[time-nuts] FLL errors
Magnus Danielson
magnus at rubidium.dyndns.org
Sun Aug 30 02:47:04 UTC 2015
Hi,
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.
Cheers,
Magnus
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