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From: John Larkin
Subject: Re: Phase locked loops and confusion.
Date: Fri, 22 Nov 2002 18:21:09 -0800
Organization: Posted via Supernews, http://www.supernews.com
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On Sat, 23 Nov 2002 01:06:45 GMT, Chuck Simmons
>PLLs are fairly common and seem fairly simple but I found over the past
>week that confusion abounds and since I have seen this confusion before,
>I wonder if it is common.
>Our digital team (yes, 2 engineers) has a need for a PLL but for various
>reasons, an all digital loop could not be considered and, therefore, I
>was dragged in as resident analog stuff guru. The y showed me the loop
>they were proposing which consisted of digital equivalents (pretty exact
>really) of a phase detector driving up and down charge pumps dumping
>into a capacitor followed by a filter followed by the VCO (which is
>analog). I was asked what the filter needed to be and I mumbled that
>they had two integrators so it should probably be a classic lead/lag and
>proceeded to draw response curves on the whiteboard indicating what was
>The next day, I was told that all of the papers they could find on the
>web used a low pass filter. My comment was an unenthusiastic "Oh." I was
>correctly interpreted and a printed article soon appeared on my desk.
>Suddenly I saw the confusion. The proposed loop had a phase detector,
>charge pumps and capacitor which is Kp/s. The article used a phase
>detector that was Kp. So the article loop has Kp*Kf/s while the proposed
>loop has Kp*Kf/s^2. A low pass is perfectly fine for the article loop
>but not workable for the proposed loop.
>I saw this confusion some years ago as well because the phase detector,
>charge pumps and capacitor is used a lot in practice but many articles
>don't mention this variant. This leads to confusion about what sort of
>filter is actually needed. The charge pumps and capacitor type has the
>advantage that it will drive the steady state phase error to zero even
>when the desired frequency is greatly different from the VCO center
Yeah, the charge pump adds another integrator to the loop. I don't
know why the charge pump thing is so popular; most of them have nasty
deadbands. A charge pump pd doesn't inherently drive the loop in the
right direction when the freq error is large; it takes a bit of
additional logic (like in the 4046) to do that.
My favorite phase detector is a d-type flipflop operating in bang-bang
mode: input signal on clock, reference osc on D. This is really
interesting to analyze and to design a filter for. Gain is infinite,
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