gusucode.com > 现代通信系统——使用MATLAB(+全部程序) > 现代通信系统——使用MATLAB(+全部程序)/现代通信系统——使用MATLAB(+全部程序)/Matlab 程序/Chapter3/ip_03_11.m
% MATLAB script for Illustrative Problem 3.11. % Demonstration script for frequency modulation. The message signal % is m(t)=sinc(100t). echo on t0=.2; % signal duration ts=0.001; % sampling interval fc=250; % carrier frequency snr=20; % SNR in dB (logarithmic) fs=1/ts; % sampling frequency df=0.3; % required freq. resolution t=[-t0/2:ts:t0/2]; % time vector kf=100; % deviation constant df=0.25; % required frequency resolution m=sinc(100*t); % the message signal int_m(1)=0; for i=1:length(t)-1 % integral of m int_m(i+1)=int_m(i)+m(i)*ts; echo off ; end echo on ; [M,m,df1]=fftseq(m,ts,df); % Fourier transform M=M/fs; % scaling f=[0:df1:df1*(length(m)-1)]-fs/2; % frequency vector u=cos(2*pi*fc*t+2*pi*kf*int_m); % modulated signal [U,u,df1]=fftseq(u,ts,df); % Fourier transform U=U/fs; % scaling [v,phase]=env_phas(u,ts,250); % demodulation, find phase of u phi=unwrap(phase); % Restore original phase. dem=(1/(2*pi*kf))*(diff(phi)/ts); % demodulator output, differentiate and scale phase pause % Press any key to see a plot of the message and the modulated signal. subplot(2,1,1) plot(t,m(1:length(t))) xlabel('Time') title('The message signal') subplot(2,1,2) plot(t,u(1:length(t))) xlabel('Time') title('The modulated signal') pause % Press any key to see plots of the magnitude of the message and the % modulated signal in the frequency domain. subplot(2,1,1) plot(f,abs(fftshift(M))) xlabel('Frequency') title('Magnitude spectrum of the message signal') subplot(2,1,2) plot(f,abs(fftshift(U))) title('Magnitude-spectrum of the modulated signal') xlabel('Frequency') pause % Press any key to see plots of the message and the demodulator output with no % noise. subplot(2,1,1) plot(t,m(1:length(t))) xlabel('Time') title('The message signal') subplot(2,1,2) plot(t,dem(1:length(t))) xlabel('Time') title('The demodulated signal')