[ntp:questions] NTP Drifts +ve and -ve

Unruh unruh-spam at physics.ubc.ca
Wed Aug 20 22:42:26 UTC 2008


"Maarten Wiltink" <maarten at kittensandcats.net> writes:

>"Unruh" <unruh-spam at physics.ubc.ca> wrote in message
>news:yoWqk.8851$%b7.4796 at edtnps82...

>> [...] (actually since it is
>> a second order critically damped system, this is not really accurate. The
>> correction action goes to zero faster than that, overshots by something
>> like 20%  and then comes back to zero). ...

>Never thought I'd be picking nits about this, but isn't that a strongly
>damped system? ISTR critical damping being defined as not overshooting.

A critically damped system is one whose solution is (A+Bt) e^(-gt)
If B is negative it overshoots. If B is 0 it approaches 0 as rapidly as
possible. If B is positive it may actually increase before it decreases.
My vague recollection is tha tthe parameters were chosen for ntp to be
critically damped, but the initial conditions are in general such that B is
negative.  An underdamped system will always have oscillations (infinitely
many but decreasing in amplitude. A critically or overdamped system can
overshoot as well, but has the problem that in general it approaches
equilibrium more slowly than a critically damped one.


>Groetjes,
>Maarten Wiltink





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