# [time-nuts] Discipline an oscillator with NTP?

Jose Camara camaraq1 at quantacorp.com
Sat Jul 23 19:42:45 UTC 2011

```	I think the original question - "is it possible" has been answered -
yes, it can be (and has been) done.

The real question becomes what specs can one achieve using a
specific feedback loop (and what is the best method to discipline). After
one year of NTP queries, assume you have a 100ms jitter on the network time,
you could at most tweak your oscillator, based on past performance, to 6E-9.
This would be simple for a perfectly stable oscillator just in need of
frequency adjustment. It gets more complicated if it is actually a mere
mortal, aging, drifty oscillator (like most of us). You'd have to start
modeling the drift, drift of the drift, temperature, etc. - similar to what
HP calls 'Smart Clock' to use the NTP only as long interval calibrator and
oscillator drift estimator.
I don't see much use in this exercise, jitter is too high (even
after massaging, averaging, voting, etc.) to get to time-nuts worthiness in
less than weeks or months' time. It is the same as instead of 1 pulse per
second, GPS gave us one pulse per month (but with 10ms uncertainty).

The generic question becomes: given one reference of such Allan
Variance, how can it be combined with another one (of different Allan
Variance spectrum) to generate a device that is better than both (typically
we want the short term stability of one, disciplined by the better long term
of another).

The mathematicians on duty could estimate the best achievable plot
for a sample HP oven osc trained by NTP (with some periodic query,
filtering, etc.)

Same problem as "Can I set my watch by the Rooster's call every
morning?"  (no DST needed here!)

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bob Camp
Sent: Saturday, July 23, 2011 11:18 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Discipline an oscillator with NTP?

Hi

WWV "as transmitted" is massively more accurate than it is as received.
There are a lot of NTP servers out there with offsets in the ms to fraction
of a ms range. Even if your path was perfect, those issues would keep you
from getting to the us level. You could indeed build up some custom servers
and take care of the issue. At least  as I understood the original question,
random servers on the net were the time source. I assume you would pick them
for short path to your location, and then reject any that did really stupid
stuff.

Bob

On Jul 23, 2011, at 2:02 PM, Chris Albertson wrote:

> On Sat, Jul 23, 2011 at 10:40 AM, Bob Camp <lists at rtty.us> wrote:
>
>> There's also the basic "do I trust the server" issue. You can indeed
trust WWV as transmitted.
>
> NTP's clock selection algorithm is pretty good.  If you choose a
> diverse set of servers then NTP will only use the subset of them that
> are self consistent.  "Pool" servers are assigned randomly so even if
> there were many bad servers in the world the chance of randomly
> picking five that were are "bad" in the exact same way is about zero.
> Typically when a server has a problem it does not match another
> randomly selected ntp server.
>
> So I think you can trust the consensus time from a set of five
> randomly selected pool servers.  It would be far easier to spoof WWV,
> just set up a transmitter.
>
> --
>
> Chris Albertson
> Redondo Beach, California
>
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