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From: SNIPrf_man_frTHIS@yahoo.com (Frank Raffaeli)
Subject: Re: Phase locked loops and confusion.
Date: 23 Nov 2002 00:01:22 -0800
NNTP-Posting-Date: 23 Nov 2002 08:01:22 GMT
John Larkin wrote in message news:...
> On Sat, 23 Nov 2002 01:06:45 GMT, Chuck Simmons
> >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.
> >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
> Yeah, the charge pump adds another integrator to the loop. I don't
> know why the charge pump thing is so popular; most of them have nasty
> deadbands. A charge pump pd doesn't inherently drive the loop in the
> right direction when the freq error is large; it takes a bit of
> additional logic (like in the 4046) to do that.
> My favorite phase detector is a d-type flipflop operating in bang-bang
> mode: input signal on clock, reference osc on D. This is really
> interesting to analyze and to design a filter for. Gain is infinite,
> sort of.
John is right about the dead-bands. In the really good PLL's, it's a
non-linear region, never completely dead. They are there to reduce
noise, but the whole assumption of a linear control system goes out
the window. I think Toshiba was the first to implement this type of
topology (low noise) very successfully in commercial markets. One of
my first assignments as a rookie IC designer was to design a PLL IC
... I guess because I had done some discrete PLL designs, and the
budget had been cut ... tempting the experienced guys to take early
retirement. I remember it took about twice as long as I told my
supervisor ..... with twice the predicted noise and current draw to
Frank W. Raffaeli
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