Re: PLL Basics

On Feb 14, 10:36 pm, Jerry Avins <j...@xxxxxxxx> wrote:
John wrote:
On Feb 13, 2:30 pm, Jerry Avins <j...@xxxxxxxx> wrote:
Muzaffer Kal wrote:
On Fri, 13 Feb 2009 11:36:50 -0500, Jerry Avins <j...@xxxxxxxx> wrote:
Of course filters are used in a PLL implementation. That doesn't mean a
PLL is a filter, any more it is a VCO.
I'm curious as to how you make the distinction. What makes a PLL a
non-filter? It has an H(s), it takes a noisy periodic signal and
produces cleaned up, frequency shaped version of it. How is that
qualitatively different from an RC filter?
It's also a signal generator, no? The OP asked a a simple,question that
(it seemed to me) embodied a misconception that I tried to dispel. To
answer your question, a simple RC filter isn't bandpass.

....

Furthermore, a PLL produces an output signal when the input is zero.
Is that different enough?

Bingo! Why didn't I think of that?


i agree that it's not very good semantics to call a PLL a "filter".
normally in this EE context, i want a filter to be an LTI system, at
least in the ideal. (so a servo might be called a filter.)

what i see in Tim's POV is that if it's phase going in and phase
coming out or frequency going in and frequency coming out (or maybe
mix it, frequency going in and phase coming out or vise versa), if you
are looking at those parameters only (not the sinusoidal signal
itself), THEN *maybe* you can call a PLL a servo and a filter acting
on those parameters. when you go from frequency to phase, there
definitely is an integrator in there. and the phase-discriminator is
like a subtractor (like in negative feedback). but if it's the actual
sinusoidal signal going in and the VCO sinusoidal signal coming out
(not the parameters describing these sinusoids), then i agree with
Jerry, et. al. but if it's the phase or frequency going in and out,
then maybe Tim et. al. ain't so far off.

r b-j
.

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