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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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slide1

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:

Eng. Mohammed kamel Abu-Foul

contents
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
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
Type of AM Modulation
  • DSB-SC
  • DSB-LC
  • SSB
  • VSB
part 1
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
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
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
The results (part 1a)

Two Sym. pulses on the signal frequency

code part 1b modulated signal
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
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 results (part 1b)

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

coherent detector
Coherent detector

S(t)

w(t))

v(t)

LPH

cos(2.π.fc.t)

code part 1c 1 coherent demodulator before filtering
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));
code part 1c 1 coherent demodulator before filtering1
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)')
the results part 1c 1
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

code part 1c 2 coherent demodulator after filtering
Code (Part 1c-2) coherent demodulator “after filtering”.

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); %
code part 1c 2 coherent demodulator after filtering1
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)')
code part 1c 2 coherent demodulator after filtering2
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)')
the results part 1c 2
The results (part 1c-2)

There are a signal in 0 as real signal only

code part 1d
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 1d1
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

slide25

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
If we change R

R=3.2e(3)

R=3.2e(4)

part 2
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 envelope1
Result of envelope

Not as real signal

comment in part 2
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
Conclusion
  • The experiment is a good simulate for AM signals.
  • We must make sure of the code because any error causes fail in compiling