INTERNATIONAL UNIVERSITY
SCHOOL OF ELECTRICAL ENGINEERING (EE)
EE115IU
PRINCIPLES OF COMMUNICATIONS SYSTEMS
LABORATORY
Lab 3
FM MODULATION
Full name: ……………………………….
Student number: ………………………...
Class: …………………………………….
Date: ……………………………………...
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INTERNATIONAL UNIVERSITY
SCHOOL OF ELECTRICAL ENGINEERING (EE)
I. Objectives
The objective of this lab is to explore the concepts FM signals. Specifically, we will simulate
them by using MATLAB.
II. Required Equipment
1. Computer
2. MATLAB software
III. Discussion of Fundamentals
The modulation of FM signal is given by:
u FM (t)= A c cos ( 2 π f c t+ β f sin2 π f m t )
: modulation index
: frequency deviation constant
Or
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INTERNATIONAL UNIVERSITY
SCHOOL OF ELECTRICAL ENGINEERING (EE)
IV. Problems
The modulating signal that is the input into an FM modulator is
m ( t ) =10 cos 32 πt .
The output of the FM modulator is
( )
t
u ( t )=10 cos 4000 πt+2 π k f ∫ m ( τ ) dτ ,
−∞
where k f =8.
The output of the FM modulator u(t) is passed through an ideal bandpass filter (BPF)
centered at f c =2000 Hz with a bandwidth of 62 Hz. (See the below figure)
1. Plot m(t) and u(t) in time domain and frequency domain. Explain results!
2. Plot u_out(t) in time domain and frequency domain. Explain results!
Hint: Using bandpass command to build an ideal bandpass filter in MATLAB
V. Review Questions
1. What is Frequency Modulation (FM) and how does it differ from Amplitude Modulation
(AM)?
Frequency modulation is a technique or a process of encoding information on a
particular signal (analogue or digital) by varying the carrier wave frequency in
accordance with the frequency of the modulating signal. As we know, a modulating
signal is nothing but information or message that has to be transmitted after being
converted into an electronic signal.
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INTERNATIONAL UNIVERSITY
SCHOOL OF ELECTRICAL ENGINEERING (EE)
2. Explain the process of frequency modulation. How does the carrier frequency change in
response to the message signal?
Step 1: Carrier wave : a high frequency sinusoidal wave is used as the carrier. This wave
is m(t)
Step 2: message wave: this is input signal that contains the information to be transmitted.
It is a typically a lower frequency signal m(t)
Step 3: frequency deviation: In frequency modulation, the carrier frequency is shifted
above or below its unmodulated value base on the instantaneous amplitude of the
message signal.
Step 4: modulated signal: the frequency of the carrier wave at any give time is f(t)=
f_c+f_f*m(t)
How the Carrier Frequency Changes
When the amplitude of the message signal increases, the carrier frequency shifts
proportionally higher than its base frequency.
When the amplitude of the message signal decreases, the carrier frequency shifts
proportionally lower than its base frequency.
If the message signal amplitude is zero, the carrier frequency remains at its unmodulated
value f_c.
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INTERNATIONAL UNIVERSITY
SCHOOL OF ELECTRICAL ENGINEERING (EE)
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