[ntp:questions] Allan deviation survey

unruh unruh at wormhole.physics.ubc.ca
Tue Sep 14 12:52:24 UTC 2010


On 2010-09-14, David L. Mills <mills at udel.edu> wrote:
> Miroslav,
>
> I think we are talking right past each other. Both Chrony and NTP 
> implement the clock discipline using a second-order feedback loop that 

chrony does not use a second-order feedback loop it is a high order, and
variable order feedback loop. It remembers not just the slope and
offset, as does ntp, but also past values of the errors. It is, as far
as I can tell, stable ( poles in the lower have complex t plane.)
The variable order also makes it non-linear.

The high order and the non-linearity both make it very different from
ntp.

> can minimize error in both time and frequency, although each uses a 
> different loop filter. Chrony uses a least-squares technique; NTP uses a 
> traditional phase-lock loop. The response of these loops is 
> characterized by risetime and overshoot, or alternatively time constant 
> and damping factor. If Chrony were designed to have similar risetime and 
> overshoot characteristics and equivalent time constant, when operated 
> under the same conditions (trace 1) it will perform in a manner similar 
> to NTP. That was and is my claim.
..




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