LPFbut(N)

function  LPFbut(N) 
	%  LPFbut(N) assumes a cutoff frequency of 1 radian/sec
	%  and an order of N.
	[zs,ps,ks] = buttap(N); 
	[b,a] = zp2tf(zs,ps,ks);
	[mag,ph,w] = bode(b,a); 
	db = 20*log10(mag);
	clg
	subplot(2,2,1), zplane(zs,ps);
	subplot(2,2,3), step(b,a);
	subplot(2,2,2), semilogx(w,db); grid
	subplot(2,2,4), semilogx(w,ph); grid