A list,

1 Modulation principle

Conventional double-sideband AM is also called standard AM, or AM for short. Assuming that the average value of the modulated signal M (t) is 0, an AM signal can be obtained by adding a DC component A0 to it and multiplying it by the carrier.

The modulation model is shown in the figure below:



2 demodulation principle

For AM signals, there are two demodulation methods: coherent demodulation and incoherent demodulation. In general, because its envelope and modulation signal M (T) shape, waveform fluctuation is exactly the same. Therefore, the envelope detection method can be used to recover the original signal.

The envelope detector is shown in the figure below:



Among them, the principles used are the single guiding diode, the high frequency bypass characteristics of capacitor and the straight isolation characteristics of capacitor.

Ii. Source code

n=1024; fs=n; % Set sampling frequency fs=1024Hz
s=320*pi; % produces a modulated signal m(t) I =0:1:n- 1;
t=i/n;
t1=(t- 1/7). ^2; t3=(t- 3/7). ^2; t4=(t4 -/7). ^2;
t6=(t- 6/7). ^2;
m=exp(-s*t1)+exp(-s*t3)+exp(-s*t4)+exp(-s*t6); % produces a modulated signal c=cos(2*pi*100*t); % Generate carrier signal carrier frequency FC =100Hz x=m.*c; % ssine-wave amplitude modulation (DSB) % demodulation wp =0.1*pi; ws=0.12*pi; Rp=1; As=15; % Design butterworth digital lowpass filter [N,wn]=buttord(WP/PI, WS/PI,Rp,As); [b,a]=butter(N,wn); m1=filter(b,a,y); M1 = % filtering2*m1; M=fft(m,n); % Calculate the frequency characteristics of the above signals and filters [H, W]=freqz(b,a,n,'whole');
f=(-n/2:1:n/2- 1); % drawingCopy the code

3. Operation results

Fourth, note

Version: 2014a complete code or write plus 1564658423