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%---------------------------------------------------------------------------- 
% exa030701, for example 3.7.1 
% to discuss the resolution of DTFT;  
%---------------------------------------------------------------------------- 
clear all; 
 
% 观察数据长度N的变化对DTFT分辨率的影响 
 f1=2;f2=2.02;f3=2.07;fs=10; 
 w=2*pi/fs; 
 N=256; 
 x=sin(w*f1*(0:N-1))+sin(w*f2*(0:N-1))+sin(w*f3*(0:N-1)); 
 f=0:fs/N:fs/2-1/N; 
 X=fft(x);  
 X=abs(X); 
 subplot(221) 
 plot(f(45:60),X(45:60));grid on; 
 xlabel('Hz') 
% 
 N=N*4; 
 x=sin(w*f1*(0:N-1))+sin(w*f2*(0:N-1))+sin(w*f3*(0:N-1)); 
 f=0:fs/N:fs/2-1/N; 
 X=fft(x);  
 X=abs(X); 
 subplot(222) 
 plot(f(45*4:4*60),X(4*45:4*60));grid on; 
 xlabel('Hz')