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Re: [Ntp] Antw: [EXT] Re: Temperature Compensation for NTP?

Kurt Roeckx <kurt@roeckx.be> Thu, 10 December 2020 20:09 UTC

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Date: Thu, 10 Dec 2020 21:09:27 +0100
From: Kurt Roeckx <kurt@roeckx.be>
To: Ulrich Windl <Ulrich.Windl@rz.uni-regensburg.de>
Cc: Hal Murray <hmurray@megapathdsl.net>, "ntp@ietf.org" <ntp@ietf.org>
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Subject: Re: [Ntp] Antw: [EXT] Re: Temperature Compensation for NTP?
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On Thu, Dec 10, 2020 at 08:56:27AM +0100, Ulrich Windl wrote:
> >>> Kurt Roeckx <kurt@roeckx.be> schrieb am 09.12.2020 um 20:45 in Nachricht
> <X9Epanr9dQLoSN1F@roeckx.be>:
> > On Tue, Dec 08, 2020 at 03:32:26PM ‑0800, Hal Murray wrote:
> >> kurt@roeckx.be said:
> >> > I think defining NTP as a impulse response is the wrong approach. It's
> an
> >> > implementation detail. If other systems use different algorithms, it
> should
> >> > only have an effect on the uncertainty. If it causes big problems, it's
> an
> >> > implementation problem. 
> >> 
> >> Not quite.  It's easy to get problems when chaining PLLs.  I don't 
> > understand 
> >> it well enough to explain and I don't have a handy reference.
> >> 
> >> The typical problem is that somebody tweaks a time constant so his system 
> >> recovers faster, then months later something crazy happens when triggered
> by 
> > 
> >> the right input data.
> > 
> > As far as I know, NTP's impulse response overshoots to have a
> > faster response. And gettings a chain to work with overshooting
> 
> I'm not deep in that subject, but couldn't a non-overshooting response be
> compensated by feeding samples at a higher rate (smaller "poll")?

I'm not sure what you mean with compensated.

If you have a higher sample frequency with the same impluse resonse,
you will also faster change your phase to match. The same impulse
response at different sample frequency means that it takes the
same amount of samples.

Note that if you want the same impulse response at different
sample frequencies, you need different feedback parameters.

So with a damped / undershooting response and a higher sample
frequency you can have a faster response.


Kurt

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