[time-nuts] Project on precise timing over Ethernet

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Sep 11 20:50:01 UTC 2009

Hal Murray wrote:
>> Actually, you can see this as a Shannon information channel, analog or
>> digital.
> What sort of bandwidth do I need to run a PLL over a long link?
> I assume it takes enough to cover all the sources of error:
>   drift in the master
>   drift in the local oscillator
>   drift in the communication link
> Does it depend on my target accuracy?  Do I need more or less bandwidth for a 
> better answer?  Am I even asking a sensible question?
> What sort of changes in delays to I get on 10 km of fiber or coax or twisted 
> pair?  Is temperature the only interesting variable?
The delay tempco of a transmission line depends primarily on the thermal
characteristics of its dielectric.
The thermal expansion of the conductors can also be important.
Conventional coax has a delay tempco in the 50-100ppm/C range unless
exotic dielectrics such a fused silica powder are used.
With a plastic dielectric water diffuses lowly into and out of the
dielectric with a long time constant unless the cable is hermetically
> Can I measure tides with a 10 km link?  Or is that swamped by temperature?
> How about microwaves?  Does the speed of light in air change with temperature 
> or humidity or whatever?
Yes the speed of light (and microwaves) depends on temperature, humidity
as well as pressure and Carbon dioxide content.


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