On Sat, 14 Feb 2009 10:53:14 -0500, Jerry Avins wrote:> Allan Herriman wrote: >> Jerry Avins <jya@ieee.org> wrote in >> news:ZSjll.144$xK6.118@newsfe12.iad: >> >>> Muzaffer Kal wrote: >>>> On Fri, 13 Feb 2009 11:36:50 -0500, Jerry Avins <jya@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 >> >> 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. > > JerryI'm going to beg to differ with you, but only in the sense that if I want to slap a PLL into some larger block diagram and call it a filter, because _in the context of that diagram_ it acts like a filter, I'm going to do it. Certainly one must not get too metaphorical in one's speech, lest it lose all meaning. On the other hand if you approach my mother with a bandpass filter full of coils and caps she'll tell you "that's not a filter", and if you insist she just might pour muddy water into it and challenge you to find the clear water coming out. So I claim that if you can call that collection of coils and caps a "filter" because it does to information what a collection of sieves might do to muddy water, then I can (in some venues at least) call a PLL a "filter" because it extends the "filter" metaphor to the behavior of the phase or frequency of the signal coming in to and going out of it. -- http://www.wescottdesign.com
PLL Basics
Started by ●February 13, 2009
Reply by ●February 14, 20092009-02-14
Reply by ●February 14, 20092009-02-14
On Sat, 14 Feb 2009 10:53:14 -0500, Jerry Avins wrote:> Allan Herriman wrote: >> Jerry Avins <jya@ieee.org> wrote in >> news:ZSjll.144$xK6.118@newsfe12.iad: >> >>> Muzaffer Kal wrote: >>>> On Fri, 13 Feb 2009 11:36:50 -0500, Jerry Avins <jya@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 >> >> 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. > > JerryI'm going to beg to differ with you, but only in the sense that if I want to slap a PLL into some larger block diagram and call it a filter, because _in the context of that diagram_ it acts like a filter, I'm going to do it. Certainly one must not get too metaphorical in one's speech, lest it lose all meaning. On the other hand if you approach my mother with a bandpass filter full of coils and caps she'll tell you "that's not a filter", and if you insist she just might pour muddy water into it and challenge you to find the clear water coming out. So I claim that if you can call that collection of coils and caps a "filter" because it does to information what a collection of sieves might do to muddy water, then I can (in some venues at least) call a PLL a "filter" because it extends the "filter" metaphor to the behavior of the phase or frequency of the signal coming in to and going out of it. -- http://www.wescottdesign.com
Reply by ●February 14, 20092009-02-14
On Sat, 14 Feb 2009 10:53:14 -0500, Jerry Avins wrote:> Allan Herriman wrote: >> Jerry Avins <jya@ieee.org> wrote in >> news:ZSjll.144$xK6.118@newsfe12.iad: >> >>> Muzaffer Kal wrote: >>>> On Fri, 13 Feb 2009 11:36:50 -0500, Jerry Avins <jya@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 >> >> 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. > > JerryI'm going to beg to differ with you, but only in the sense that if I want to slap a PLL into some larger block diagram and call it a filter, because _in the context of that diagram_ it acts like a filter, I'm going to do it. Certainly one must not get too metaphorical in one's speech, lest it lose all meaning. On the other hand if you approach my mother with a bandpass filter full of coils and caps she'll tell you "that's not a filter", and if you insist she just might pour muddy water into it and challenge you to find the clear water coming out. So I claim that if you can call that collection of coils and caps a "filter" because it does to information what a collection of sieves might do to muddy water, then I can (in some venues at least) call a PLL a "filter" because it extends the "filter" metaphor to the behavior of the phase or frequency of the signal coming in to and going out of it. -- http://www.wescottdesign.com
Reply by ●February 14, 20092009-02-14
