[time-nuts] Dithering vs. locking all the clocks to the OCXO?

Hal Murray hmurray at suespammers.org
Thu Jul 6 02:40:02 EDT 2006


> With the fibre-based two-way time transfer. For shorter distances you
> can do well on coax, but for the distance range you require you really
> want to go fibre. That world is a bit different but can be made sense
> off. 

How do I setup 2 clocks so they are ticking within 1 ns of eachother?

If both clocks were next to eachother on a bench, I'd connect them to a scope 
(or equivalent box) and adjust a knob until the signals lined up.  Note that 
1 ns is small enough so that I have to make sure the cables are the same 
length and the amplifier delays are matched.  I can swap inputs to check that.

But what if the clocks are 10 km apart and all I have is two fibers between 
the sites?  Is there some common recipe for synchronizing this setup?

Let's assume the fibers are the same length.  I'm not sure that's accurate at 
the ns level.  You can probably swap fibers to check.

One approach is to make a symmetrical setup: send your signal to the other 
site, compare the signal from the other site with yours, adjust one knob 
(pick one) until the offsets match.  That's ugly since you now have to 
measure an offset rather than tune for a null.  Is there a way to avoid that?



What's the thermal coefficient of delay for fibers?


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