PLL for Zero Delay LPF
in [Design]
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From: Glenn Kenroy on 23 Jun 2010 22:48 On Wed, 23 Jun 2010 16:58:39 -0700, dplatt(a)radagast.org (Dave Platt) wrote: >>>Does your filter really need to be realtime? Yes please, for this application. >You can split the signal in half, and feed >part of it to a very narrow-bandwidth "band-pass" filter which lets >through only the 60 Hz component, Or, as I replied to John's post, how about deriving this signal instead from a second sensing coil tuned to 60Hz? Then adjust amplitude, invert and mix with to the output of sensor #1 to null the 60Hz component there. Glenn Kenroy
From: John Larkin on 23 Jun 2010 22:51 On Thu, 24 Jun 2010 02:34:38 GMT, glennkenroy(a)protech.com (Glenn Kenroy) wrote: >On Wed, 23 Jun 2010 16:12:30 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>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. >> > >You mean perhaps to use two identical sensing coils; one tuned to ELF >and the second to 60Hz? The output from the second could then be >adjusted in amplitude and inverted to null the 60Hz component in the >output of the second. Actually, I was thinking of using the local power line. But a divided-down crystal oscillator would work as well. It only needs to be pretty close to 60 Hz. > >>Does your definition of "real time" allow any time delay? If so, a >>lowpass filter with some delay and phase equalization might work. >> > >Yes, but I am still uncertain how the phase equalization would be >implemented in terms of actual circuitry. Ask google! John
From: Tim Wescott on 23 Jun 2010 23:49 On 06/23/2010 03:21 PM, 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? A zero _phase shift_ notch is easy, but a notch filter is hardly low delay, and the narrower it is the more delay it must have -- if you can't figure out why, ask. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Paul Keinanen on 24 Jun 2010 01:18 On Wed, 23 Jun 2010 19:51:32 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Thu, 24 Jun 2010 02:34:38 GMT, glennkenroy(a)protech.com (Glenn >Kenroy) wrote: >>You mean perhaps to use two identical sensing coils; one tuned to ELF >>and the second to 60Hz? The output from the second could then be >>adjusted in amplitude and inverted to null the 60Hz component in the >>output of the second. > >Actually, I was thinking of using the local power line. But a >divided-down crystal oscillator would work as well. It only needs to >be pretty close to 60 Hz. If the device is mains powered, why not use it directly as a reference for cancellation. You may need the ability to shift this reference phase, since the pick up coil may also pick up magnetic fields from other phases, creating a resultant, which is not in phase with your equipment power.
From: Jeroen Belleman on 24 Jun 2010 03:40
Glenn Kenroy wrote: > On Wed, 23 Jun 2010 16:58:39 -0700, dplatt(a)radagast.org (Dave Platt) > wrote: > > >>>> Does your filter really need to be realtime? > > Yes please, for this application. > >> You can split the signal in half, and feed >> part of it to a very narrow-bandwidth "band-pass" filter which lets >> through only the 60 Hz component, > > Or, as I replied to John's post, how about deriving this signal > instead from a second sensing coil tuned to 60Hz? Then adjust > amplitude, invert and mix with to the output of sensor #1 to null the > 60Hz component there. A second sensing coil doesn't necessarily pick up exactly the same interference as the first, so you're probably better off notch-filtering the signal from a single sensor. Apart from that, *carefully* *read* Dave Platt's explanation. This really is fundamental. Using a 2nd sensor tuned to 60Hz does not change that. Jeroen Belleman |