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ip_03_05.m
Package: 现代通信系统matlab版源代码.zip　[view]
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Upload Date: 2013-03-02
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Communication Document

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Matlab

ip_03_05.m：Code Content
  1. % MATLAB script for Illustrative Problem 3.5.
  2. % Demonstration script for DSB-AM demodulation. The message signal
  3. % is +1 for 0 < t < t0/3, -2 for t0/3 < t < 2t0/3, and zero otherwise.
  4. echo on
  5. t0=.15;                                 % signal duration
  6. ts=1/1500;                              % sampling interval
  7. fc=250;                                 % carrier frequency
  8. fs=1/ts;                                % sampling frequency
  9. t=[0:ts:t0];                            % time vector
  10. df=0.3;                                 % desired frequency resolution
  11. % message signal
  12. m=[ones(1,t0/(3*ts)),-2*ones(1,t0/(3*ts)),zeros(1,t0/(3*ts)+1)];
  13. c=cos(2*pi*fc.*t);                      % carrier signal
  14. u=m.*c;                                 % modulated signal
  15. y=u.*c;                             % mixing
  16. [M,m,df1]=fftseq(m,ts,df);              % Fourier transform 
  17. M=M/fs;                                 % scaling
  18. [U,u,df1]=fftseq(u,ts,df);              % Fourier transform 
  19. U=U/fs;                                 % scaling
  20. [Y,y,df1]=fftseq(y,ts,df);              % Fourier transform
  21. Y=Y/fs;                                 % scaling
  22. f_cutoff=150;                           % cutoff freq. of the filter
  23. n_cutoff=floor(150/df1);                % Design the filter.
  24. f=[0:df1:df1*(length(y)-1)]-fs/2;
  25. H=zeros(size(f));                    
  26. H(1:n_cutoff)=2*ones(1,n_cutoff);    
  27. H(length(f)-n_cutoff+1:length(f))=2*ones(1,n_cutoff);
  28. DEM=H.*Y;                   % spectrum of the filter output
  29. dem=real(ifft(DEM))*fs;             % filter output
  30. pause % Press a key to see the effect of mixing.
  31. clf
  32. subplot(3,1,1)
  33. plot(f,fftshift(abs(M)))
  34. title('Spectrum of the Message Signal')
  35. xlabel('Frequency')
  36. subplot(3,1,2)
  37. plot(f,fftshift(abs(U)))
  38. title('Spectrum of the Modulated Signal')
  39. xlabel('Frequency')
  40. subplot(3,1,3)
  41. plot(f,fftshift(abs(Y)))
  42. title('Spectrum of the Mixer Output')
  43. xlabel('Frequency')
  44. pause % Press a key to see the effect of filtering on the mixer output.
  45. clf
  46. subplot(3,1,1)
  47. plot(f,fftshift(abs(Y)))
  48. title('Spectrum of the Mixer Output')
  49. xlabel('Frequency')
  50. subplot(3,1,2)
  51. plot(f,fftshift(abs(H)))
  52. title('Lowpass Filter Characteristics')
  53. xlabel('Frequency')
  54. subplot(3,1,3)
  55. plot(f,fftshift(abs(DEM)))
  56. title('Spectrum of the Demodulator output')
  57. xlabel('Frequency')
  58. pause % Press a key to compare the spectra of the message and the received signal.
  59. clf
  60. subplot(2,1,1)
  61. plot(f,fftshift(abs(M)))
  62. title('Spectrum of the Message Signal')
  63. xlabel('Frequency')
  64. subplot(2,1,2)
  65. plot(f,fftshift(abs(DEM)))
  66. title('Spectrum of the Demodulator Output')
  67. xlabel('Frequency')
  68. pause % Press a key to see the message and the demodulator output signals.
  69. subplot(2,1,1)
  70. plot(t,m(1:length(t)))
  71. title('The Message Signal')
  72. xlabel('Time')
  73. subplot(2,1,2)
  74. plot(t,dem(1:length(t)))
  75. title('The Demodulator Output')
  76. xlabel('Time')