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

Poul-Henning Kamp phk at phk.freebsd.dk
Thu Jul 6 06:49:28 UTC 2006


In message <20060706064003.72211BDF0 at ip-64-139-1-69.sjc.megapath.net>, Hal Murr
ay writes:

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

That's how you do it.  You can either have one end fixed (free-running)
and the other end slaved to it, or you have have both goal-seeking with
twice the timeconstant.

The latter is the easier way since the setup i symmetric:  Both ends
attemt to reach the same offset between received time and local time.

You can either do it by having the both strive for zero offse and
let the mutual fight find the average for you, or you can have
communication between them to sort out the goal value.

Striving for zero is simpler (you don't need to actually measure
the offset, an analog solution will do) but more noisy and requires
longer time constants gain.


-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.




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