tk=1;
z=1;
num=[z];
zeta=0.05;den=[tk^2 2*zeta*tk 1];[m1,p1,w1]=bode(num,den);
zeta=0.2;den=[tk^2 2*zeta*tk 1];[m3,p3,w3]=bode(num,den);
zeta=0.5;den=[tk^2 2*zeta*tk 1];[m4,p4,w4]=bode(num,den);
zeta=1.0;den=[tk^2 2*zeta*tk 1];[m6,p6,w6]=bode(num,den);

subplot(211)
semilogx(w1,20*log10(m1),'k')
hold on
semilogx(w3,20*log10(m3),'k--')
semilogx(w4,20*log10(m4),'k-.')
semilogx(w6,20*log10(m6),'k:')
hold off
xlabel('Frequency (rad/sec)'), ylabel('Magnitude (dB)')
%grid

subplot(212)
semilogx(w1,p1,'k')
hold on
semilogx(w3,p3,'k--')
semilogx(w4,p4,'k-.')
semilogx(w6,p6,'k:')
hold off
%grid
xlabel('Frequency (rad/sec)'), ylabel('Phase (deg)')
h=legend('\zeta=0.05','\zeta=0.2','\zeta=0.5','\zeta=1.0',1);
axes(h)

set(1, 'PaperUnits', 'centimeters')
set(1, 'PaperPosition', [3 10 11 9])
print -deps 'obr437.eps'