[time-nuts] Low-long-term-drift clock for board levelintegration?

Chris Albertson albertson.chris at gmail.com
Tue Feb 21 18:05:04 UTC 2012

On Tue, Feb 21, 2012 at 9:16 AM, Bob Camp <lists at rtty.us> wrote:
> Hi
> The only saving feature on a local LAN is that you get to run a *lot* of
> data if you wish to.

Also, by definition a "local LAN" does not have a router.

BTW, Is the O.P. still here?   I wonder if he is re-thinking his
requirements.  I think his plan was to do timing by comparing the
real-time time stamps at each end of a network connection.   That
would require  very, very good time keeping that is not even possible
without GPS.   Perhaps he has now given up on that approach and has
gone to using relative time and round trip timing which would work
even using a $2   TTL "can oscillator".

If the OP is still here, try this analogy:   You want to know how long
a swimmer takes to swim the length of the pool.   There are two

(1) Your method: Have two coaches, one on each end of the pool each
with a wrist watch.  The first coach says "go" and notes the time.
The second coach waits for the swimmer then writes down the time when
the swimmer touches the end of the pool.  Later you subtract time 1
from time 2.    This can work but requires some very good watches that
are in perfect sync

(2) Easy method: First coach says go and starts a stop watch.  Second
coach yells "done" when the swimmer reaches the end and the first
coach stops the watch.   No precision time sync is required but we do
need a backward communication path and we need to know the delay in
that path.   The first coach can measure the round trip delay by
occasionally yelling "test" and listening for a reply from the second
coach.  Then he assumes the one-way delay is 1/2 the round trip.

I suggest using method #2 for your network timing.   Method #1 is
simply to expensive and technically hard especially on a $300 budget
with "no clue" installers.

Now we can move the discussion to measuring the speed of the backwards
channel with some method better than just taking 1/2 the round trip.

Chris Albertson
Redondo Beach, California

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