[time-nuts] 1 pps Accuracy in two locations

[Bruce Griffiths]

PSOF fiber has a much lower tempco:
https://accelconf.web.cern.ch/accelconf/p01/PAPERS/MPPH011.PDF

Bruce
> On 17 January 2020 at 00:08 Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
> 
> 
> 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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