% % Filename: example17.m % % Description: M-file for demonstrating the DFT approximation of % continous-time frequency content. % figure(1); clear; clf; % clear memory and figure t = 0:0.0000005:0.00015; % CT and CT signal x = (1 + cos(2*pi*20000*t)).*cos(2*pi*100000*t); subplot(2,2,1); plot(t,x); xlabel('tMATLAB Plot Generated:'); ylabel('x(t)'); title('CT AM Signal'); f = [-100000 100000 -80000 80000 -120000 120000]; X = [pi pi pi/2 pi/2 pi/2 pi/2];% CTFT mag spectrum of CT signal subplot(2,2,2); stem(f,abs(X),'^'); xlabel('f '); ylabel('|X(f)|'); title('CTFT Mag Spectrum of CT AM Signal'); T = 0.0000025; % compute and plot DT signal n = 0; for m = 0:5:length(x)-5, xn(n+1) = x(m+1); nvec(n+1) = n; n = n + 1; end; subplot(2,2,3); stem(nvec,xn,'filled'); xlabel('n'); ylabel('x[n]'); title('x[n] from Sampling CT AM Signal'); xnpad = [xn zeros(1,20)]; % compute & plot padded DT signal nvecpad = [nvec 60:79]; subplot(2,2,4); stem(nvecpad,xnpad,'filled'); xlabel('n'); ylabel('x[n]'); title('Padded x[n]'); figure(2); clf; % open & clear figure 2 Xk = dft(xnpad); % compute and plot DFT mag spectrum k = nvecpad; stem(k/(80*T), abs(Xk),'filled'); xlabel('f = k/NT '); ylabel('|Xk|'); title('Magnitude Spectrum from DFT of Sampled Signal');