State variable notch filter.

I was trying to turn a previous SV filter into a notch, (Freq ~3.3 kHz, Q~10-20, see my mech. PID thread.) I was thinking of just subtracting the BP from the input. But almost all the web stuff I could find said add the HP and LP. (That seemed crazy to me, at high Q both those are peaking.. but then again the SV filter seems a bit crazy to me... two integraters in a row, come on. :^)

So I first summed the LP and HP, (it was easier, than the BP, no invert and divide the signal by the Q.) Amazingly it worked. Unfortunately the depth of the notch varied inversely with the Q... At high Q the notch was not deep. (It's probably losses in the capacitors.) At a Q of 20 and 500 mV input min on the notch was 25 mV.

Depressing.

Anyway I then flipped over the BP, attenuated it by ~Q (with a pot to tweak the attenuation) and summed it with the input. That made a much better notch. ~5mV at Q=20, (500 mV in) I didn't have time to check other Q's.

The thing that stinks about this notch, is that the high Q means I have to throw away dynamic range on the input. Is there a high Q, lower gain band pass filter.... (I'm seeing Tim W., "Why not... A/D -> uC -> D/A " or do you also suffer dynamic range issues when implementing a notch in digital space?)

George H.

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George Herold
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Instead of tinkering why don't you do the math ?>:-}

It's Veal Marsala and wine time, so I'll crank you out something later, with orthogonal adjustments...

Can we start with assuming you have 4 OpAmp sections? ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

Nevermind, it's already done, see GyratorFilterNotch.pdf on the S.E.D/Schematics Page of my website. Just add an input buffer and an output buffer (using up 4 OpAmp sections) and you're done. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

Yeah.. well I'm now using 5 or 6. There are 4 (a qaud) in the original SV. The first is a buffer, 10M to ground. And now I've tacked a floating dual on top. An inverter (and divider) on the BP that I then sum with the input. (I'll post a pic, typical hair ball...) I think as long as I put some gain behind the filter section, the dynamic range loss is not a problem.

I really love the SV filters separate Q and frequency adjustment.

George H.

Reply to
George Herold

[snip]

So do my gyrator filters... far nicer than SV. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

Tim W. already suggested a Twin-T. One of the nice things about it is that the null is provided by the passive network, and is (insofar as I understand it) more reliably deep. I suppose that centering the resonance on the null is still going to be an issue, and will get more sensitive to component variations as you turn up the Q.

Or, you can use Phil Hobb's suggestion of an LC circuit -- but there again, I suspect you'll find that non-ideal components will either cause the null to not be complete, or will cause the resonance to not be centered on the null.

I think you'll find that you suffer dynamic range issues no matter what you use. You certainly do with a digital filter -- as the poles get closer to the unit circle, the necessary precision of the coefficients as well as the data path width needed goes up. The nice thing about a digital filter, however, is that you don't have to redesign it every time you build a board, or change passives vendors.

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Tim Wescott 
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Tim Wescott

Thanks Tim, absolutely correct you suggested an (active) Twin T. I looked a little at the twin T, it has more components that I have to tweak if I need to change Q and frequency. I also read that it suffers from the same issue I saw with the SV notch. and this is that the depth decreases as the Q increases. (Though I didn't try it.) For the LC suggestion, I guess I'm liking JT's Gyrator, or something like that.

George H.

Reply to
George Herold

This is the point where I'd do a sensitivity analysis of the whole circuit, component by component and taking op-amp phase shift into account. Getting a null is one of those (big number) - (big number) = 0 things, so it doesn't take a very large percentage-wise deviation from perfection to ruin your nice perfect null.

The rule of thumb that was handed to me a long, long time ago by a guy I still trust is that when you're trying to cancel things out in an electronic circuit, getting 40dB of suppression is about the most you can realistically expect. If you must have more than that then you either need to go digital (and make damned sure it'll really work) or you need to expect that you're just building a lab queen that'll never work right in the field.

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Tim Wescott 
Control systems, embedded software and circuit design 
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Tim Wescott

See "StateVariableFilter(P+1).pdf" on the S.E.D/Schematics Page of my website... doesn't have dynamic range related to Q AND minimizes the effects of dissipation factor in the capacitors.

(Came up with this trick ~1970 when I was making (hybrid) low audio frequency filters... where dissipation factor can really screw you over.) ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

May work. I'd still want to see a component-by-component sensitivity analysis if I thought really deep nulls in my notches were important.

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Tim Wescott 
Control systems, embedded software and circuit design 
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Tim Wescott

For critical work you'll need COG's and low-tolerance resistors, plus one or two pots if you're really persnickety, one for f0 and one for Q.

I've been known to avoid the trimming during manufacture by using a series string of stagger-tuned notches. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

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