[time-nuts] Frequency over fiber (was WWV and legal issues)

Bob kb8tq kb8tq at n1k.org
Sun Sep 2 09:08:12 EDT 2018


> On Sep 2, 2018, at 7:17 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> Hi Gerhard,
> I see that this became a separate thread.
> On 09/02/2018 01:38 AM, Gerhard Hoffmann wrote:
>> Am 01.09.2018 um 20:40 schrieb Magnus Danielson:
>>> One should first know that there is a lot of papers now on frequency
>>> transfer over fiber. The stability achieved on the best ones so far
>>> greatly below that of the optical clocks that they want to compare.
>> Please, in a nutshell: what are the worst offenders:
>> - tranceivers (mechanical, temp, other misfeatures)
>> - cables ( bending, temp, mechanical stress)
>> - others?
>> In the case of transceivers: are there desirable modifications
>> that would alleviate the problems?
> Acoustical sensitivity, low frequency changes.
> For optical clocks and frequency transfer, just the vibration from
> traffic and other activity causes disturbances which disturbes the group
> delay. What is done for these links is to actively compensate then using
> a return path and closing the loop with a controller, very much like a
> PLL. The length of the loop limits the bandwidth and hence how high up
> the compensation can be done, so for longer stretches, this needs to be
> repeated. They have now built links from PTB to SYRTE and NPL.
> Temperature shifts are slower, but also compensated though the active loop.

I suspect there’s a longer list of “slow” environmental effects that are also taken 
care of with the compensation setup. One would guess that crossing a active  
fault line would be “interesting”. 


> Close proximity to strong power-currents have also been shown to cause
> modulations, so separate from power-cables if you can.
> Remember that the end nodes have very stable clocks, so their effects
> can be taken out of the equation. For other setups, such as telco
> operation, that's a completely different ballgame.
> Cheers,
> Magnus
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