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From: firstname.lastname@example.org (Bob Wilson)
Subject: Re: How to increase PLL order?
Date: Wed, 11 Dec 2002 04:09:20 -0000
Organization: Your Organization
X-Newsreader: WinVN 0.99.9 (Released Version) (x86 32bit)
In article ,
>On Tue, 10 Dec 2002 18:08:18 -0800, "Christopher R. Carlen"
>>Bill Sloman wrote:
>>> According to chapter 2 of Floyd M. Gardner's "Phaselock Techniques"
>>> (ISBN 0-471-04294-3) you need a second order control loop, which is to
>>> say an integrator in the feed-back path (in addition to the integrator
>>> formed by the voltage-controlled oscillator) to get a zero phase error
>>> at an arbitrary (but stable) frequency.
>>> The nature of the phase detector has nothing to do with Christopher
>>> Carlen's problem. A digital phase-frequency detector comes into lock
>>> much more nicely and quite a lot faster than a simple
>>> multiplying/exclusive-OR phase detector, but once the loop is locked
>>> both forms of phase detector give an error signal directly
>>> proportional to phase error, and you have to feed this into an
>>> integrator this if you want an arbitrary voltage output (and
>>> frequency) at zero phase error.
>>Yeah, that's what I think too!
>>> To get a zero phase error when tracking a linearly varying frequency
>>> you have to go to a third order loop (with two integrators in the
>>> feed-back path) and to get zero phase error when tracking a frequency
>>> which is increaising or decreasing at a constantly
>>> accelerating/decelerating rate you need a fourth order loop, and so
>>Well fortunately, I just need the second order.
>>Thanks for the input.
>A 4046 type II (digital/charge pump) detector driving an R-C or R-C//C
>filter does give a type 2 closed loop: zero steady-state phase error.
But it is sensitive to noise and will slip a cycle whenever it misses an
edge. I presonally prefer using the exor (PD I), and stuffing lots of gain
in the form of a simple integrator in the loop. The result is that phase
shift across the lock range is now essentially zero (just like PD II), but
it tolerates massive amounts of noise.
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