1. Intelligent Islamic Electrical Engineers (IIEE)
Communications LAB I Amplitude Modulation DSB-LC (Full AM) presentation
UNDER SUBERVISION:
Eng. Mohammed k. Abu Foul
Design by :
Intelligent Islamic Electrical Engineers (IIEE)
presentation by:
Mahmood A. El-Tatar
Islame E. Ashoor
Mohaned H. Eqdeeh

2. Signals in Communications
Base band
Bandpass
Amplitude
modulation
Angle
Phase MOD. PM
Frequency MOD. FM
DSB-SC MOD.
DSB+C MOD.
SSB MOD.
VSB MOD.

3. Analog Communication system
Information source
Signal modulator
Propagation channel
Signal demodulator
Information output

4. Amplitude Modulation:
There are 4 main types of AM signals:
Full Am – 2 sidebands and carrier.
Double sideband suppressed carrier (DSB-SC).
Single sideband AM (SSB).
Vestigial sideband AM (VSB).

5. Amplitude Modulation DSB-LC (Full AM)
Full AM is sometimes referred to as Double Side Band – Large Carrier (DSLC)
clearly indicating its main feature as having a large amplitude carrier component
added to the signal spectrum.
Type of modulators :
1-multiplier mod.
2-nonlinear mod.
3- switching mod.

6. Modulating a signal
Block diagram :
Equations :

7. Process of modulation

8. Modulation index If we consider : We defined modulation index to be
f(t)= Am cos(wm*t), c(t)= Ac cos(wc*t)
Φ(t)= (Am+Ac) cos(wm*t) cos(wc*t)
We defined modulation index to be
M= Am/Ac
Thus we have:
Φ(t)= Ac(1+m cos(wm*t))* cos(wc*t)
In order to detect the signal with no distortion we require m ≤1

9. Matlap code fc=20000; ts=1/(10*fc); t=[0:2000]*ts; fs=1/ts; m=.25;
Ac=4;
x = (cos(2*pi*2000*t)).^3
figure(1)
subplot(221)
plot(t,x)
title('plot of baseband signal x(t)')
xlabel('time (t)')
ylabel('x(t)')
Xf=fftshift(fft(x))/length(t);
deltax=fs/length(Xf);
fx=-fs/2:deltax:fs/2-deltax;
subplot(222)
plot(fx,abs(Xf))
title('the fourier transform of x(t)')
xlabel('frequency (f)')
ylabel('X(f)')
y=(1+m*x)*Ac.*cos(2*pi*fc*t);
subplot(223)
plot(t,y)
%%title('the modulated signal y(t)=(1+m*x)*Ac.*cos(2*pi*fc*t)')
ylabel('y(t)')
yf=fftshift(fft(y))/length(t);
delta=fs/length(yf);
f=-fs/2:delta:fs/2-delta;
subplot(224)
plot(f,abs(yf))
ylabel('Y(f)')

10. Demodulation of Full AM
(1) Coherent Demodulation:
consists of multiplying received signal with a local carrier and filtering out unwanted
components centered around 2wc.

11. Filters Types Of Filters:
We will use the Butterworth filter in our study .
Types Of Filters:
LPF
BPF
HPF
BRF

12. HOW TO MAKE LPF
To get the Butterworth filter order and cutoff frequency we use this code :
Wp=5000/fs;
Ws=20000/fs;
Rp=-1;
Rs=-100;
[N, Wn] = BUTTORD(Wp, Ws, Rp, Rs)
[num,den]=butter(N,Wn)
v=filter(num,den,w)
Where:
Wp :Passband corner frequency.
Ws: Stopband corner frequency .
Rp&Rs : ripple factor .
N : Filter order
Wn: cutoff frequency
num, den: filter transfer function

13. Matlap code w=y.*cos(2*pi*fc*t) 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)')
Wp=5000/fs;
Ws=20000/fs;
Rp=-1;
Rs=-100;
[N, Wn] = BUTTORD(Wp, Ws, Rp, Rs)
[num,den]=butter(N,Wn)
v=filter(num,den,w)
figure(4)
plot(t,v)
title('the demodulated signal after LPF v(t)')
ylabel('v(t)')
Vf=fftshift(fft(v))/length(t);
deltav=fs/length(Vf);
fv=-fs/2:deltav:fs/2-deltav;
plot(fv,abs(Vf))
title('the fourier transform of v(t)')
ylabel('V(f)')

14. (2) Envelope Detector
The output of the detector follows the envelope of the modulated signal.
On positive half-cycle of RF input signal
Φ(t), the diode is forward biased and the
capacitor C charges up rapidly to the peak
value of RF input signal
When RF input falls below the output
voltage then the diode becomes reversebiased
and the capacitor C discharges
through the load resistor RL

15. 1/fc << RC<< 1/B
RC effective
Choice of R and C:
1/fc << RC<< 1/B
Where B is the highest frequency in f(t) (original signal)
Where fc is the carrier frequency

16. Matlap code R=3.2e3; C=01e-6; RC=R*C; Vout(1)=y(1); for i=2:length(y);
if y(i)>Vout(i-1)
Vout(i)=y(i);
else
Vout(i)=Vout(i-1)*exp(ts/RC);
end
figure(3)
plot(t,Vout,t,y)
xlabel('time (t)')
ylabel('v(t)')

17. RC effective by Matlap
RC too large
RC too small

18. conclusion
Clearly, main advantage of Full AM modulation is very simple and cost effective demodulation of the signal at the receiving end.
But AM is as wasteful of spectrum as DSB-SC and requires a large carrier amplitude which in turns require more overall power to be transmitted.
(Transmission of carrier is a waste of power since it does not carry information except its phase and frequency).

19. GUI TOOL
This tool is used to give a simple understanding of this experiment and generally of the amplitude modulation.
You can use it
ENJOY

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IIEE
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