clear all; close all; fs=8e5;%抽樣頻率 fm=20e3;%基帶頻率 n=2*(6*fs/fm); final=(1/fs)*(n-1); fc=2e5; % 載波頻率 t=0:1/fs:(final); Fn=fs/2;%耐奎斯特頻率 %用正弦波產生方波 %========================================== twopi_fc_t=2*pi*fm*t; A=1; phi=0; x = A * cos(twopi_fc_t + phi); % 方波 am=1; x(x>0)=am; x(x<0)=-1; figure(1) subplot(321); plot(t,x); axis([0 2e-4 -2 2]); title('基帶信號'); grid on car=sin(2*pi*fc*t);%載波 ask=x.*car;%載波調制 subplot(322); plot(t,ask); axis([0 200e-6 -2 2]); title('PSK信號'); grid on; %===================================================== vn=0.1; noise=vn*(randn(size(t)));%產生噪音 subplot(323); plot(t,noise); grid on; title('噪音信號'); axis([0 .2e-3 -1 1]); askn=(ask+noise);%調制后加噪 subplot(324); plot(t,askn); axis([0 200e-6 -2 2]); title('加噪后信號'); grid on; %帶通濾波 %====================================================================== fBW=40e3; f=[0:3e3:4e5]; w=2*pi*f/fs; z=exp(w*j); BW=2*pi*fBW/fs; a=.8547;%BW=2(1-a)/sqrt(a) p=(j^2*a^2); gain=.135; Hz=gain*(z+1).*(z-1)./(z.^2-(p)); subplot(325); plot(f,abs(Hz)); title('帶通濾波器'); grid on; Hz(Hz==0)=10^(8);%avoid log(0) subplot(326); plot(f,20*log10(abs(Hz))); grid on; title('Receiver -3dB Filter Response'); axis([1e5 3e5 -3 1]); %濾波器系數 a=[1 0 0.7305];%[1 0 p] b=[0.135 0 -0.135];%gain*[1 0 -1] faskn=filter(b,a,askn); figure(2) subplot(321); plot(t,faskn); axis([0 100e-6 -2 2]); title('通過帶通濾波后輸出'); grid on; cm=faskn.*car;%解調 subplot(322); plot(t,cm); axis([0 100e-6 -2 2]); grid on; title('通過相乘器后輸出'); %低通濾波器 %================================================================== p=0.72; gain1=0.14;%gain=(1-p)/2 Hz1=gain1*(z+1)./(z-(p)); subplot(323); Hz1(Hz1==0)=10^(-8);%avoid log(0) plot(f,20*log10(abs(Hz1))); grid on; title('LPF -3dB response'); axis([0 5e4 -3 1]); %濾波器系數 a1=[1 -0.72];%(z-(p)) b1=[0.14 0.14];%gain*[1 1] so=filter(b1,a1,cm); so=so*10;%add gain so=so-mean(so);%removes DC component subplot(324);