C: 3-POLE BUTTERWORTH ACTIVE FILTERS, BY KERWIN; PUBLISHED
C:  IN HUELSMAN, "ACTIVE FILTERS", MC GRAW-HILL 1970, CH. 2

C:  ENTER ZERO FOR R OR FOR C, IF EITHER IS UNKNOWN.  TO CALL
C:  GRAPH ROUTINE, ENTER A NEGATIVE VALUE FOR R.  EXISTING 
C:  VALUES OF R, C, OR K MAY BE RETAINED BY TYPING 'ALT MODE' KEY.
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C-FOCAL,1969

01.02 S RI(1)=1;S RI(2)=10;S RI(3)=1;F I=1,3;S CI(I)=1/RI(I)
01.05 T !! "R IN KILOHMS, C IN MICROFD., K IN KILOHZ.
01.10 S PI=3.14159;T ! "ENTER ZERO FOR THE UNKNOWN.
01.20 A !!?R?;I (R)4.01;A ?   C?;I (-R*C)2.1;A ?   K?
01.30 S RF=2*3.14159*K*1E3;I (R)1.13,1.4
01.35 S K1=R*1E3;S K2=1/K1*RF;G 1.5
01.40 S K2=C*1E-6;S K1=1/K2*RF
01.50 F I=1,3;S R(I)=RI(I)*K1;T !"R(",%1,I,") ",%5.05,R(I)*1E-3
01.60 T " K.OHMS";F I=1,3;S C(I)=CI(I)*K2;T !"C(",%1,I,") ";D 3
01.70 G 1.2

02.10 S RC=R*1E3*C*1E-6;S K=1/2E3*PI*RC;T !,%5.05,K," K.HZ.
02.20 G 1.2

03.10 I [C(I)-1E-9]3.5
03.30 T %5.05,C(I)*1E6," MFD.";R
03.50 T %5.05,C(I)*1E12," PICOFD."

04.01 I (K)1.2,1.2
04.03 T !!"-40.......-30.......-20.......-10........0   DB.",!
04.05 F EX=-.6,.05,-.05;D 5;D 6
04.07 S BD=37;D 6.1;T "*     DESIGN FR.",K,!!
04.10 F EX=.05,.05,.6;D 5;D 7
04.20 Q
04.30 S X=FSQT[1+X^6];S X=-20*FLOG(X)/FLOG(10)

05.10 S WC=FEXP[EX*FLOG(10)];S X=WC;D 4.3;S DB=FITR(40+X)
05.20 S X=1/WC;D 4.3;S BD=FITR(40+X)

06.10 F I=0,BD;T " "
06.20 T "H";F I=BD+1,DB;T " "
06.30 T "L";F I=DB,50;T " "
06.40 T %4.04,WC*K,!!

07.10 F I=0,DB;T " "
07.20 T "L";F I=DB+1,BD;T " "
07.30 T "H";F I=BD,50;T " "
07.40 D 6.4

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