On Sat, 14 Feb 2009 10:56:27 -0500, Jerry Avins wrote:> YTach wrote: >> On Feb 13, 1:16 pm, Michelot <mhostett...@voila.fr> wrote: >>> Bonsoir, >>> >>> Thanks very much Jerry, Allan, HardySpicer and Tim for your simple >>> words. >>> >>> Sorry for my misconception, PLL is really not a filter. The word >>> "bandwidth" (in Hz) is not only reserved to fiters, and there are >>> certainly many other objects which have also a bandwidth, as PLL e.g. >>> >>>> Hence, if it can be called a PLL, it is low pass. >>> So, it is low pass when it is locked, and when we make varied the >>> input reference frequency. Is it correct ? >>> >>> If we haven't frequency multiplication or division, for the input >>> reference frequency Fin = F0 we have Fout = k F0. >>> >>> If now, we have Fin = F0 + f(t), with f the frequency change according >>> to the time, I think we would get Fout = kF0 + T(f) f(t), with T(f) a >>> low pass transfert function. Is it correct like this? >>> >>> Best regards, >>> Michelot >> >> Bandwidth of PLL in simple words describe how fast the PLL circuit will >> react to any phase change. To choose the appropriate bandwidth, there >> is a tradeoff between stability and performance (defined by the time to >> lock to a phase). So the higher the bandwidth of your PLL, the faster >> it will converge to the phase of its input signal but it become less >> stable to sudden changes in phase. > > Exactly. Not everything with a bandwidth is a filter. Servos have > bandwidth too. A PLL is a kind of servo. > > JerryAn active filter is a kind of servo. Can I not call it a "filter" if it has an amplifier with frequency selective feedback? And I have had occasion to design servo systems whose performance depended on embodying specific transfer functions to noisy measurement data, to follow the good stuff while filtering out the bad. So I'm still with the "PLL as filter" crowd. -- http://www.wescottdesign.com
Reply by ●February 14, 20092009-02-14
Tim Wescott wrote:> On Sat, 14 Feb 2009 10:53:14 -0500, Jerry Avins wrote: > >> Allan Herriman wrote: >>> Jerry Avins <jya@ieee.org> wrote in >>> news:ZSjll.144$xK6.118@newsfe12.iad: >>> >>>> Muzaffer Kal wrote: >>>>> On Fri, 13 Feb 2009 11:36:50 -0500, Jerry Avins <jya@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 >>> 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 > > I'm going to beg to differ with you, but only in the sense that if I want > to slap a PLL into some larger block diagram and call it a filter, > because _in the context of that diagram_ it acts like a filter, I'm going > to do it.More power to you! Context is all.> Certainly one must not get too metaphorical in one's speech, lest it lose > all meaning. On the other hand if you approach my mother with a bandpass > filter full of coils and caps she'll tell you "that's not a filter", and > if you insist she just might pour muddy water into it and challenge you > to find the clear water coming out. > > So I claim that if you can call that collection of coils and caps a > "filter" because it does to information what a collection of sieves might > do to muddy water, then I can (in some venues at least) call a PLL a > "filter" because it extends the "filter" metaphor to the behavior of the > phase or frequency of the signal coming in to and going out of it.What comes out of a Brita filter and what goes into an IF strip is what goes into them, less some unwanted stuff. If a frequency-doubling PLL is a filter, then so is a full-wave rectifier. "Certainly one must not get too metaphorical in one's speech, lest it lose all meaning." Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●February 14, 20092009-02-14
Tim Wescott wrote:> On Sat, 14 Feb 2009 10:56:27 -0500, Jerry Avins wrote: > >> YTach wrote: >>> On Feb 13, 1:16 pm, Michelot <mhostett...@voila.fr> wrote: >>>> Bonsoir, >>>> >>>> Thanks very much Jerry, Allan, HardySpicer and Tim for your simple >>>> words. >>>> >>>> Sorry for my misconception, PLL is really not a filter. The word >>>> "bandwidth" (in Hz) is not only reserved to fiters, and there are >>>> certainly many other objects which have also a bandwidth, as PLL e.g. >>>> >>>>> Hence, if it can be called a PLL, it is low pass. >>>> So, it is low pass when it is locked, and when we make varied the >>>> input reference frequency. Is it correct ? >>>> >>>> If we haven't frequency multiplication or division, for the input >>>> reference frequency Fin = F0 we have Fout = k F0. >>>> >>>> If now, we have Fin = F0 + f(t), with f the frequency change according >>>> to the time, I think we would get Fout = kF0 + T(f) f(t), with T(f) a >>>> low pass transfert function. Is it correct like this? >>>> >>>> Best regards, >>>> Michelot >>> Bandwidth of PLL in simple words describe how fast the PLL circuit will >>> react to any phase change. To choose the appropriate bandwidth, there >>> is a tradeoff between stability and performance (defined by the time to >>> lock to a phase). So the higher the bandwidth of your PLL, the faster >>> it will converge to the phase of its input signal but it become less >>> stable to sudden changes in phase. >> Exactly. Not everything with a bandwidth is a filter. Servos have >> bandwidth too. A PLL is a kind of servo. >> >> Jerry > > An active filter is a kind of servo. Can I not call it a "filter" if it > has an amplifier with frequency selective feedback?You can call it a filter because, active or not, it filters and does no more.