Dual to AllPass.?

Wouldn't it be nice. No, in general that violates causality. (I'll be a bit less dogmatic about that than previously, since we talked about this several months back.)

A symmetric FIR filter centered around t=0 might qualify, if you don't need it to be real-time.

Cheers

Phil Hobbs

I was hoping you had a change of heart ;-)

(I've decided, therefore, that I've been fed some bad data from a real device :-( ...Jim Thompson

if it doesn't need to be real time run in through the filter twice, once fo rward once reverse. Double the filter order, zero phase

-Lasse

Couldn't you make a lowpass filter, then fix its phase shift with an all-pass?

You could make an approximation of a constant-delay filter (like Phil's symmetric FIR). But you can't keep the phase constant here in the real world.

You can flatten it out, or fix it in some restricted region (as in your example from last year) but you can't get rid of it everyplace. Group delay != real delay, but it's the leading order approximation.

Cheers

Phil Hobbs

Jim didn't specify exactly zero phase, over an infinite frequency span; neither request would be reasonable. But it should be practical to make a lowpass filter, followed by an allpass network, that has minimal phase shift up to, say,

I doubt that very much. If you can do that to an LC-type lowpass, you'll put Tim Wescott and Co. out of business.

Cheers

Phil Hobbs

I didn't say LC. I was thinking 1st order, RC, because that case is easy to think about.

But it could be done to a higher-order filter, just more work.

Point is, that the answer to Jim's question is that, yes, it can be done within reasonable practical limits.

Well I had LTspice up and running. I tried setting this up, RC low pass, followed by all pass. (no luck all I got was more phase shift.) (I tried both versions of the all pass, cap or resistor to ground on non-inverting input.)

I went to make sure I was doing the all pass correctly and found this, totally unrelated... but all pass as Quadrature generator.

A bit like the phase sequence filter.

George H.

I think that's in AoE, or at least Win posted that here years ago. ...Jim Thompson

First order RC filters don't have real delay--you can cancel out a lowpass with a highpass, over whatever bandwidth you like, if you can stand the loss.

Cheers

Phil Hobbs

Hey on a related question, Is there a filter that has a minimum pass shift until you get right near the corner? (I'm afraid the answer is no, but I thought I'd ask.)

George H.

Bessel, for one. And there are fancier filters that have better rolloff and are sort of phase linear some distance into the stopband.

Williams' book has a number of forms, and lots of nice graphs.

Of course, higher order filters have more time delay, which is itself phase shift vs frequency. The Bessel type filters just have approximately constant delay vs frequency.

Sure. An elliptic, or even just a sharpish lowpass with a bunch of sharp notches to improve the shape factor.

Notch filters aren't used enough IMO--I like using them inside control loops for resonant actuators.

Cheers

Phil Hobbs

I remember trying to work with a wire made of metglas, 0.7 MS/m conductivity and perm of 1,000,000

Used as a 10 micron wire at DC you got low resistance, but any attempt at frequency the resistance skyrocketed. From memory no phase shift. Eddy currents are like a distributed network. The idea is drive a fixed resistor with the 'wire', and at DC you get some maximum voltage through, then as you go up in frequency almost nothing. Again, from memory, no phase shift, or at least very little phase shift.

Forgot to ask. Are you after a physically realizable implementation, or are you after a theoretical implementation?

Have to check my models to see range and phase shift etc.

Is this a 'curisoity' question? Or some important piece in something?

Trying to model excess phase conditions in an OpAmp, but I've decided the data is hokey... some definite non-physical stuff. ...Jim Thompson

shift.)

non-inverting input.)

er-phase-shifter

done within

I can give you one that does 20 Hz to 20 kHz with 4 opamps.

