function VTB2_3(z,rmin,rmax,opt)%VTB2_3 Steady state magnitude and phase of a single degree of freedom % damped system.  % VTB2_3(zeta,rmin,rmax) plots the response of a single degree of % freedom system with damping ratio zeta between the frequency % ratios rmin and rmax.% zeta can be a list of damping ratios.% VTB2_3(zeta,rmin,rmax,opt) allows plotting of magnitude% (opt=1), phase (opt=2), or magnitude and phase (opt=3, default).%% Example:% vtb2_3([0:.2:1],0,2,3)if ishold==0	clfendif nargin==1	opt=z;	z=[.0:.05:.5];	rmin=0;	rmax=4;	disp('Demo mode. Type ''help vtb2_3'' to learn how to enter values.')endif nargin==0 	z=[.0:.2:2];	rmin=0;	rmax=4;	disp('Demo mode. Type ''help vtb2_3'' to learn how to enter values.')	opt=3;endif nargin==3, opt=3;endr=rmin:(rmax-rmin)/1000:rmax;z=z+eps;for i=1:length(z)	A0(i,:)=1./(1-r.^2+2*j*r*z(i));	a{i}=['\zeta = ' num2str(z(i))];endif opt==3 	subplot(2,1,1)endif opt ~=2semilogy(r,abs(A0))%plot(r,log10(abs(A0)))%labels=str2num(get(gca,'yticklabel'));%for i=1:length(labels)%	labels(i,:)=10^labels(i);%end%labels%labels=num2str(labels)%set(gca,'yticklabel',labels,'yticklabelmode','auto')legend(char(a),1)xlabel('Frequency Ratio')ylabel('Normalized Amplitude')title('Normalized Amplitude versus Frequency Ratio')%set(gca,'YTick',0:22.5:180)grid onzoom onend%pauseif opt==3subplot(2,1,2)endif opt ~=1plot(r,-angle(A0)/pi*180)legend(char(a),4)xlabel('Frequency Ratio')ylabel('Phase lag (\circ)')title('Phase versus Frequency Ratio')set(gca,'YTick',0:22.5:180)grid onzoom onaxis(axis+[0 0 -10 10])endbreak% Old codefor i=1:length(z)	z=zz(i);	A0=(1)./sqrt((1-r.^2).^2+(2*z*r).^2);%(2.30)	c	hold onendxlabel('Frequency Ratio')ylabel('Normalized Amplitude')title('Normalized Amplitude versus Frequency Ratio')grid onzoom onhold offpausefor i=1:length(zz)	z=zz(i);	phi=atan2(2*z*r,1-r.^2);%(2.30)	plot(r,phi/pi*180)	hold on	endxlabel('Frequency Ratio')ylabel('Phase lag (\circ)')title('Phase versus Frequency Ratio')set(gca,'YTick',0:22.5:180)grid onzoom onhold off%legend(char(a),4)