% 第三章: 例 3.18a:
%
% 模擬信號
Dt = 0.00005;
t = -0.005:Dt:0.005;
xa = exp(-1000*abs(t));
% 離散時間信號
Ts = 0.0002; n = -25:1:25;
x = exp(-1000*abs(n*Ts));
% 離散時間付利葉變換
K = 500; k = 0:1:K;
w = pi*k/K;
X = x * exp(-j*n'*w);
X = real(X);
w = [-fliplr(w), w(2:K+1)];
X = [fliplr(X), X(2:K+1)];
subplot(1,1,1)
subplot(2,1,1);plot(t*1000,xa);
a=axis;text(a(2)+.2,a(3),'t 毫秒'); ylabel('x1(n)')
title('離散信號'); hold on
stem(n*Ts*1000,x); gtext('Ts=0.2 msec'); hold off
subplot(2,1,2);plot(w/pi,X);
xlabel('以pi為單位的頻率'); ylabel('X1(w)')
title('離散信號付利葉變換')