[ntp:questions] NTP Drifts +ve and -ve
David L. Mills
mills at udel.edu
Thu Aug 21 17:25:23 UTC 2008
Not quite. The loop filter is a second-order polynomial with damping
factor as described in rfc-1305, the web documentation and my book. The
coefficients are chosen for a slightly underdamped characteristic
yielding an overshoot of about 7 percent at all poll intervals.
> "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.
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