1. Amplitude modulation DSB-LC (full AM)
Islamic University of Gaza
Electrical Engineering Department
Communication I laboratory
Amplitude modulation DSB-LC (full AM)
Submitted by:
Adham Abu-Shamla Mohammed Hajjaj
Submitted to:
Eng. Mohammed kamel Abu-Foul

2. Contents: The objective of this experiment.
Quick review about the AM modulation and (DSB-LC)
Part 1 code and its comments and results (step by step)
Part 2 code and its comment and results
Conclusion.

3. Objective:
Understanding AM modulation, Double Side Band-Large Carrier (DSB-LC) “known as Full AM”.
Using MATLAB to plot the modulated signal.
To simulate coherent demodulator and an envelope detector to obtain the real signal using MATLAB.

4. Quick review about the AM modulation and (DSB-LC)
What is AM modulation
It’s a type of modulation, that’s used to transmit the signal with high frequency.
Why Modulation
FDM(frequency division multiplexing)
Hard ware limitation (λ=c/f),(L(Antenna Hight)=λ/10)

5. Type of AM Modulation
DSB-SC
DSB-LC
SSB
VSB

6. (AM) and (FM) :

7. Part 1
Use MATLAB to simulate this block (AM block) (f(t)=cos(2π2000t),Ac=4,m=0.25,fc=20Khz)
f(t) m
Ac.cos(2.π.fct)
S(t)=Ac[1+mf(t)] .cos(2.π.fct) X +

8. Part 1 (a) code: the input signal
Sampling period
Carrier freqyancy
DC shift
Modulation index µ=mp/A
Sampling frequancy
fc=20000; % Carrier frequency
ts=1/(10*fc);
t=[0:2000]*ts;
fs=1/ts; % Sampling frequency
m=.25; % Modulation index
Ac=4; % DC shift
x=cos(2*pi*2000*t); % the original signal
figure(1)
subplot(211)
plot(t,x)
title('plot of baseband signal x(t)')
xlabel('time (t)')
ylabel('x(t)')
plot the real signal
Comment:
In the above code we chose ts =1/(10*fs) to avoid overlapping in signal, then we make Fourier transform to plot the magnitude spectrum.

9. Code cont. Xf=fftshift(fft(x)); Xf=Xf/length(Xf);
deltax=fs/length(Xf);
fx=-fs/2:deltax:fs/2-deltax;
subplot(212)
plot(fx,abs(Xf))
title('the fourier transform of x(t)')
xlabel('frequency (f)')
ylabel('X(f)')
Fourier Transform of a real signal
Plot the magnitude spectrum

10. Two Sym. pulses on the signal frequency
The results (part 1a)
Two Sym. pulses on the signal frequency

11. Code (part 1b): modulated signal
y=(1+m*x)*Ac.*cos(2*pi*fc*t); % (Modulated signal)
figure(2)
subplot(211)
plot(t,y)
title('the modulated signal y(t)=(1+m*x)*Ac.*cos(2*pi*fc*t)')
xlabel('time (t)')
ylabel('y(t)')
yf=fftshift(fft(y));

12. Code (part 1b): modulated signal cont.
yf=yf/length(yf);
delta=fs/length(yf);
f=-fs/2:delta:fs/2-delta;
subplot(212)
plot(f,abs(yf))
title('the fourier transform of the modulated signal Y(f)')
xlabel('frequency (f)')
ylabel('Y(f)')
Plot the magnitude spectrum

13. The same signal shifted at fc and the magnitude divided by 2
The results (part 1b)
The same signal shifted at fc and the magnitude divided by 2

14. Coherent detector
S(t)
w(t) )
LPH
v(t)
cos(2.π.fc.t)

15. Code (Part 1c-1) coherent demodulator “before filtering”.
w=y.*cos(2*pi*fc*t); % Coherent demodulated signal
figure(3)
subplot(211)
plot(t,w)
title('plot of demodulated signal w(t) before LPF')
xlabel('time (t)')
ylabel('w(t)')
wf=fftshift(fft(w));

16. Code (Part 1c-1) coherent demodulator “before filtering”.
wf=wf/length(wf);
delta=fs/length(wf);
f=-fs/2:delta:fs/2-delta;
subplot(212)
plot(f,abs(wf))
title('fourier transform of the demodulated signal W(f)')
xlabel('frequency (f)')
ylabel('W(f)')

17. The results (part 1c-1)
The modulated signal shifted at 2*fc and the magnitude divided by 2
There are a signal in 0 as real signal

18. Code (Part 1c-2) coherent demodulator “after filtering”.
Fn. that return the order of the filter and the cutt off frequency
Fn. that return the transfer function of the Butterworth filter
losses due to rippels
The Upper frequency of the transient region ( must be between 0 and 1 )
the lower frequency of the transient region ( must be between 0 and 1 )
Wp=5000/fs;
Ws=20000/fs;
Rp=-1;
Rs=-100;
[N, Wn] = BUTTORD(Wp, Ws, Rp, Rs); %
[num,den]=butter(N,Wn); %

19. Code (Part 1c-2) coherent demodulator “after filtering”.
Filtering process
v=filter(num,den,w);
figure(4)
subplot(211)
plot(t,v)
title('the demodulated signal after LPF v(t)')
xlabel('time (t)')
ylabel('v(t)')

20. Code (Part 1c-2) coherent demodulator “after filtering”.
Vf=fftshift(fft(v));
Vf=Vf/length(Vf);
deltav=fs/length(Vf);
fv=-fs/2:deltav:fs/2-deltav;
subplot(212)
plot(fv,abs(Vf))
title('the fourier transform of v(t)')
xlabel('frequency (f)')
ylabel('V(f)')

21. There are a signal in 0 as real signal only
The results (part 1c-2)
There are a signal in 0 as real signal only

22. Envelope detector

23. Code (part 1d) %C=0.1e-6F,R=3.2e2%%%%% c=0.1e-6; r=3.2e2; RC=r*c;
Vc=ones*(1:length(y));
Vc(1)=y(1);
for i=2:length(y)
if y(i)>=Vc(i-1)

24. We can explain this code by the following flow chart
Code (part 1d)
Vc(i)=y(i);
else
Vc(i)=Vc(i-1).*exp(-ts/RC);
end
figure(5)
plot(t,y,t,Vc)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
We can explain this code by the following flow chart

25. Vc(i)=Vc(i-1).*exp(-ts/RC);
start
Vc(1)=y(1)
i=2
Y(i)>=Vc(i-1)
YES NO
Vc(i)=y(i)
Vc(i)=Vc(i-1).*exp(-ts/RC);
i=length(Y) NO
YES
start

26. The results (C=0.1e-6F,R=3.2e2)

27. If we change R
R=3.2e(3)
R=3.2e(4)

28. we will show the results of this part and comment the reason
Repeat part 1 with Ac=1, m = 2
After we make the simulation, the result is the same in part 1, but we saw some difference in the envelope detector .
we will show the results of this part and comment the reason

29. Result of envelope
overlap

30. Result of envelope
Not as real signal

31. Comment in part 2
The reason of part 2 become like this becouse the Ac is not enough to alternate the signal up to zero so envelope detector can’t get the real signal

32. Conclusion The experiment is a good simulate for AM signals.
We must make sure of the code because any error causes fail in compiling

33. Thank you