Re: Can PLL Freq Error be zero?

From: Larry Brasfield (donotspam_larry_brasfield_at_hotmail.com)
Date: 03/18/05 

Date: Fri, 18 Mar 2005 08:10:37 -0800

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote
 in message news:lb3k31hsc5qrld5nlmm8et5dfdms5pr3fg@4ax.com...
[on use of 'S' in an expression unrelated to Laplace transforms]
> ... so it was perfectly obvious to me that S = divider ratio. There's only
> 26 letters on my keyboard, and it's not my fault that some lunatic
> Frenchman decided to use S for something else.

(Chuckle) If he was a proper Frenchman, he more
likely used much prettier, flourished versions of 's'
that would be trickier to replicate with machinery.

> Actually, I never use that Laplace stuff. I must have learned it once
> in ancient times, because I didn't flunk very many of my EE courses,
> but nowadays a quickie Bode plot is enough to stabilize simple loops,
> and if it gets more complicated, or gets nonlinear, I just simulate.

Don't take this as advice to the effect "You should
relearn and use that Laplace stuff." But I would urge
a different way of looking at that tool than as a poor
substitute for a simulator. For complex loops, it can
be a real assist while trying to understand what to do
for them, especially when you have expression for
the system poles and zeros. I have rarely used the
Laplace (or Fourier) tranform to actually compute a
frequency or time domain response. But, as I see it,
there is no good substitute, (except from a get-it-done
quickly standpoint!), for an s-plane representation of
the system and its pieces.

> I had an engineer a while back who was a whiz at this stuff... pages
> of equations, root locus, polynomials in s-domain, all neatly solved.

I was interviewed by a fellow once who, noting
the phrase "root locus" in my resume, got a gleam
in his eye and pulled out a special question he had
worked up for such claims. After I ascertained that
he really wanted me to work out the solution, right
there in his office, (while he did other work for 15
minutes or so), I did the work and handed it over
to him. He went immediately to the bottom line
and informed me it was wrong. I acknowledged
that possibility, but invited him to go with me thru
the derivation to find the error, a challenge he took
up readily. After another 30 minutes scrutinizing
each step, getting to the bottom without finding any
identifiable error, he pulled out a little work***
that I imagine was his derivation. We found his
error in short order, leading to a reaction that I
considered carefully later. The reaction? A plain
and apparently sincere thank-you, accompanied by
a smile. Needless to say, he was not the reason I
turned down the company's generous offer.

> But the answers usually made no sense, and when I pointed it out to
> him he got pissed off (ie, angry in American.)

One reason to have a few different analytical tools at
hand is for checking. 'Making sense', (when carefully
done and not confused for bare supposition), is one
of the better ones for that purpose.

> As soon as I finished
> paying for the legal fees to get him a green card, he quit.

A good decision. People who find it difficult to deal
with their own errors usually make poor engineers. 

-- 
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.