[time-nuts] FLL errors

Bob Camp kb8tq at n1k.org
Sat Aug 29 16:49:39 EDT 2015


Interesting question ….

> On Aug 29, 2015, at 5:24 AM, Neville Michie <namichie at gmail.com> wrote:
> A PLL locks on to the nearest cycle,
> is a Time Locked Loop different?

At the most basic level, no. Phase is already commonly looked at in units
of time. Yes that’s a bit odd at first. ADEV (for example) is a phase based
measure that uses units of time.  

> 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.

Which creates all sorts of issues in the loop. GPS time rather than UTC (with leap seconds )
is used in every GPSDO system I’ve seen for this reason. 

> 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?

Because you *always* get into application specific details, sure. As a more 
general concept: 

Time locked loop -> has a static time error in a gain only loop. 

In your example of leap seconds, that’s going to require something a bit different
than a classic PLL. In the non-leap second case (GPS time) it’s effectively what
is done in a GPSDO. There it is commonly referred to as phase lock. Since some
of the details there are a bit weird, maybe a new term is called for. 


> Cheers,
> Neville Michie
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