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Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
References: <3DDED492.2B9873B0@webaccess.net> <0lGD9.514$pN1.email@example.com> <3DDF7F94.25D6A972@webaccess.net>
Subject: Re: Phase locked loops and confusion.
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Sat, 23 Nov 2002 16:10:47 -0000
NNTP-Posting-Date: Sat, 23 Nov 2002 16:11:02 GMT
Chuck Simmons wrote:
> 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, explicitly.
> The blocks are a VCO which can be considered an integrator (Kf/s)
> pretty obviously.
No problem here.
>There ia a phase detector block which can have an
> integrator lumped into it.
I don't follow this. The phase detector block itself does not have a 1/s
>There is a filter block which, for
> convenience, is between the phase detector and the VCO. If the phase
> detector has no lumped integrator,
Which it dont?
> the filter can be a wire because
> the 1st order loop is stable.
Agreed, if there is no filter, the VCO 1/s is enough (but poor) to lock
> 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
Yes, If there is a filter, then this filter (1/(1+s.tau), with the vco
1/s will require a lead in the filter.
>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.
Well I'm confused on what you mean by having a phase detector lumped
as/with an integrator.
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