[time-nuts] Frequency over fiber (was WWV and legal issues)
magnus at rubidium.dyndns.org
Sun Sep 2 07:17:20 EDT 2018
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.
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.
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