communication engineering lab report, 5th sem, electrical engineering, IIT Delhi, eep306

1. 29-8-12Umang Gupta, 2010EE50564
VivekMangal, 2010EE50566
IndraBhushan, 2010EE50548
Jitendra Kumar Meena, 2009EE50483
Experiment
Frequency Modulation and Demodulation
Aim: To study and observe the frequency modulation and demodulation using VCO and PLL.
Introduction:
In communication system, it is often that one has to send a speech/video signal of lower frequency
content. It is very difficult to transmit the signal at low frequency through the medium. Hence the
signal is modified to high frequency signal so that it can be sent through the medium. Once the
signal is modified on the receiver’s end it can be sent through the medium. Now the receiver on
receiving the signal applies demodulation techniques to get the message signal.
One such method is frequency modulation. Let the message signal be m(t).
We define frequency modulated signal,
x(t)=cos(2πfct+ )
fcis the carrier frequency. K is constant.
This is called frequency modulation, as the information content is encoded in frequency.
Now this can be demodulated in many ways by using a multiplier and applying Hilbert transform or
by PLL using VCO
Observations and remarks:
Matlab Outputs:
CODE:
clearall
fc=10000;
fm=500;
t=(0.000001:.000001:2/fm);
x=cos(2*pi*t.*fc+15*sin(2*pi*t.*fm)); //frequency modulated signal
figure(1);
plot(t,x);
title('frequency modulation (fc= 10KHz and fm=500 Hz)');
xlabel('time '), ylabel('signal or voltage');
holdon;
gridon;
y=10*demod(x,fc,10*fc,'fm'); //function to demodulate the signal
figure(2);
plot (t,y);
title('demodulated signal(fc= 10KHz and fm=500 Hz)');
xlabel('time '), ylabel('signal or voltage');
holdon;
gridon;
Taking fc=10KHz and fm=500Hz
We get following outputs
Modulated Signal for two msg periods. Note the change in frequncy.

2. Demodulated signal.
Note that there is a DC component which needs to be filtered
Experimental Obeservations:
The modulated signal for step square wave is as:

3. For a sinusoid, the modulated wave output is:
Demodulated signal o/p is as follows:

4. There are some distortions in the demodulated signal. This may because of the tuning of the
PLL.
Conclusion
The results of FM modulations are well within the predicted boundaries. But it is not an
efficient way of transmission as it takes up larger bandwidth. The PLL implementation of FM
is used in radios and related communications.