Reply by robert bristow-johnson●February 15, 20092009-02-15
On Feb 14, 10:36�pm, Jerry Avins <j...@ieee.org> wrote:
> John wrote:
> > On Feb 13, 2:30 pm, Jerry Avins <j...@ieee.org> wrote:
> >> Muzaffer Kal wrote:
> >>> On Fri, 13 Feb 2009 11:36:50 -0500, Jerry Avins <j...@ieee.org> 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
Reply by Jerry Avins●February 14, 20092009-02-14
John wrote:
> On Feb 13, 2:30 pm, Jerry Avins <j...@ieee.org> wrote:
>> Muzaffer Kal wrote:
>>> On Fri, 13 Feb 2009 11:36:50 -0500, Jerry Avins <j...@ieee.org> 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
>> --
>> Engineering is the art of making what you want from things you can get.
>> �����������������������������������������������������������������������
>
> Furthermore, a PLL produces an output signal when the input is zero.
> Is that different enough?
Bingo! Why didn't I think of that?
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Jerry Avins●February 14, 20092009-02-14
Tim Wescott wrote:
...
> So if I have some timing signal plus noise, and I want to regenerate the
> phase and frequency of the timing signal less the noise, wouldn't the PLL
> that I use precisely meet your definition of "filter"?
Yes. But not every PLL meets that description.
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by Jerry Avins●February 14, 20092009-02-14
Randy Yates wrote:
> Glen Herrmannsfeldt <gah@ugcs.caltech.edu> writes:
>> [...]
>> I probably believe that different people have legitimately different
>> definitions of what is and isn't a filter. In the DSP context, it might
>> be a little more restrictive than in other contexts.
>
> I would define a filter as any LTI system; as someone else said,
> anything with a T(s) is a "filter." That includes linear control
> systems, of which a PLL is a type.
>
>> Would you consider a half wave rectifier a filter?
>
> I would not.
>
> This thread is interesting because it seems that a PLL isn't a filter
> (that's what I was thinking at first). I now believe it is.
even one that has an output that is a harmonic of the input? LTI?
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Randy Yates●February 14, 20092009-02-14
Randy Yates <yates@ieee.org> writes:
> [...]
> Similarly for the input signal; we don't really use the frequency of the
> input signal in the model, but the phase.
N.B.: "Phase" in this context means the *argument*.
Now my mind is starting to get wrapped around the axle again... If you
have noise on the input signal (in frequency space), then how does the
model faithfully represent that in phase space? It seems you'd have to
embed the specifics of your phase detector into your model to account
for such things. And of course this isn't linear, but once you get to
phase space, it's linear.
Hmmm... As my ex-boss used to say, "Clear as mud."
--
% Randy Yates % "I met someone who looks alot like you,
%% Fuquay-Varina, NC % she does the things you do,
%%% 919-577-9882 % but she is an IBM."
%%%% <yates@ieee.org> % 'Yours Truly, 2095', *Time*, ELO
http://www.digitalsignallabs.com
Reply by Glen Herrmannsfeldt●February 14, 20092009-02-14
Jerry Avins wrote:
(snip)
> 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.
Did we decide a half wave rectifier was a filter? (It removes
half of the signal.) Yet the output has frequency components
that the input didn't have. You could even filter some out with
RLC filters, and create multiples of the original. That is done
in optics to make optical frequency multipliers. (Optical PLL's
not being so easy to make, though you might have a tunable laser
with a feedback loop.)
-- glen
Reply by Randy Yates●February 14, 20092009-02-14
spope33@speedymail.org (Steve Pope) writes:
> Randy Yates <yates@ieee.org> wrote:
>
>>I would define a filter as any LTI system; as someone else said,
>>anything with a T(s) is a "filter." That includes linear control
>>systems, of which a PLL is a type.
>
> I like this definition, but I'm not so sure a voltage-controlled
> oscillator is a linear component. It has output frequencies
> that do not appear at its inputs.
>
> I'll have to think about this one a little more. :-)
That's a good point, Steve. We really have to define our model.
It is true that the VCO you can buy from MF Electronics and plop
down on your board isn't a linear component. But in Gardner's
model of a PLL [gardner], he operates in "phase space," i.e.,
he's using the argument of the cos(\omega t + \phi) as the
output of the VCO, so that the VCO transfer function is
K/s.
Similarly for the input signal; we don't really use the frequency of the
input signal in the model, but the phase.
This model is then LTI.
--Randy
@book{gardner,
title = "Phaselock Techniques",
author = "Floyd M. Gardner",
publisher = "Wiley",
edition = "second",
year = "1979"}
--
% Randy Yates % "Midnight, on the water...
%% Fuquay-Varina, NC % I saw... the ocean's daughter."
%%% 919-577-9882 % 'Can't Get It Out Of My Head'
%%%% <yates@ieee.org> % *El Dorado*, Electric Light Orchestra
http://www.digitalsignallabs.com
Reply by Steve Pope●February 14, 20092009-02-14
Randy Yates <yates@ieee.org> wrote:
>I would define a filter as any LTI system; as someone else said,
>anything with a T(s) is a "filter." That includes linear control
>systems, of which a PLL is a type.
I like this definition, but I'm not so sure a voltage-controlled
oscillator is a linear component. It has output frequencies
that do not appear at its inputs.
I'll have to think about this one a little more. :-)
Steve
> [...]
> I probably believe that different people have legitimately different
> definitions of what is and isn't a filter. In the DSP context, it might
> be a little more restrictive than in other contexts.
I would define a filter as any LTI system; as someone else said,
anything with a T(s) is a "filter." That includes linear control
systems, of which a PLL is a type.
> Would you consider a half wave rectifier a filter?
I would not.
This thread is interesting because it seems that a PLL isn't a filter
(that's what I was thinking at first). I now believe it is.
--
% Randy Yates % "Bird, on the wing,
%% Fuquay-Varina, NC % goes floating by
%%% 919-577-9882 % but there's a teardrop in his eye..."
%%%% <yates@ieee.org> % 'One Summer Dream', *Face The Music*, ELO
http://www.digitalsignallabs.com
Reply by John●February 14, 20092009-02-14
On Feb 13, 2:30�pm, Jerry Avins <j...@ieee.org> wrote:
> Muzaffer Kal wrote:
> > On Fri, 13 Feb 2009 11:36:50 -0500, Jerry Avins <j...@ieee.org> 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
> --
> Engineering is the art of making what you want from things you can get.
> �����������������������������������������������������������������������
Furthermore, a PLL produces an output signal when the input is zero.
Is that different enough?
John