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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Phase locked loops and confusion.
Date: Sat, 23 Nov 2002 01:06:45 GMT
NNTP-Posting-Date: Fri, 22 Nov 2002 17:06:45 PST
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
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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