PLL for Zero Delay LPF
in [Design]
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From: Joerg on 23 Jun 2010 12:50 Vladimir Vassilevsky wrote: > > > Joerg wrote: > >> Vladimir Vassilevsky wrote: >> >>> >>> Glenn Kenroy wrote: >>> >>> >>>> Excuse me for resuscitating this topic, but I was wondering if anyone >>>> could elaborate further on the following concept (see excerpt below) >>>> generously provided by MooseFET? >>>> My original post was about how to design a _zero_ delay LP filter for >>>> geomagnetic signals below 50Hz. >>>> I am hoping for a bit more detail to assist me in coming up with a >>>> workable schematic. >>> >>> You need to contact Harry Potter or some democrat, as they believe they >>> can make miracles. Several folks already explained why zero delay filter >>> is impossible. There is no way to make zero delay filter, although it is >>> possible to play different tricks with phase; PLL is one of those. >>> >> >> >> If someone came up with a negative delay filter, now that would be >> something :-) > > > > There was a good thread in comp.dsp about filters with negative dfi/dW, > i.e. negative group delay. This is indeed possible. Those filters > forecast the future by predicting the waveform, but they can't see into > the future. > That sounds eerily similar to what some people on Wall Street used to claim :-) Many pension funds with union ties still do :-( -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: langwadt on 23 Jun 2010 12:58 On 23 Jun., 17:12, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 23 Jun 2010 09:09:51 GMT, glennken...(a)protech.com (Glenn > > > > Kenroy) wrote: > >Excuse me for resuscitating this topic, but I was wondering if anyone > >could elaborate further on the following concept (see excerpt below) > >generously provided by MooseFET? > > >My original post was about how to design a _zero_ delay LP filter for > >geomagnetic signals below 50Hz. > > >I am hoping for a bit more detail to assist me in coming up with a > >workable schematic. > > >Glenn Kenroy > > >****** > > >"The first step is to make a PLL that locks onto the 60Hz. You want > >the VCO in the PLL to be running at many times the 60Hz frequency. > >I am going to suggest 7200 times, but faster is likely better. 7200 > >times just makes the explanation easier. > > >Important frequencies: > > >60*8*3*5 = 7200 > > >7200 / 15 = 60*8 > >7200 / 3 = 5*60*8 > >7200 / 5 = 3*60*8 > > >I will assume that you have the PLL locked to the 60Hz. > > >You will be making the same circuit 3 times. It uses the CD4051 > >The 8 times the frequency goes to a counter that makes the > >CD4051 scan through a group of capacitors. > > >Each of the 8 outputs of the CD4051 connects to one end of > >a capacitor. The other end of the capacitor is grounded. > > >If the common point is fed with a resistor. This makes a circuit > >that will charge up the capacitors until they match the 60Hz > >input waveform." > > >****** > > The commutating capacitor thing is cute, but it makes a bandpass > filter, not a lowpass. And you'd get better signal quality using a > commercial switched-capacitor filter, which can also be clocked by a > PLL. > > Does your filter really need to be realtime? If you're acquiring and > digitizing the data, it can be post-processed, which allows a > near-ideal lowpass to be applied. > > A zero delay realtime lowpass filter is impossible. > > John if post-processing, run the data through the filter twice, once forwards once backwards, you get double the filter order and zero phase -Lasse
From: Dave Platt on 23 Jun 2010 14:04 In article <88emqkF2spU2(a)mid.individual.net>, Joerg <news(a)analogconsultants.com> wrote: >If someone came up with a negative delay filter, now that would be >something :-) Someone did. He was, of course, arrested by the Causality Police for violation of the Temporal Accords. As per their usual practice, they arrested him *before* he invented it. :-) -- Dave Platt <dplatt(a)radagast.org> AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads!
From: Spehro Pefhany on 23 Jun 2010 14:27 On Wed, 23 Jun 2010 11:04:49 -0700, dplatt(a)radagast.org (Dave Platt) wrote: >In article <88emqkF2spU2(a)mid.individual.net>, >Joerg <news(a)analogconsultants.com> wrote: > >>If someone came up with a negative delay filter, now that would be >>something :-) > >Someone did. > >He was, of course, arrested by the Causality Police for violation of >the Temporal Accords. As per their usual practice, they arrested him >*before* he invented it. > > :-) It just requires capacitors made with resublimated Thiotimoline dielectric.
From: John Larkin on 23 Jun 2010 14:39
On Wed, 23 Jun 2010 09:58:14 -0700 (PDT), "langwadt(a)fonz.dk" <langwadt(a)fonz.dk> wrote: >On 23 Jun., 17:12, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Wed, 23 Jun 2010 09:09:51 GMT, glennken...(a)protech.com (Glenn >> >> >> >> Kenroy) wrote: >> >Excuse me for resuscitating this topic, but I was wondering if anyone >> >could elaborate further on the following concept (see excerpt below) >> >generously provided by MooseFET? >> >> >My original post was about how to design a _zero_ delay LP filter for >> >geomagnetic signals below 50Hz. >> >> >I am hoping for a bit more detail to assist me in coming up with a >> >workable schematic. >> >> >Glenn Kenroy >> >> >****** >> >> >"The first step is to make a PLL that locks onto the 60Hz. You want >> >the VCO in the PLL to be running at many times the 60Hz frequency. >> >I am going to suggest 7200 times, but faster is likely better. 7200 >> >times just makes the explanation easier. >> >> >Important frequencies: >> >> >60*8*3*5 = 7200 >> >> >7200 / 15 = 60*8 >> >7200 / 3 = 5*60*8 >> >7200 / 5 = 3*60*8 >> >> >I will assume that you have the PLL locked to the 60Hz. >> >> >You will be making the same circuit 3 times. It uses the CD4051 >> >The 8 times the frequency goes to a counter that makes the >> >CD4051 scan through a group of capacitors. >> >> >Each of the 8 outputs of the CD4051 connects to one end of >> >a capacitor. The other end of the capacitor is grounded. >> >> >If the common point is fed with a resistor. �This makes a circuit >> >that will charge up the capacitors until they match the 60Hz >> >input waveform." >> >> >****** >> >> The commutating capacitor thing is cute, but it makes a bandpass >> filter, not a lowpass. And you'd get better signal quality using a >> commercial switched-capacitor filter, which can also be clocked by a >> PLL. >> >> Does your filter really need to be realtime? If you're acquiring and >> digitizing the data, it can be post-processed, which allows a >> near-ideal lowpass to be applied. >> >> A zero delay realtime lowpass filter is impossible. >> >> John > >if post-processing, run the data through the filter twice, >once forwards once backwards, you get double the filter >order and zero phase > > >-Lasse Or just make a FIR approximation to an ideal lowpass filter. Time lag doesn't matter when you're processing offline. Or Matlab it or something. John |