[time-nuts] looking for good description/generalized model for time adjustments

[Lux, James P (337C)]

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of Christian Vogel
Sent: Wednesday, July 29, 2009 11:51 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] looking for good description/generalized model for time adjustments

Hi Lux,
> The real problem is that the "master" clock might be a crummy 50ppm XO, and all us "slaves" all have TCXOs that are good to a few ppm.  It's almost the inverse problem from disciplining a XO with GPS.
>   
I have the impression that you are confused by your own weird 
terminology ;-). If you have a rather good clock yourself, and want to 
somehow characterize external events with their own sketchy time-of-day 
estimation, then please don't call this cheap thing the "master".


---> But it *is* the master, by fiat.   Here's a scenario: A schedule is published that says:
(MT = Master's time)
At 12:00.001MT Box A puts out a pulse
At 12:00.002MT Box B puts out a pulse
At 12:01.001MT Box A puts out a pulse.

Box A and Box B MUST follow Master Time, no matter how crummy it is. 



Just let your internal clock ("precise") run undisturbed and capture the 
precise time for all the external events received from the "cheap" 
outside machines. Then timestamp a number of external/internal events 
and calculate the average relative external clock frequency. Filter the 
last N events and use the relative frequencies to extrapolate events to 
be expected in the future. It's basically just a PLL done in software.

--- That is the general solution.  What I'm looking for is a bit more formalism (presumably someone has studied this and done a paper?)  And, (so I don't have to write them) if someone has done some nice figures and examples..


--specific example..

A Single board computer with non TCXO clock (on order of 10ppm variability over short term) is the "system controller" and sets the time schedules.  It sends commands to devices which have TCXO clocks (on order of 1ppm short term variability) saying "at time X do action Y".  It also periodically sends messages to the devices saying "At the tone, my time is Z".

What we would like is a) some algorithms to do this (and NTP type PLLs are a decent way) and b) some formalism and rigor to choose operating parameters (e.g. update every 1 second or every day or whatever)