% IIR filter design example
%
% Bandstop Butterworth filter
% Passband Ripple = 3 dB
% Stopband Attenuation = 20 dB
% Lower passband = 0 - 50 Hz
% Upper passband = 450 - 500 Hz
% Stopband = 200 - 300 Hz
% Sampling frequency = 1 kHz
Fs = 1000;          % Sampling frequency
FN = Fs/2;          % Nyquist frequency
Ap = 3;             % Passband ripple
As = 20;            % Stopband attenuation
wp = [50/FN, 450/FN];        % Normalized passband edge
ws = [200/FN, 300/FN];       % Normalized stopband edge
[N,wc] = buttord(wp, ws, Ap, As);
[b,a] = butter(N,wp,'stop');   % Compute numerator & denominator coefficients
[zz,pz,kz] = butter(N,wp,'stop'); % Compute zeros, poles, and gain
subplot(2,1,1)
[H,f] = freqz(b,a,512,Fs);  % Compute frequency response
plot(f, abs(H))
xlabel('Frequency (Hz)')
ylabel('Magnitude Response')
subplot(2,1,2)
zplane(b,a)         % Plot zeros and poles