[time-nuts] GPSDO & Crystal Aging

Chris Albertson albertson.chris at gmail.com
Fri Apr 11 19:38:37 UTC 2014

Look at what NTP does to select "good" clocks when it has many to choose
from.   It does not simply average them.

It looks at the noise in each one and then sees which clocks have
overlapping error bars.  It assumes that all good clocks have the same time
within limits of their precision.   Then from the good clocks there is a
second level weeding out process then finally it does a weighted average of
the remainders where I think those with less jitter get more weight.

It would not be impossible to do this with 10MHz oscillators.   Certainly a
simple average is not a good idea as a broken unit can pull the entire
average way down.  I think you'd have to check reasonableness first and
eliminate outliers   I think today you might simply digitize the signals
and figure out which were best using software.

In short the output is "ensemble time" (not "average time") but there is a
careful selection of who is allowed to be  member of the ensemble.

I used a joke last week to explain to a class why we don't use averages,
with no other qualifications.  The joke is "Bill Gates walks into a bar....
 What's the average net worth of everyone in the bar?  Maybe $250 million."

My point was that it is hard to describe a population that is not Gaussian
distributed.  "Stuck" and jumping crystals are not Gaussian.  You'd have to
detect the misbehaving devices.

David Allan had
> this interesting concept to the effect that if
> you had a sufficient number of wristwatches
> (maybe 1000) and you averaged them together
> you could somehow get a quality clock, or at
> least 31.6 times better.  Kind of like the
> notion of 1000 monkeys with 1000 typewriters...

Chris Albertson
Redondo Beach, California

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