[time-nuts] FLL errors

Magnus Danielson magnus at rubidium.dyndns.org
Sat Aug 29 22:47:04 EDT 2015


On 08/29/2015 11:24 AM, Neville Michie wrote:
> A PLL locks on to the nearest cycle,
> is a Time Locked Loop different?

Yes and now. In a signal conveying time, rather than letting a rising 
edge denote "0 degrees of phase" you have some even time measure 
occuring, of some known nominal rate. You know what "time" it was on the 
time-scale, so that you know how much your local replica time-scale is 
off when compared. This time difference does go beyond the nearest 
cycle, but typically for locked situations is the nearest cycle.

Don't ask how I know, I just know.

> If the decoded time from a GPS system is used discipline
> an oscillator then leap seconds would have to have
> a frequency transient to maintain lock.

No, as GPS time in itself does not have leap-seconds, it's nominally the 
TAI time-scale offset. GPS signal conveys the difference between GPS 
time and UTC, and thuse the UTC can be conveyed.

> If you use the output to say drive a radio telescope monitoring
> a distant object you would want Earth’s rotation to be phase or
> sidereal Time locked. I realise that for such a task far more complex
> computation would be required.
> So is a time locked loop a valid concept?

Yes, whenever the enumeration of cycles to some time-scale is relevant.


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