1) The document describes an experiment on Quadrature Amplitude Modulation (QAM). 2) QAM is introduced as a digital modulation technique that modulates two analog/digital messages by changing the amplitude of two carrier waves that are out of phase. 3) Matlab code is provided to simulate QAM modulation and demodulation of signals with different frequencies to demonstrate how QAM works.

1. Umang Gupta (2010EE50564)
Indra Bhushan (2010EE50548)
Vivek Mangal (2010EE50566)
Jitendra Kumar Meena (2009EE5 )
5-9-12
Experiment
Quadrature Amplitude Modulation
Aim: To study the working of phase locked loop and identify the lock and capture ranges.
Introduction:
QAM is an Analog as well as a digital modulation. It can modulate two analog/digital
message signals by changing (modulating) of carrier waves, using the amplitude
modulation or amplitude shift keying. The two carrier waves usually are sinusoids and out
of phase and so called Quadrature amplitude modulation. The modulated waves are
summed, and the resulting waveform is a combination of two amplitude modulated signals.
QAM is used extensively as a modulation scheme for digital telecommunication systems.

2. Observations and remarks:
Theoritical Results:
Matlab code to simulate the qam modulation in two sinusoids:
clear all
fc=10000;
fm1=500;
fm2=100;
t=(0.000001:.000001:1/fm2);
m1=sin(2*pi*t.*fm1);
m2=sin(2*pi*t.*fm2);
x1=m1.*cos(2*pi*t.*fc);
x2=m2.*sin(2*pi*t.*fc);
x=x1+x2;
figure(1);
plot(t,x);
title('QAM modulation (fc= 10KHz, fm1=500Hz and fm2=100Hz )');
xlabel('time '), ylabel('signal or voltage');
hold on;
grid on;
figure(2);
plot(t,x1);
title('DSBSC modulation (fc= 10KHz and fm=500 Hz)');
xlabel('time '), ylabel('signal or voltage');
hold on;
grid on;
figure(3);
plot(t,x2);
title('DSBSC modulation (fc= 10KHz and fm=100 Hz)');
xlabel('time '), ylabel('signal or voltage');
hold on;
grid on;
fnorm=2000/100000;
[b,a] = butter(10, fnorm, 'low');
y1=x.*sin(2*pi*t.*fc);
y1l = filtfilt(b, a, y1);
y2=x.*cos(2*pi*t.*fc);
y2l = filtfilt(b, a, y2);
figure(4)
plot(t,y1l);
title('QAM demodulation');

3. xlabel('time '), ylabel('message signal 1');
hold on;
grid on;
figure(5)
plot(t,y2l);
title('QAM demodulation');
xlabel('time '), ylabel('message signal 2');
hold on;
grid on;
The outputs of the simulation are as follows-
QAM signal for Fc=100KHZ AND 100 AND 500 HZ SIGNAL

4. Individual demodulated waves. QAM is the sum of the two.

5. Demodulated signals

6. Practical observations: (QAM of cont. signal)
QAM signal
Signal 3 is QAM.
Signal 1 is input, signal 2, which is in quadrature is not shown
Vector/constellation output of the signal

7. Demodulated signal

8. QAM of Discrete signal
Modulated signal

9. Vectored/constellation output
Demodulated Signals, note the delay in the output
1 is i/p and 4 is o/p

10. 2 is i/p and 4 is o/p
Conclusion: QAM increases the efiifciency of Amplitude modualtion as two signals can be
transmitted over the same bandwidth and hence is popularly preferred over simple
amplitude of DSBSC modulation. But however some extra hardware is required to
demodulate QAM.