Version 4 SHEET 1 1056 1048 WIRE 496 -448 368 -448 WIRE 368 -416 368 -448 WIRE 496 -416 496 -448 WIRE 208 -336 176 -336 WIRE 624 -336 560 -336 WIRE 800 -336 704 -336 WIRE 176 -272 -224 -272 WIRE 368 -272 368 -336 WIRE 368 -272 176 -272 WIRE 416 -272 368 -272 WIRE 688 -272 416 -272 WIRE 784 -272 688 -272 WIRE 560 -160 560 -336 WIRE -16 -144 -96 -144 WIRE 96 -144 96 -336 WIRE 96 -144 64 -144 WIRE 688 -128 688 -272 WIRE 176 -112 176 -272 WIRE 560 -112 560 -160 WIRE 656 -112 560 -112 WIRE 96 -96 96 -144 WIRE 144 -96 96 -96 WIRE 800 -96 800 -336 WIRE 800 -96 720 -96 WIRE 896 -96 800 -96 WIRE 976 -96 976 -144 WIRE 288 -80 288 -336 WIRE 288 -80 208 -80 WIRE 352 -80 352 -96 WIRE 352 -80 288 -80 WIRE 480 -80 480 -160 WIRE 480 -80 352 -80 WIRE 608 -80 560 -80 WIRE 656 -80 608 -80 WIRE 128 -64 64 -64 WIRE 144 -64 128 -64 WIRE 480 -64 480 -80 WIRE 560 -64 560 -80 WIRE -96 -32 -96 -144 WIRE -16 -32 -96 -32 WIRE 64 -32 64 -64 WIRE 976 -32 976 -96 WIRE 128 -16 128 -64 WIRE 176 -16 176 -48 WIRE 368 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96 336 WIRE 192 368 96 368 WIRE 688 368 688 320 WIRE 720 368 688 368 WIRE 336 384 336 288 WIRE 336 384 256 384 WIRE 400 384 400 320 WIRE 400 384 336 384 WIRE 832 384 832 256 WIRE 832 384 784 384 WIRE 896 384 832 384 WIRE 976 384 976 304 WIRE 192 400 176 400 WIRE 720 400 704 400 WIRE -304 416 -304 352 WIRE -176 416 -304 416 WIRE -96 416 -96 336 WIRE 0 416 -96 416 WIRE 400 416 400 384 WIRE 512 416 480 416 WIRE 624 416 592 416 WIRE 688 416 624 416 WIRE 704 416 704 400 WIRE 704 416 688 416 WIRE -448 464 -448 432 WIRE 688 464 688 416 WIRE 80 480 80 416 WIRE 144 480 80 480 WIRE 176 480 176 400 WIRE 176 480 144 480 WIRE 752 480 752 416 WIRE 864 480 864 16 WIRE 864 480 752 480 WIRE 976 480 976 384 WIRE 224 496 224 416 WIRE 288 496 224 496 WIRE -96 512 -96 416 WIRE 16 512 -96 512 WIRE 80 512 80 480 WIRE 144 512 144 480 WIRE 400 544 400 416 WIRE 624 544 624 416 WIRE 624 544 464 544 WIRE 688 560 688 528 WIRE -448 592 -448 544 WIRE -368 592 -368 336 WIRE -368 592 -448 592 WIRE 976 592 976 560 WIRE -448 624 -448 592 WIRE 32 640 16 640 WIRE 144 640 144 576 WIRE 144 640 112 640 WIRE 16 672 16 640 WIRE 688 672 688 640 WIRE 16 800 16 752 WIRE 688 800 688 752 WIRE -176 848 -176 416 WIRE 288 848 288 496 WIRE 288 848 -176 848 WIRE 368 848 368 -16 WIRE 368 848 288 848 WIRE 864 848 864 480 WIRE 864 848 368 848 WIRE 896 848 864 848 WIRE 896 880 896 848 WIRE 896 1008 896 960 FLAG -448 624 0 FLAG 80 256 0 FLAG 16 800 0 FLAG 560 256 0 FLAG 688 800 0 FLAG 976 96 0 FLAG 976 592 0 FLAG 896 1008 0 FLAG 496 -416 0 FLAG 976 -144 +45deg FLAG 976 304 -45deg FLAG 352 -96 +deg1 FLAG 416 272 -deg1 SYMBOL Opamps\\LT1028 -304 256 R0 SYMATTR InstName U1 SYMBOL Opamps\\LT1028 176 -144 R0 SYMATTR InstName U2 SYMBOL Opamps\\LT1028 688 -160 R0 SYMATTR InstName U3 SYMBOL Opamps\\LT1028 224 320 R0 SYMATTR InstName U4 SYMBOL Opamps\\LT1028 752 320 R0 SYMATTR InstName U5 SYMBOL voltage -448 448 R0 WINDOW 123 24 132 Left 0 WINDOW 39 24 160 Left 0 SYMATTR Value2 AC 1 0 SYMATTR SpiceLine Rser=100 Cpar=20p SYMATTR InstName V1 SYMATTR Value PULSE(-0.5 0.5 2e-6 1e-6 1e-6 PARAM stepon stepper) SYMBOL res -256 160 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value 10k SYMBOL res 192 -352 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 100k SYMBOL res 304 -352 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 5.6k SYMBOL res 80 -160 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 22k SYMBOL res 80 -48 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 56k SYMBOL res 64 320 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 