On 11/07/2010 03:06 PM, Poul-Henning Kamp wrote:
If you have a microcontroller, isn't it simpler to just look for
convergence in the development of the error term ?
Possibly.
You're probably not in any big hurry for exact convergence anyway,
so overdampen the PLL and simply check that the redidual decreases
towards zero.
Once you have locked the PLL, the fastest way to detect loss of
lock, in particular if you use too high time-constant, is the
too low frequency of zero crossings in the residual.
There are many ways to observe phase-lock.
If you to level detect of the AC part of the beat signal, it will go
from a fairly constant level to a very close to zero level. This takes
no significant amount of hardware to detect, and there is several method
to observe this "Loss Of Beat-tone". The motivation is that as long as
the frequency error is large enough, the controlled oscillator will
"slip", when the frequency comes within limits the PLL locks in rather
than track in the remaining frequency error without a slip.
Thus, the slip detection could also use a form of slip time-out RC link.
Regardless of method (amplitude or beat frequency) the time-constant
must be roughly matched to the lock-in frequency limit of the PLL.
Cheers,
Magnus
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