[time-nuts] 1 pps Accuracy in two locations

[Bruce Griffiths]

Except for some low tempco single mode fibers the delay tempco is on the order of 10ppm/K:
https://library.nrao.edu/public/memos/edtn/EDTN_168.pdf

Bruce 
> On 16 January 2020 at 23:29 Magnus Danielson <magnus at rubidium.se> wrote:
> 
> 
> Hi,
> 
> On 2020-01-15 23:34, Attila Kinali wrote:
> > On Wed, 4 Dec 2019 09:40:34 -0000
> > <martyn at ptsyst.com> wrote:
> >
> >> I'm always being asked to provide equipment that can produce two 1 pps
> >> outputs aligned to each other to within a few ps.
> >>
> >> These two 1 pps pulses are not in the same location and could be 100 metres
> >> to a few km away.
> > As others have written, getting down to a few ps is not feasible, at least
> > not with the amount of money your customers are likely willing to pay.
> > To get down to these levels you will need to pull fibres from one location
> > to another and using special circuitry to activly compensate variation
> > in length due to temperature changes and vibration, even for burried fibres.
> > Just to put into perspective what your customers are asking for: in 1ps
> > light travels 300µm in vacuum/air or ~150µm in fibre/coax.
> 
> Let me correct that a little.
> 
> For fibre the relative dielectrics of the silica glass is just about
> 2.25 giving the index just about 1.5, which then gives the 300 um / 1.5
> to about 200 um. I am known to indicate the length of 1 ns in fibre
> betwen my index finger and thumb, roughly 2 dm, giving the delay for 1 m
> to be about 5 ns, letting the round-trip-time for 1 m be 10 ns which is
> a very handy number for rule of thumb conversions for fibre. If you look
> in more detail, the actual property depend on the wavelength being used
> and the temperature of the fibre, as this changes the actual delay.
> While first degree compensation is trivial in two-way systems, you end
> up having calibration issues.
> 
> Coax is less easy. If you have the normal RG58 crap, it aligns to about
> the same numbers as fiber, as the dielectrics is about the same.
> However, for more phase-stable cables with lower dielectric loss one
> simply has less dielectrics to start with, such as foam or other form of
> support for center conductor. That gives the relative dielectric go
> towards 1 and thus the velocity factor with that. It's much more a "it
> depends".
> 
> Other than that, I agree with the general analysis of Attila, it is
> close to my experience, and I've been working on these things
> commercially for over 10 years now. If you want to know how things works
> (or rather not work) in a telecom, it is even more painful than this.
> 
> So end conclusion being, if you required precision of 1 ps from a timing
> system, you are likely going to have one very expensive system and it
> will be a pain to operate, it may be worth considering if you are doing
> it the right way. I've seen requirements in the 10s of ps for a fixed
> system setup, but that is while challenging kind of doable, but then
> that requires quite a bit of additional control loops and knowing what
> one does.
> 
> Cheers,
> Magnus
> 
> 
> 
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