State variable filter Q

What kind of "sharp filtering" are you trying to get (i.e. f, Q)? SV filters (2 integrators and a summer) are generally among the best- behaved, most predictable active filters {though I believe JT is partial to gyrator-based filters}

Yes. Gyrator-based filters have Q-independent component sensitivity, usually 1:1 or less; and frequency and Q adjustments are independent of each other. ...Jim Thompson

See the bottom of:

_That_ is the peak i am talking about. Also attached is a Spice of the circuit with modified values to get neat 7.8Hz.

Need to know how to tune and get that peak.

Hmmm. All you need to know seems be in the tutorial. The center frequency is established by the RC of the two identical integrators:

fcenter = 1/(2pi*RC)^2

The Q is related to the DC gain vs the gain at the center frequency.

Simple case: 1. All resistors = 10k and C = 1uF, Fc = 15.9 Hz, Gain = 1 2. Input R = 1k, other resistors = 10k and C = 1uF, Fc = 15.9 Hz, Gain = 10, Q = 10 3. Case 2 but with C = 0.5uF, Fc = 31.8 Hz ...

Not hard (?)

BUT in Spice, all i have to do is change the summer from "opamp" to any available LT opamp, and *poof* _no_ peak - just a broad hump with different frequency at the max. So, to get to reality, it looks dang near impossible.

On 11/30/2015 4:32 AM, Robert Baer wrote:

Well, I just grabbed an LT1001A and swapped it into your schematic with the Q=10 31.8 Hz from above and the picture came out the same as with "opamp". Maybe you need to fetch a fresh cup of coffee?

-- Grizzly H.

Version 4 SHEET 1 880 852 WIRE -176 -16 -384 -16 WIRE 448 -16 -96 -16 WIRE -384 48 -384 -16 WIRE -336 48 -384 48 WIRE -208 48 -256 48 WIRE 0 80 -48 80 WIRE 112 80 64 80 WIRE 336 80 288 80 WIRE 448 80 448 -16 WIRE 448 80 400 80 WIRE -384 144 -384 48 WIRE -304 144 -384 144 WIRE -208 160 -208 48 WIRE -208 160 -240 160 WIRE -176 160 -208 160 WIRE -48 160 -48 80 WIRE -48 160 -96 160 WIRE 0 160 -48 160 WIRE -512 176 -576 176 WIRE -384 176 -432 176 WIRE -304 176 -384 176 WIRE 112 176 112 80 WIRE 112 176 64 176 WIRE 160 176 112 176 WIRE 288 176 288 80 WIRE 288 176 240 176 WIRE 336 176 288 176 WIRE 0 192 -16 192 WIRE 448 192 448 80 WIRE 448 192 400 192 WIRE 480 192 448 192 WIRE 336 208 320 208 WIRE -16 256 -16 192 WIRE 320 256 320 208 WIRE -576 272 -576 256 WIRE -384 336 -384 176 WIRE -176 336 -384 336 WIRE 112 336 112 176 WIRE 112 336 -96 336 WIRE 480 336 112 336 FLAG 480 336 Va FLAG -576 272 0 FLAG -16 256 0 FLAG 320 256 0 FLAG 480 192 Vb FLAG -272 128 +15V FLAG 32 144 +15V FLAG 368 160 +15V FLAG -272 192 -15V FLAG 32 208 -15V FLAG 368 224 -15V FLAG -624 -16 0 FLAG -496 -16 0 FLAG -624 -96 -15V FLAG -496 -96 +15V SYMBOL voltage -576 160 R0 WINDOW 123 -127 33 Left 2 WINDOW 39 0 0 Left 2 WINDOW 0 -97 76 Left 2 SYMATTR Value2 AC 1V SYMATTR InstName V1 SYMATTR Value "" SYMBOL res -528 192 R270 WINDOW 0 31 5 VTop 2 WINDOW 3 -8 43 VBottom 2 SYMATTR InstName R1 SYMATTR Value 1K SYMBOL res -352 64 R270 WINDOW 0 38 49 VTop 2 WINDOW 3 -6 37 VBottom 2 SYMATTR InstName R2 SYMATTR Value 10K SYMBOL res -192 0 R270 WINDOW 0 45 51 VTop 2 WINDOW 3 -12 45 VBottom 2 SYMATTR InstName R4 SYMATTR Value 10K SYMBOL res -192 352 R270 WINDOW 0 45 51 VTop 2 WINDOW 3 -12 45 VBottom 2 SYMATTR InstName R6 SYMATTR Value 10K SYMBOL cap 64 64 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 -22 76 VTop 2 SYMATTR InstName C1 SYMATTR Value {C} SYMBOL cap 400 64 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 -20 78 VTop 2 SYMATTR InstName C2 SYMATTR Value {C} SYMBOL res -80 144 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 -59 56 VTop 2 SYMATTR InstName R3 SYMATTR Value {R} SYMBOL res 256 160 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value {R} SYMBOL Opamps\\LT1001A -272 96 R0 SYMATTR InstName U2 SYMBOL Opamps\\LT1001A 32 112 R0 SYMATTR InstName U3 SYMBOL Opamps\\LT1001A 368 128 R0 SYMATTR InstName U4 SYMBOL voltage -624 -112 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 44 Left 2 SYMATTR InstName V2 SYMATTR Value -15 SYMATTR SpiceLine Rser=0.1 SYMBOL voltage -496 -112 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 44 Left 2 SYMATTR InstName V3 SYMATTR Value 15 SYMATTR SpiceLine Rser=0.1 TEXT -304 -152 Left 2 !.ac lin 10000 1 100 TEXT -296 -112 Left 2 !.lib opamp.sub TEXT -104 -112 Left 2 !.param R=10K C=0.5u

I've used SV-filters a lot for audio stuff. Q's to 50. You have to stay well inside the Q*freq

In your AC Analysis, change your "number of points per decade" to about

No where near the same! My peak is near the 7.8Hz i want and it is SHARP; ~6.2Hz, -29.8dB. Your "peak" is (as i said) broad as all hell and way different in frequency; ~31.6Hz, +20dB.

So, you imply that REAL parts give results wildly different thanwhat SPICE shows - which then may imply the so-called math tweaks ("calculations") are not so useful as well?

Makes little difference other than make it look prettier. Part change still makes it suck.

R1 = 0.35k (voltage gain = Q = 10000/350 = 28.586) C = 2u Fc = 1/(2*pi*10k*2u) = 7.957...

Switch the plot to linear on both axes - looks better.

He's channeling Bob Pease - "The best programming language is solder".

Plus rosin smells nice. Like sitting next to a campfire. Maybe with a hot cup of coffee you made yourself from freshly ground beans.

Grin, I did add the smiley face. And I think I was just watching a Pease video (linked to in the 610 thread) (What's all this femto amp stuff?) Where Paul Grohe is showing off his prototype tester for the LMC6001.

George H.