know your signal level, and then you're all set.

For, say, 100 MHz reference, SNR = -20*log10(628E6 * 1E-12)  = 64 dB








>>
>> Can you share with us what their application is?
> This is indeed a good question. What is the actual requirements and
> conditions.
>>
>>
>>> So they are asking for two of my GNSS frequency standards with 1 pps
>>> outputs.
>>>
>>> The 1 pps outputs being derived from the rubidium oscillator (which
>>> is aligned to GPS/GNSS)
>>>
>>> The best I think I can achieve is in the low ns range.
>>
>> Right. It will be ns, not ps. Forget about using GNSS for ps level
>> timing.
> Sub ns is possible but painful, but you are not deep sub ns. This is a
> very well studied issue for national timing laboratories and part of the
> fundament keeping these labs tied to the major labs to compare for the
> full EAL/TAI/UTC time-scale contribution. Major labs also have two-way
> satellite links, which is better.
>>
>>
>>> Does anyone know how this can be achieved?
>>
>> Google for papers by high-end national timing laboratories. Words
>> like: active temperature stabilized (phase stabilized) bidirectional
>> optical fiber links.
>>
>> Very possible, very expensive, quite common now. I'd guess most of the
>> timing centers in Europe are linked this way.
> 
> Well, Europe has come further on the optical links than US, but not a
> dominant feature, even if the ambition is there. The map shown regularly
> looks somewhat more connected than reality gives. I go to these
> conferences, and see the progress. Also, there is two types of links,
> the frequency links (now stable down to 1E-19), and the time-links. For
> time you end up with lots of issues that you can just ignore as you do
> frequency, essentially you need to re-learn. For distances like these,
> look at White Rabbit for sure. It is being run for some links on long
> distances and it is pretty good, but for big-scale usage it does not
> make economical sense.
> 
> US/North America has extremely little links being done. The only one I
> know was really reported was from GPS(MC) to UTC(NIST) local to
> Colorado. As I recall it, that was only a temporary setup.
> 
> Cheers,
> Magnus
> 
> 
> 
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