[time-nuts] FLL errors
Bob Camp
kb8tq at n1k.org
Sat Aug 29 16:49:39 EDT 2015
Hi
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.
Bob
>
> Cheers,
> Neville Michie
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
More information about the time-nuts
mailing list