The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Phase locked loops and confusion.
References: <3DDED492.2B9873B0@webaccess.net> <0lGD9.514$pN1.firstname.lastname@example.org>
Date: Sat, 23 Nov 2002 13:16:18 GMT
NNTP-Posting-Date: Sat, 23 Nov 2002 05:16:18 PST
Kevin Aylward wrote:
> Chuck Simmons wrote:
> > 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 needed.
> > 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 don't understand what you are actually illustrating here. What exact
> terms are going with what. Where is every "s" term in what block,
The blocks are a VCO which can be considered an integrator (Kf/s) pretty
obviously. There ia a phase detector block which can have an integrator
lumped into it. There is a filter block which, for convenience, is
between the phase detector and the VCO. If the phase detector has no
lumped integrator, the filter can be a wire because the 1st order loop
is stable. It is usually a low pass filter. If the phase detector has a
lumped integrator, charge pumps and cap, the filter needs to be a
lead/lag or something of the sort to stabilize Kp*Kf/s^2. Generally, I
am talking about the confusion I see that comes about because a fairly
common phase detector strategy lumps an integrator becoming Kp/s instead
of Kp. This leads to confusion about how to stabilize the loop.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup