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From: "Bill Sloman"
Subject: Re: Phase locked loops and confusion.
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Sat, 23 Nov 2002 04:35:56 GMT
NNTP-Posting-Date: Sat, 23 Nov 2002 15:35:56 EST
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"Chuck Simmons" wrote in message
> 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
One solution is a good textbook. I've got
by Floyd Martin Gardner
John Wiley & Sons; ISBN: 0471042943; 2 edition (April 1979)
This is still available from Amazon at $110 new or from $33 used.
Recent discussions seem to favour more recent authors. Floyd Gardener
is good on setting out the transfer function of the whole loop, but if you
read the Motorola or Philips application notes carefully you can get the
Bill Sloman, Nijmegen
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