> And I have had occasion to design servo systems whose performance > depended on embodying specific transfer functions to noisy measurement > data, to follow the good stuff while filtering out the bad.I don't think I've ever designed a servo that didn't contain a filter.> So I'm still with the "PLL as filter" crowd.A PLL is a kind of servo. Does that mean that a servo is a kind of PLL? Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●February 14, 20092009-02-14
Jerry Avins wrote: (snip)> What comes out of a Brita filter and what goes into an IF strip is what > goes into them, less some unwanted stuff. If a frequency-doubling PLL is > a filter, then so is a full-wave rectifier. "Certainly one must not get > too metaphorical in one's speech, lest it lose all meaning."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. Would you consider a half wave rectifier a filter? I was just last night at the EMP, Experience Music Project, http://www.empsfm.org/ One exhibit is on special effects pedals, mostly used with electric guitars. There are four effects pedals connected in series such that you can turn them on and off, adjust the controls on each, and listen to the result. The first is the distortion pedal, which I believe is traditionally a half wave rectifier, though there may be other ways to do it. Next is the delay (or echo) pedal where you can vary the delay, amount of the delayed signal, and I believe the amount of the delay fed back into the input (recursive delay). The third is the chorus pedal, which I believe gives a time varying delay. The last is the Wah-Wah pedal, well described here: http://en.wikipedia.org/wiki/Wah-wah_pedal The Wah-Wah is pretty interesting. It seems that it is just a bandpass filter with a variable passband. The interesting effects come from varying the passband while playing a note. The story in the wikipedia page is that someone was redesigning a guitar amplifier that had a three position mid range boost switch, as a transistorized amplifier, and put a potentiometer in instead of the three position switch. Then in testing, started turning the knob while playing notes through it. Only a short time later it was installed in a pedal housing. Anyway, the museum exhibit allows one to turn on and off each one, vary the knobs, and listen to the results. Some people might call each of these filters, others might not. -- glen http://en.wikipedia.org/wiki/Wah-wah_pedal
Reply by ●February 14, 20092009-02-14
On Sat, 14 Feb 2009 15:35:15 -0500, Jerry Avins wrote:> Tim Wescott wrote: >> On Sat, 14 Feb 2009 10:53:14 -0500, Jerry Avins wrote: >> >>> Allan Herriman wrote: >>>> Jerry Avins <jya@ieee.org> wrote in >>>> news:ZSjll.144$xK6.118@newsfe12.iad: >>>> >>>>> Muzaffer Kal wrote: >>>>>> On Fri, 13 Feb 2009 11:36:50 -0500, Jerry Avins <jya@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 >>>> 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 >> >> I'm going to beg to differ with you, but only in the sense that if I >> want to slap a PLL into some larger block diagram and call it a filter, >> because _in the context of that diagram_ it acts like a filter, I'm >> going to do it. > > More power to you! Context is all. > >> Certainly one must not get too metaphorical in one's speech, lest it >> lose all meaning. On the other hand if you approach my mother with a >> bandpass filter full of coils and caps she'll tell you "that's not a >> filter", and if you insist she just might pour muddy water into it and >> challenge you to find the clear water coming out. >> >> So I claim that if you can call that collection of coils and caps a >> "filter" because it does to information what a collection of sieves >> might do to muddy water, then I can (in some venues at least) call a >> PLL a "filter" because it extends the "filter" metaphor to the behavior >> of the phase or frequency of the signal coming in to and going out of >> it. > > What comes out of a Brita filter and what goes into an IF strip is what > goes into them, less some unwanted stuff. If a frequency-doubling PLL is > a filter, then so is a full-wave rectifier. "Certainly one must not get > too metaphorical in one's speech, lest it lose all meaning." > > JerrySo 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"? -- http://www.wescottdesign.com
