Butterworth filter

   illustrates the design of a Butterworth bandstop filter.

% Program P7_1

% Design of a Butterworth Bandstop Digital Filter

Ws = [0.4 0.6]; Wp = [0.3 0.7]; Rp = 0.4; Rs = 50;

% Estimate the Filter Order

[N1, Wn1] = buttord(Wp, Ws, Rp, Rs);

% Design the Filter

[num,den] = butter(N1,Wn1,’stop’);

% Display the transfer function

disp(’Numerator coefficients are ’);disp(num);

disp(’Denominator coefficients are ’);disp(den);

% Compute the gain response

[g,w] = gain(num,den);

% Plot the gain response

plot(w/pi,g);grid

axis([0 1 -60 5]);

xlabel(’\omega /\pi’); ylabel(’Gain, dB’);

title(’Gain Response of a Butterworth Bandstop Filter’);