From: Jim Thompson
Subject: Re: Phase locked loops and confusion.
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Date: Sat, 23 Nov 2002 16:54:48 GMT
NNTP-Posting-Date: Sat, 23 Nov 2002 11:54:48 EST
Organization: Cox Communications
On Sat, 23 Nov 2002 01:06:45 GMT,
Chuck Simmons ,
In Newsgroup: sci.electronics.design,
Entitled: "Phase locked loops and confusion.",
Wrote the following:
|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
For a good phase detector see...
Subject: Phase/Frequency Detector - DualD-PFD.pdf
Phase detectors don't have integrators, but VCO's are represented by
Thus, if you add any filter at all, it *must* be lead-lag, or it's
Why don't you post the complete PLL system requirements, type of input
signal, type of output signal, frequencies; is it 1:1, or is there a
divider in the loop?
Then we can see who, in this group, really knows PLL's ;-)
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| Jim-T@analog_innovations.com Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
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