On 09/16/2012 10:30 PM, Poul-Henning Kamp wrote:
In message<34d5c3ce-6b3d-4944-996a-7637373b2...@gmail.com>, Dennis Ferguson wr
ites:
I'm not sure there could be a difference between the goals of
frequency accuracy and time accuracy that would effect that time
constant.
It does.
A PLL more or less corresponds to an "PI" regulation, where a FLL
only needs to have the "I" term.
Because you don't have the interaction between the P and I terms,
the I-timeconstant can be longer.
The balance between P and I is important to establish the damping of
your PI regulator.
A good PI-based PLL actually combines the FLL and PLL domains by
differentiating the phase detector and feeding that through a scaling
into the integrator, adding to the phase-detector scaled by the I
factor. That way you can get very good pull-in properties which then
gently goes over to PLL properties. When PLL lock is achieved the FLL
scale-factor can be removed, as it only contributes noise.
A strict FLL would have the differentiated phase scaled and added into
the frequency steering, after the PI-regulators integrator. This D term
would set the frequency right, but the integrator would not learn the
frequency as quick and there would be tracking errors until it learns.
This differentiated phase aided integrator solves the bad pull-in
behaviour for the case when the reference signal and the oscillators
signal is far apart.
Cheers,
Magnus
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