22k SYMBOL res 96 400 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R7 SYMATTR Value 39k SYMBOL res 192 272 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R8 SYMATTR Value 100k SYMBOL res 352 272 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R9 SYMATTR Value 5.6k SYMBOL res 576 -176 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R10 SYMATTR Value 12k SYMBOL res 576 -80 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R11 SYMATTR Value 47k SYMBOL res 608 304 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R12 SYMATTR Value 12k SYMBOL res 608 400 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R13 SYMATTR Value 3.9k SYMBOL res 720 -352 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R14 SYMATTR Value 47k SYMBOL res 784 240 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R15 SYMATTR Value 47k SYMBOL res 112 64 R0 SYMATTR InstName R16 SYMATTR Value 47k SYMBOL res 64 144 R0 SYMATTR InstName R17 SYMATTR Value 4.7k SYMBOL res 0 656 R0 SYMATTR InstName R18 SYMATTR Value 2.0k SYMBOL res 128 624 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R19 SYMATTR Value 33k SYMBOL res 672 656 R0 SYMATTR InstName R20 SYMATTR Value 1.5k SYMBOL res 672 544 R0 SYMATTR InstName R21 SYMATTR Value 22k SYMBOL res 544 128 R0 SYMATTR InstName R22 SYMATTR Value 22k SYMBOL cap 112 -16 R0 SYMATTR InstName C1 SYMATTR Value 1n SYMBOL cap 544 48 R0 SYMATTR InstName C2 SYMATTR Value 100n SYMBOL cap 128 512 R0 SYMATTR InstName C3 SYMATTR Value 27n SYMBOL cap 672 464 R0 SYMATTR InstName C4 SYMATTR Value 470p SYMBOL cap 560 0 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C5 SYMATTR Value 100n SYMBOL cap 48 48 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C6 SYMATTR Value 1n SYMBOL cap 80 496 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C7 SYMATTR Value 27n SYMBOL res 496 400 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R23 SYMATTR Value 47k SYMBOL cap 464 528 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C8 SYMATTR Value 470p SYMBOL res 992 -112 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R24 SYMATTR Value 2k SYMBOL res 992 368 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R25 SYMATTR Value 2k SYMBOL res 960 -48 R0 SYMATTR InstName R26 SYMATTR Value 100k SYMBOL res 960 464 R0 SYMATTR InstName R27 SYMATTR Value 100k SYMBOL voltage 368 -320 R180 WINDOW 0 24 104 Left 0 WINDOW 3 24 16 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 24 76 Left 0 SYMATTR InstName V3 SYMATTR Value 15 SYMATTR SpiceLine Rser=0.1 SYMBOL voltage 896 976 R180 WINDOW 0 24 104 Left 0 WINDOW 3 24 16 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 24 -12 Left 0 SYMATTR InstName V2 SYMATTR Value 15 SYMATTR SpiceLine Rser=0.1 SYMBOL res -464 336 R0 SYMATTR InstName R28 SYMATTR Value 10k TEXT -480 1032 Left 0 !;ac oct 4 20 20000 TEXT -480 928 Left 0 ;* Tuning parts (nominal) R3 (5.6k), R9 (5.6k), R13 (3.9k), R17 (3.3k), R18 (2.0k), R20 (1.8K) TEXT 272 -360 Left 0 ;* TEXT 152 184 Left 0 ;* TEXT 320 264 Left 0 ;* TEXT 88 696 Left 0 ;* TEXT 584 384 Left 0 ;* TEXT 760 696 Left 0 ;* TEXT -472 712 Left 0 !.step oct param stepon 25u 25m 2 TEXT -472 752 Left 0 !.step oct param stepper 50u 50m 2 TEXT -472 872 Left 0 !.tran 100m

span; neither

lowpass

shift up to, say

easy to

done within

the corner?

I think an inverse Chebychev can do that. I like the smooth pass band and low phase ripple in the pass band. Almost all the cobblies are in the stop band. Bessels seem to be easier to realize though but have different characteristics in corner sharpness and stop band loss.

?-)