Article Filtering of ECG signal using Butterworth Filter and its fea...

The poles for Butterworth low-pass digital filters are on a cirlce centred on z=1.

Pampa, just run this MATLAB code (below). Enjoy :-)

%===

%Butterworth filter - by F.S. Schlindwein

N=4; % order - change here for 1, 2, 3, 4, ...

Np=512;

step=2/512;

frequencies=0:step:2-step;

%frequencies=0:2/512:2-2/512

Wn=0.2;

Fs=2;

%[zb,pb,kb] = butter(N,Wn,'low');

[Bb,Ab] = butter(N,Wn,'low');

%[sosb,gb] = zp2sos(zb,pb,kb);

%Hb = dfilt.df2sos(sosb,gb);

%grpdelay(Hb);

Hb = freqz(Bb,Ab,Np,'whole',Fs);

figure(1);

plot(frequencies(1:256), abs(Hb(1:256)));

grid on;

set(gca,'YLim',[0 1.1])

title('Gain of Butterworth');

print -deps -tiff -r600 Butter_gain;

%set(gca,'YLim',[0 5])

figure(2);

zplane(Bb,Ab);

title('Poles and Zeroes of Butterworth');

angleb=angle(Hb);

figure(3);

plot(frequencies(1:256), unwrap(angle(Hb(1:256))));

grid on;

title('Phase of Butterworth');

figure(4);

[Gd,W] = grpdelay(Bb,Ab,Np,Fs);

plot(frequencies(1:Np-8)/2,Gd(1:Np-8));

title('Group delay of Butterworth');