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.griffi...@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 <mag...@rubidium.se> wrote: > > > > > > Hi, > > > > On 2020-01-15 23:34, Attila Kinali wrote: > > > On Wed, 4 Dec 2019 09:40:34 -0000 > > > <mar...@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 > > > > > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@lists.febo.com > > To unsubscribe, go to > > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.