[time-nuts] Holdover

lincoln lincoln at ampmonkeys.com
Fri Aug 19 14:48:16 EDT 2016

Hello Bob,
	At work we make internet clock that are governed by a number of ITU and IEEE standards. As others have stated correctly, there is no standard. There are a number of behaviors that can be configured depending on application.

	1. Always a PPS out, TOD string carries a valid or "do not use" flag - default, actually used about 1/3 of the time
	2. Output only when clock is "synced" , That is  the error is small and the slope of the changes made is small, for at least 4 observational windows. 
	3. Like number 2 but adds the notion of Holdover. That is when sync is broken, if you had been synced for "enough" time, keep output if you hold over counter has not expired. 

Now what to do when your source is re-established..
	If the offset is large, or if the PPS has been squelched, or the reference source has changed, jam the 1pps, and steer from there. 
	If the offset is small, ( some ┬Ás, ) steer the 1pps , with the max rate of steering set by how much we can screw up the frequency, default is 10 ppb  
	Loop bandwidth is set by the source, PTP / IEEE 1588 sources bandwidth can be as narrow as  1~3 mHz , GNSS has much wider bandwidth, 
	All of the clock class mappings, limits and thresholds are configurable at some level. 	


On Aug 15, 2016, at 21:35 , Bob Stewart <bob at evoria.net> wrote:

> It's been pointed out to me that I didn't understand the function of the 1PPS of a time standard.  I confess that somehow I had confused the term to be timing standard; which would be an entirely different thing.  But, this is time-nuts, so I should have realized...
> Anyway, is there a standard, or at least an accepted practice, for how holdover is handled in a time standard?  Not "how it's done", as in algorithms, but what is expected by the user.  I can see at least 2 ways: time warping (which would be especially bad if the time standard had gotten ahead in time) and nudging back to the correct time.  The case of warping is obvious.  But, are there other methods, and is there some standard for how quickly the time output of the time standard, and of course the 1PPS pulse, is nudged back to the correct time?
> Bob - AE6RV
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