%實驗一:頻譜分析與采樣定理
%陳友興
T=0.0001;               %采樣間隔T=0.0002
F=1/T;                 %采樣頻率為F=1/T
L=0.01;                %記錄長度L=0.01   
N=L/T;                 
t=0:T:L;             
a=37;
f1=0:F/N:F;
f2=-F/2:F/N:F/2;
%%%%%%%%%%%%%%%%%%%%%%%%%
x1=cos(100*pi*a*t);
y1=T*abs(fft(x1));
y11=fftshift(y1);
figure(1),
subplot(3,1,1),plot(t,x1);title('正弦信號');
subplot(3,1,2),stem(f1,y1);title('正弦信號頻譜');
subplot(3,1,3),stem(f2,y11);title('正弦信號頻譜');
%%%%%%%%%%%%%%%%%%%%%
x2=exp(-a*t);
y2=T*abs(fft(x2));
y21=fftshift(y2);
figure(2),
subplot(3,1,1),stem(t,x2);title('指數(shù)信號');
subplot(3,1,2),stem(f1,y2);title('指數(shù)信號頻譜');
subplot(3,1,3),stem(f2,y21);title('指數(shù)信號頻譜');
%%%%%%%%%%%%%%%%%%%%%
x3=x1.*x2;
y3=T*abs(fft(x3));
y31=fftshift(y3);
figure(3),
subplot(3,1,1),stem(t,x3);title('兩信號相乘');
subplot(3,1,2),stem(f1,y3);title('兩信號相乘頻譜');
subplot(3,1,3),stem(f2,y31);title('兩信號相乘頻譜');