MATLAB CODE TO DESIGN A BUTTERWORTH LOWPASS FILTER

CODE:

clear all

alpha=.4;  %passband attenuation in dB

alphas=30; %stopband attenuation in dB

fp=400; %passband freq. in hz

fs=800; %stopband freq. in hz

F=2000; %sampling freq. in hz

omp=2*fp/F;oms=2*fs/F; % To find cutoff freq. and order of the filter

[n,wn]=buttord(omp,oms,alpha,alphas) %syatem function of the filter

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

w=0:.01:pi;

[h,om]=freqz(b,a,w,'whole');

m=abs(h);

an=angle(h);

subplot(2,1,1);plot(om/pi,20*log(m));grid;

ylabel('gain in dB');

Xlabel('NORMALISED FREQUENCY')

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

ylabel('phase in radians')

xlabel('noramlised frequency')

OUTPUT:

n =4

wn =0.5821

b =  0.1518    0.6073    0.9109    0.6073    0.1518

a =1.0000    0.6418    0.6165    0.1449    0.0259