Re: PLL Basics

Allan Herriman wrote:
Jerry Avins <jya@xxxxxxxx> wrote in news:ZSjll.144$xK6.118@xxxxxxxxxxxx:

Muzaffer Kal wrote:
On Fri, 13 Feb 2009 11:36:50 -0500, Jerry Avins <jya@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.

Jerry

Hi Jerry,

You could try to think in a different domain. The PLL does indeed act like a lowpass filter (and the OP was not suffering from any misconception), provided that one thinks about the phase of the output as it is related to the phase of the reference. The "signal generator" aspect is a red herring; the maths works just as well (and is a lot easier to manipulate) if the output frequency is zero.

Of course, the output isn't really at DC, and the lowpass filter shape is actually a bandpass (centered around the carrier frequency).

I see what you're saying, but I think it's a stretch. The output of a PLL needn't have the same frequency as the reference. It's one thing to recognize that a PLL has filtering qualities, quite another to call it a filter.

Jerry
--
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