Reply by ●February 14, 20092009-02-14
Glen Herrmannsfeldt wrote:> Jerry Avins wrote: > (snip) > >> What comes out of a Brita filter and what goes into an IF strip is >> what goes into them, less some unwanted stuff. If a frequency-doubling >> PLL is a filter, then so is a full-wave rectifier. "Certainly one must >> not get too metaphorical in one's speech, lest it lose all meaning." > > 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. > > Would you consider a half wave rectifier a filter? > > I was just last night at the EMP, Experience Music Project, > > http://www.empsfm.org/ > > One exhibit is on special effects pedals, mostly used with electric > guitars. There are four effects pedals connected in series such that > you can turn them on and off, adjust the controls on each, and listen > to the result. > > The first is the distortion pedal, which I believe is traditionally > a half wave rectifier, though there may be other ways to do it. > > Next is the delay (or echo) pedal where you can vary the delay, > amount of the delayed signal, and I believe the amount of the delay > fed back into the input (recursive delay). > > The third is the chorus pedal, which I believe gives a time > varying delay. > > The last is the Wah-Wah pedal, well described here: > > http://en.wikipedia.org/wiki/Wah-wah_pedal > > The Wah-Wah is pretty interesting. It seems that it is just > a bandpass filter with a variable passband. The interesting > effects come from varying the passband while playing a note. > The story in the wikipedia page is that someone was redesigning > a guitar amplifier that had a three position mid range boost > switch, as a transistorized amplifier, and put a potentiometer > in instead of the three position switch. Then in testing, started > turning the knob while playing notes through it. Only a short time > later it was installed in a pedal housing. > > Anyway, the museum exhibit allows one to turn on and off each one, > vary the knobs, and listen to the results. > > Some people might call each of these filters, others might not.Borderline cases can help to sharpen definitions, but I don't think that they usually add much to general discussions. This particular discussion began when someone who didn't seem to understand PLLs very well sought to characterize them as filters. I sought to disabuse him, not because PLLs are orthogonal to filters, but because a filter paradigm is not a fruitful way to understand PLLs. My attempt may have been unnecessary, but I still claim it is correct. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●February 14, 20092009-02-14
Jerry Avins wrote:> Tim Wescott wrote: > >> On Sat, 14 Feb 2009 10:53:14 -0500, Jerry Avins wrote: >> >>> Allan Herriman wrote: >>> >>>> Jerry Avins <jya@ieee.org> wrote in >>>> news:ZSjll.144$xK6.118@newsfe12.iad: >>>> >>>>> Muzaffer Kal wrote: >>>>> >>>>>> On Fri, 13 Feb 2009 11:36:50 -0500, Jerry Avins <jya@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 >>>> >>>> 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 >> >> >> I'm going to beg to differ with you, but only in the sense that if I >> want to slap a PLL into some larger block diagram and call it a >> filter, because _in the context of that diagram_ it acts like a >> filter, I'm going to do it. > > > More power to you! Context is all. > >> Certainly one must not get too metaphorical in one's speech, lest it >> lose all meaning. On the other hand if you approach my mother with a >> bandpass filter full of coils and caps she'll tell you "that's not a >> filter", and if you insist she just might pour muddy water into it and >> challenge you to find the clear water coming out. >> >> So I claim that if you can call that collection of coils and caps a >> "filter" because it does to information what a collection of sieves >> might do to muddy water, then I can (in some venues at least) call a >> PLL a "filter" because it extends the "filter" metaphor to the >> behavior of the phase or frequency of the signal coming in to and >> going out of it. > > > What comes out of a Brita filter and what goes into an IF strip is what > goes into them, less some unwanted stuff. If a frequency-doubling PLL is > a filter, then so is a full-wave rectifier. "Certainly one must not get > too metaphorical in one's speech, lest it lose all meaning." > > JerryA *ZERO JITTER* PLL from two 6AL5's ;/
