PLL for Zero Delay LPF

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

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"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."

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Reply to
Glenn Kenroy
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As I demonstrated in...

"ANALYSIS AND DESIGN OF INTEGRATED CIRCUITS" MOTOROLA Series in Solid-State Electronics ©1967 Library of Congress Catalog Card Number: 66-25486

Chapter 15 (Which I wrote :-) pp470-473 ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Jim, can I get a list of everything you have ever done so I can see just how omnipotent you really are?

Reply to
George Jefferson

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.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

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

Reply to
John Larkin

If someone came up with a negative delay filter, now that would be something :-)

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SCNR, Joerg

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Reply to
Joerg

Does the OP need zero delay or is it really _flat_phase_?

It'd probably be a PITA, but I'd guess one could conjure up something like an all-pass to flatten the phase response. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

That used to be one of our regular chores in Doppler ultrasound. Gradually the required 1% capacitor became unobtanium and then, huge sigh of relief, AD came out with the first affordable DSP for around 10 bucks. Then, the folks in purchasing started to like us again and we could share in their Christmas time bonbon cartons they got from distributors.

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Regards, Joerg

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Reply to
Joerg

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.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

That's been suggested in the original thread.

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Tim Wescott
Control system and signal processing consulting
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Reply to
Tim Wescott

That sounds eerily similar to what some people on Wall Street used to claim :-)

Many pension funds with union ties still do :-(

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Regards, Joerg

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Reply to
Joerg

if post-processing, run the data through the filter twice, once forwards once backwards, you get double the filter order and zero phase

-Lasse

Reply to
langwadt

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.

:-)

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Dave Platt                                    AE6EO
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Reply to
Dave Platt

It just requires capacitors made with resublimated Thiotimoline dielectric.

Reply to
Spehro Pefhany

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

Reply to
John Larkin

And he served his sentence before that.

John

Reply to
John Larkin

r
s

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It is - of course - too good to be true.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

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

Reply to
Glenn Kenroy

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

--=20 Deep Friar: a very philosophical monk. Website:

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Reply to
Tim Williams

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

Reply to
John Larkin

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