Version 4
SHEET 1 1924 724
WIRE 64 128 0 128
WIRE 208 128 176 128
WIRE 320 128 256 128
WIRE 0 144 0 128
WIRE 256 144 256 128
WIRE 208 160 208 128
WIRE 0 240 0 224
WIRE 208 240 208 208
WIRE 256 240 256 224
FLAG 0 240 0
FLAG 256 240 0
FLAG 208 240 0
FLAG 64 128 in
FLAG 176 128 in
FLAG 320 128 out1
SYMBOL voltage 0 128 R0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 0
SYMATTR Value2 AC 1 0
SYMATTR InstName V1
SYMATTR Value ""
SYMBOL e 256 128 R0
SYMATTR InstName E1
SYMATTR Value Laplace=1/((s/wg+1)*((s/wg)^2 + s/wg*0.6180 +1) *( (s/wg)^2 + s/wg*1.6180 + 1))
TEXT -24 32 Left 2 !.ac dec 100 10 100k
TEXT -24 -176 Left 2 ;https://www.mikrocontroller.net/topic/470031#5763378
TEXT 288 288 Left 2 ;https://en.wikipedia.org/wiki/Butterworth_filter
TEXT -24 -112 Left 4 ;Grenzfrequenz 1kHz
TEXT -24 -64 Left 3 !.param fg=1k
TEXT -24 -16 Left 2 !.param wg=2*pi*fg
TEXT 584 -120 Left 2 !.options meascplxfmt=polar\n.meas mag0 FIND mag(V(out1)) AT 10\n.meas fg FIND freq WHEN mag(V(out1))=mag0/sqrt(2)
TEXT 592 8 Left 3 ;View -> SPICE Error Log
TEXT 592 56 Left 2 ;mag0: mag(v(out1))=(1,0°) at 10\nfg: freq=(1000.03,0°) at 1000.03