PLL for Zero Delay LPF
in [Design]
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From: John Larkin on 23 Jun 2010 14:46 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. > > :-) And he served his sentence before that. John
From: Bill Sloman on 23 Jun 2010 17:47 On Jun 23, 5:16 pm, Joerg <inva...(a)invalid.invalid> 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 :-) http://en.wikipedia.org/wiki/Thiotimoline It is - of course - too good to be true. -- Bill Sloman, Nijmegen
From: Glenn Kenroy on 23 Jun 2010 18:21 On Wed, 23 Jun 2010 08:12:49 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >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. > OK, but since the primary objective is to remove 60Hz artifact from the ELF geomagnetic data (in real time), would a zero delay _notch_ filter be feasible in place of a LPF? Glenn Kenroy
From: Tim Williams on 23 Jun 2010 18:32 "Glenn Kenroy" <glennkenroy(a)protech.com> wrote in message news:4c2287e0.662703(a)news.tpg.com.au... > OK, but since the primary objective is to remove 60Hz artifact from > the ELF geomagnetic data (in real time), would a zero delay _notch_ > filter be feasible in place of a LPF? What significance is "zero delay" anyway? You need at least a fraction of a cycle to tell if the signal has some sort of frequency content, and you need a whole lot of cycles to tell with any certainty what those frequencies happen to be. You can't beat Heisenberg. Most people watching ELF load the data into a sliding FT. This gives you an animated spectrum for most frequencies, with greater uncertainty at lower frequencies, approaching cutoff. (Apparent DC offset in each FT period corresponds to any lower frequencies. Presumably, those DC samples could be FT'd again for reading spectra from the lower bands, and so on.) Now, if you're looking for a pattern (in "real time") on one of those displays, you need at least a few sample periods to get any pattern to recognize, which is maybe 100ms at least, and if you're looking for radio or whatever, you might as well save the whole thing and postprocess it. And philosophically, how would you even know if the signal is delayed by a filter? Does it have to be phase-correct with respect to other sources? That's simple to accommodate: subtract the difference out (a good reason to use a high order Bessel filter, incidentially). Time is relative, there's nothing funny about doing that. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: John Larkin on 23 Jun 2010 19:12
On Wed, 23 Jun 2010 22:21:07 GMT, glennkenroy(a)protech.com (Glenn Kenroy) wrote: >On Wed, 23 Jun 2010 08:12:49 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > > >>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. >> > >OK, but since the primary objective is to remove 60Hz artifact from >the ELF geomagnetic data (in real time), would a zero delay _notch_ >filter be feasible in place of a LPF? > >Glenn Kenroy That should be possible. Imagine a series resistor, and then a very high-Q series LC to ground. The LC is a short at 60 Hz, but a very high impedance not far away. It would introduce delay and phase errors close around 60 Hz but have little effect elsewhere. So a narrow notch, implemented some other way, like an active filter, should be possible with little affect away from the notch. Maybe notch out 60 and 120, and use a lowpass to zap higher stuff. That sounds like an elliptical sort of filter. How much 60 Hz do you need to reject? Another approach would be to have a local 60 Hz reference. Synchronously detect the i/q components in your signal, remodulate, and subtract them out. Does your definition of "real time" allow any time delay? If so, a lowpass filter with some delay and phase equalization might work. John |