On Fri, 13 Feb 2009 13:16:08 -0800, Michelot 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, > MichelotThat is correct. -- http://www.wescottdesign.com
PLL Basics
Started by ●February 13, 2009
Reply by ●February 13, 20092009-02-13
Reply by ●February 14, 20092009-02-14
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. > > JerryHi 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). Regards, Allan
Reply by ●February 14, 20092009-02-14
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, > MichelotBandwidth 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.
Reply by ●February 14, 20092009-02-14
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 -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●February 14, 20092009-02-14
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 -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
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 10:53:14 -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 10:53:14 -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
