RC ACTIVE FILTER PROGRAM BY KINCAID AND SHIRLEY
C     MODIFIED FOR FOCAL BY TOM BEAN & WILLIAM ROMANS

C         T: TYPE OF FILTER (ENTER L FOR LOWPASS;H FOR HIGHPASS)
C         N: NUMBER OF POLES (MUST BE EVEN NUMBER)
C        KH: CUTOFF FREQUENCY IN KHZ
C         R: ALLOWABLE RIPPLE IN PASSBAND
C            (0 DB. FOR BUTTERWORTH DESIGN)
C        C1: MAXIMUM CAPACITANCE VALUE IN NANOFARADS

C     R1,R2 GIVEN IN KOHMS.  

C     THIS VERSION REINITIALIZES AFTER EACH PRINT-OUT, ALLOWING
C     SEVERAL 'PASSES' AT A DESIGN.  TO RETAIN PREVIOUS VALUE OF
C     A PARAMETER, TYPE ALT MODE KEY.  TO HALT, TYPE CTRL/C.



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C-FOCAL,1969

01.02 A !? T N KH R C1 ? ,!!;S PI=3.14159;S C(6)=1;S C(0)=10
01.04 S C(1)=6.8;S C(2)=4.7;S C(3)=3.3;S C(4)=2.2;S C(5)=1.5
01.06 S LG=FLOG(10);S L1=.1*LG;S J=0;F I=1,N/2;D 3
01.08 G 1.02

03.08 I (-R)3.16;S X1=1;S X2=1;G 3.26
03.16 S AR=FEXP(R*L1)-1;S E=FSQT(AR)
03.18 S D=FEXP(FLOG(1/E+FSQT(1/AR+1))/N)
03.20 S X1=(D-1/D)/2;S X2=X1+1/D
03.26 S AR=PI/2+PI*(2*I-1)/2*N
03.30 S S=X1*FCOS[AR];S W=X2*FSIN[AR];S S2=S^2
03.32 S A=S2+W^2
03.34 S X3=2*PI*KH*C1
03.36 I (T-12)7.04;F N1=-1,9;D 4

04.04 S P=FITR(FLOG(C1)/LG)-N1;S PP=10^FABS<P>
04.06 F K=1,6;I (N/2-J)4.1,4.1;D 5
04.10 R

05.04 I (-P)5.08;S C2=C(K)/PP;S C3=C(K-1)/PP;G 5.12
05.08 S C2=C(K)*PP;S C3=C(K-1)*PP
05.12 S M=C1/C2;I (M-A/S2)5.16;D 6
05.16 R

06.02 S RN=(-S*1E3/X3)*M/A;S AR=FSQT[1-A/S2*M]
06.04 S R1=RN*(1+AR);S R2=RN*(1-AR)
06.08 I (A/S2-C1/C3)6.10;D 8
06.10 R

07.04 S C2=C1;S R1=-S*1E3/X3;S R2=R1*A/S2;D 8

08.08 T "SECT."%2,I,!;S J=I
08.10 T %6.05,?C1?,?  C2?,?  R1?,?  R2?,!!
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