filter

i make an am modulation system but i can not make a correct filter please test this system and tell me that is the wn=fm/2*fc ?true or false

function [op]=am(m,fc,fm,snr)
%am modulation
t=0:.001:1;
figure;
subplot 311;
plot(t,m);
c=2*cos(2*pi*fc*t);
tx=m.*c;
subplot 312;
plot(t,tx);
%tansmitte signal
snr=10^(snr/10);
vn=var(tx)/snr;                    %AWGN channl
n=sqrt(vn)*randn(1,1001);
tx=tx+n;
%receive signal
rx=tx.*c;
[b a]=butter(1,(fm/(2*fc)));
op=filter(b,a,rx);
subplot 313;
plot(t,op);
return