% Program 4_1
% 4-th Order Analog Butterworth Lowpass Filter Design
%
format long
% Determine zeros and poles
[z,p,k] = buttap(4);
disp('Poles are at');disp(p);
% Determine transfer function coefficients
[pz, pp] = zp2tf(z, p, k);
% Print coefficients in descending powers of s
disp('Numerator polynomial'); disp(pz)
disp('Denominator polynomial'); disp(real(pp))
omega = [0: 0.01: 5];
% Compute and plot frequency response
h = freqs(pz,pp,omega);
plot (omega,20*log10(abs(h)));grid
xlabel('Normalized frequency'); ylabel('Gain, dB');