MATLAB CODE TO DESIGN A BUTTERWORTH BAND REJECT FILTER

CODE:

clear all

alphap=2;  %passband attenuation in dB

alphas=20; %stopband attenuation in dB

wp=[.2*pi,.4*pi]; % passband freq. in radians

ws=[.1*pi,.5*pi]; % stopband freq. in radians

%to find cutoff freq. and order of the filter

[n,wn]=buttord(wp/pi,ws/pi,alphap,alphas) %syatem function of the filter

[b,a]=butter(n,wn,'STOP')

w=0:.01:pi;

[h,ph]=freqz(b,a,w);

m=20*log(abs(h));

an=angle(h);

subplot(2,1,1);plot(ph/pi,m);grid;

ylabel('Gain in dB');

Xlabel('NORMALISED FREQUENCY')

subplot(2,1,2);plot(ph/pi,an);grid

ylabel('phase in radians');

xlabel('noramlised frequency')

OUTPUT:

n =  4

wn = 0.1950    0.4082

b =  0.4086   -2.0201    5.3798   -9.1468   10.8962   -9.1468    5.3798   -2.0201    0.4086

a =1.0000   -3.8710    7.9699  -10.6417   10.0781   -6.8167    3.2579   -1.0044    0.1670