Transcript Full AM

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
120071065
120061542
120072380
Signals in Communications
Signals in Communications
Bandpass
Base band
Angle
modulation
Amplitude
modulation
DSB-SC MOD.
DSB+C MOD.
SSB MOD.
VSB MOD.
Phase MOD.
PM
Frequency MOD.
FM
Analog Communication system
Information
source
Signal
modulator
Propagation
channel
Signal
demodulator
Information
output
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).
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.
Modulating a signal
•Block diagram :
•
Equations :
Process of modulation
Modulation index
If we consider :
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
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
0.4
X(f)
0
0.2
-0.5
0
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)')
-1
0
0.005
time (t)
0.01
-2000 -1000 0 1000 2000
frequency (f)
5
Y(f)
1.5
y(t)
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)')
xlabel('time (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))
xlabel('frequency (f)')
ylabel('Y(f)')
the fourier transform of x(t)
0.5
x(t)
figure(1)
subplot(221)
plot(t,x)
title('plot of baseband signal x(t)')
xlabel('time (t)')
ylabel('x(t)')
plot of baseband signal x(t)
1
0
1
0.5
0
-5
0
0.005
time (t)
0.01
-2
-1
0
1
frequency (f)
2
4
x 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.
Filters
We will use the Butterworth filter in our study .
Types Of Filters:
LPF
BPF
HPF
BRF
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.
N : Filter order
Ws: Stopband corner frequency .
Wn: cutoff frequency
Rp&Rs : ripple factor .
num, den: filter transfer function
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)
subplot(211)
plot(t,v)
title('the demodulated signal after LPF v(t)')
xlabel('time (t)')
ylabel('v(t)')
Vf=fftshift(fft(v))/length(t);
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)')
(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
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
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
end
figure(3)
plot(t,Vout,t,y)
xlabel('time (t)')
ylabel('v(t)')
RC effective by Matlap
RC too large
RC too small
the output of the envelope detector
the output of the envelope detector
10
5
4
3
2
5
v(t)
v(t)
1
0
-1
0
-2
-3
-4
-5
-5
0
0.002
0.004
0.006
time (t)
0.008
0.01
0
0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009
time (t)
0.01
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).
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
Intelligent Islamic Electrical Engineers
IIEE
Soon … in Al-Aqsa