Islamic University of Gaza Electrical Engineering Department Communication I laboratory

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Islamic University of Gaza Electrical Engineering Department Communication I laboratory . Amplitude modulation DSB-LC (full AM) . Submitted by: Adham Abu-Shamla Mohammed Hajjaj 120063320 120063640 Submitted to:

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## Islamic University of Gaza Electrical Engineering Department Communication I laboratory

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Islamic University of Gaza

Electrical Engineering Department

Communication I laboratory

Amplitude modulation DSB-LC (full AM)

Submitted by:

120063320 120063640

Submitted to:

Eng. Mohammed kamel Abu-Foul

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

+

Part 1 (a) code: the input signal

Carrier freqyancy

Sampling period

Modulation index µ=mp/A

DC shift

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.

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

The results (part 1a)

Two Sym. pulses on the signal frequency

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));
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

The results (part 1b)

The same signal shifted at fc and the magnitude divided by 2

Coherent detector

S(t)

w(t))

v(t)

LPH

cos(2.π.fc.t)

• 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));
• 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)')
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

the lower frequency of the transient region ( must be between 0 and 1 )

The Upper frequency of the transient region ( must be between 0 and 1 )

losses due to rippels

Fn. that return the order of the filter and the cutt off frequency

Fn. that return the transfer function of the Butterworth filter

• Wp=5000/fs;
• Ws=20000/fs;
• Rp=-1;
• Rs=-100;
• [N, Wn] = BUTTORD(Wp, Ws, Rp, Rs); %
• [num,den]=butter(N,Wn); %

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)')

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)')
The results (part 1c-2)

There are a signal in 0 as real signal only

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)
Code (part 1d)
• Vc(i)=y(i);
• else
• Vc(i)=Vc(i-1).*exp(-ts/RC);
• end
• end
• figure(5)
• plot(t,y,t,Vc)
• %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

We can explain this code by the following flow chart

start

Vc(1)=y(1)

i=2

Y(i)>=Vc(i-1)

NO

YES

Vc(i)=y(i)

Vc(i)=Vc(i-1).*exp(-ts/RC);

i=length(Y)

NO

YES

start

If we change R

R=3.2e(3)

R=3.2e(4)

Part 2
• 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

Result of envelope

Not as real signal

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