close all ;
clear all;
fc=9;
N1=9;
N2=27;
n1=0:1/N1:N1
x1=sin(2*pi*n1)+3*sin(2*pi*3*n1)+5*sin(2*pi*5*n1)+7*sin(2*pi*7*n1)+9*sin(2*pi*9*n1);
n2=0:1/N2:N2
x2=sin(2*pi*n2)+3*sin(2*pi*3*n2)+5*sin(2*pi*5*n2)+7*sin(2*pi*7*n2)+9*sin(2*pi*9*n2);
wp=0.35;
ws=0.7;
rp=3;
rs=15;
[N,wc]=buttord(wp,ws,rp,rs,'s');
[Bz,Az]=butter(N,wc,'low');
wk=64;
Hk=freqz(Bz,Az,wk);
figure
subplot(2,2,1);
stem(0:1/63:1,abs(Hk),'.');
xlabel('频率');ylabel('滤波器幅度/dB');
grid on
m1=filter(Bz,Az,x1);
m2=filter(Bz,Az,x2);
y1=fft(m1,N1);
y2=fft(m2,N2);
subplot(2,2,3)
stem(0:N1-1,abs(y1),'.');
xlabel('频率');ylabel('滤波后Y2(n)频率特性')
title('N=9')
grid on
subplot(2,2,4)
stem(0:N2-1,abs(y2),'.');
xlabel('频率');ylabel('滤波后Y2(n)频率特性');
title('N=27')
grid on
figure
wp=0.75;
ws=0.5;
rp=3;
rs=15;
[N,wc]=buttord(wp,ws,rp,rs,'s');
[Bz,Az]=butter(N,wc,'high');
wk=64;
Hk=freqz(Bz,Az,wk);
subplot(2,2,1);
stem(0:1/63:1,abs(Hk),'.');
xlabel('频率');ylabel('滤波器幅度/dB');
grid on
m1=filter(Bz,Az,x1);
m2=filter(Bz,Az,x2);
y1=fft(m1,N1);
y2=fft(m2,N2);
subplot(2,2,3)
stem(0:N1-1,abs(y1),'.');
xlabel('频率');ylabel('滤波后Y1(n)频率特性');
title('N=9')
grid on
subplot(2,2,4)
stem(0:N2-1,abs(y2),'.');
xlabel('频率');ylabel('滤波后Y2(n)频率特性');
title('N=27')
grid on
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