fmdemod_4.m

matlab编程实现FM信号的解调程序

% This is a demodulation program with different FM demodulation algorithms
% fmdemod_4.m
% to check FM MODULATION with different methods


Fs = 102000; % Sampling rate of signal
Fc = 25000; % Carrier frequency
AXIS_NUM = 10000;
t = [0:AXIS_NUM]'/Fs; % Sampling times
s1 = sin(2*pi*100*t);%+2*sin(2*pi*600*t); % Channel 1
s2 = sin(2*pi*150*t)+2*sin(2*pi*900*t); % Channel 2
x = s1;%[s1,s2]; % Two-channel signal
dev = 3; % Frequency deviation in modulated signal

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        % Frequency Modulation 
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        % applying function fmmod() and fmdemod()
y = fmmod(x,Fc,Fs,dev); % Modulate both channels.
z = fmdemod(y,Fc,Fs,dev); % Demodulate both channels

        % applying analystical expressions and fmdemod()
y2 = 10*cos(2*pi*Fc*t + 2*pi*cumsum(x)*dev/Fs);
z2 = fmdemod(y2,Fc,Fs,dev); % Demodulate both channels

figure(1),clf;                              %plot demodulation signals
plot([y,y2]),title('FM modulation using different methods');
% plot([z z2]),title('FM demodulated using different way');
legend('function fmmod()','analytical expression');


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                            % FM demodulation%

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                            % method 1 : fmdemod() solution%

z = fmdemod(y2,Fc,Fs,dev);

                            % method 2 % orthorgnal demoduation
                            % 2.1 multiplying
ini_phase = 15;
ri = y2.*cos(2*pi*Fc*t+ini_phase);
rq = -y2.*sin(2*pi*Fc*t+ini_phase);

                            % 2.2 Filtering
      
b = fir1(100,0.1);                            
rif = filter(b,1,ri);
rqf = filter(b,1,rq);

                            % 2.3 Calculate instance phase and frequncy
                            % Applying arctangent methods

instance_phase = atan2(rqf,rif);
instance_freq  = (diff(detrend(unwrap(instance_phase))));
amp_adjust = std(s1)/std(instance_freq(100:end));

                            % 2.4 Applying I-Q difference calculations

rid = diff(rif);
rqd = diff(rqf);
diff_iq = (rqf(1:end-1).*rid-rif(1:end-1).*rqd)./(rif(1:end-1).^2+rqf(1:end-1).^2);
% diff_iq = (rqf(1:end-1).*rid-rif(1:end-1).*rqd);

figure(2),clf,hold on;
plot(x(200:end-200),'k*');
plot(amp_adjust*instance_freq(100:end),'r');
plot(amp_adjust*diff_iq(100:end),'g')
legend('original signal','demodulated by arctangent','demodulated by I-Q difference');

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                        %     demodulated signal filtering
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b = fir1(100,0.1);
demod_f1 = filter(b,1,amp_adjust*instance_freq);
demod_f2 = filter(b,1,amp_adjust*diff_iq);


figure(3),clf,hold on;
plot([demod_f1(200:end),demod_f2(200:end)]);
title('filtering the demodulated signal');
legend('demodulated by arctangent','demodulated by I-Q difference');

% downsampling()
down_f1 = downsample(demod_f1,4);
down_f2 = downsample(demod_f2,4);

figure(4);
plot([down_f1,down_f2]);

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                        %     The End
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