Course: BEng (Hons) in Electrical and Electronic Engineering

1. EEEC6440315
Communication Systems
Faculty of Engineering and Computer Technology
Laboratory Manual
Lecturer: Ravandran Muttiah BEng (Hons) MSc MIET
Year/Semester: Year 2 / Semester 2
Academic Session: 2020/2021
The information in this documentis important and should be noted by all students undertaking the
Bachelor of Engineering (Honours) in Electrical and Electronic Engineering
Approved by Coordinator: Endorsed By Dean:
------------------------------------------ __________________

2. AIMST University Faculty of Engineering and Computer Technology
BEng (Hons) in Electrical and Electronic Engineering Communication Systems 1
Experiment 1 - Amplitude Modulator (AM)
Theory
The AM modulation is a kind of modulation technique which is in use since the very
early days of wireless data transmission. In a radio transmission system there is a relation
between the ranges of frequencies which can be transmitted wirelessly with the length of
the transmitting antenna. The relation is inversely proportional to one another, means as
the frequency of the signal to be transmitted increases the length of the antenna can be
reduced and as the frequency of the signal to be transmitted decreases the length of the
transmitting antenna should be increased accordingly.
Using an antenna of few meters the frequencies in the range of MHz can be easily
transmitted to a distance. The basic purpose of the wireless transmitting system in early
days was to transmit the audio signals, but to transmit audio signals which fall in the
range of few KHz an antenna of more than a Kilometer height would have been
required. Since it was practically impossible to construct such a long antenna, the high
frequency signals are transmitted after they are modulated with the low frequency audio
signals.
The amplitude modulation of a high frequency signal is easy to achieve and the
demodulation is also simple compared to other techniques. The high frequency signal
which is modulated to carry the low frequency audio signals are called ‘carrier
frequency’ and the audio signals used for modulation is called ‘modulating signal’ or
‘message signal’ or ‘base band signal’.
Objective
This experiment demonstrates how to generate an Amplitude Modulation (AM) using the
simplest possible circuit. To demonstrate the AM modulation of a carrier signal with a
message signal, both the carrier signal and message signal generations must be presented.
The carrier signal and message signal used in this project are pure sine waves. The
modulator circuit has to be constructed according to the specifications.
Procedure
The entire circuit should be divided into three blocks:
(1) Carrier frequency generator (High frequency sine wave).
(2) Message frequency generator (Low frequency sine wave).
(3) AM Modulator.
The block diagram of the AM modulation used in this project is shown in figure 1.

3. AIMST University Faculty of Engineering and Computer Technology
BEng (Hons) in Electrical and Electronic Engineering Communication Systems 2
AM modulated signal
Figure1
For both the carrier signal of high frequency and the message signal of low frequency,
exactly same sine wave generating circuits are to be used but the frequencies are set to
high and low respectively with the help of their variable components.
Construct the AM circuit as shown in figure 2.
Specifications
To produce a stable AM modulation, FET based circuits has to be used. The carrier wave
can be allowed to flow through the channel of the FET and the message signal can be
used to modify the width of the channel and hence one can achieve the simplest AM
modulation. The component used here as an AM modulator is the N-channel FET,
BFW10. The carrier signal is fed through the N-channel from source to drain of the FET
which is then modulated by applying the message signal on the gate of the FET. The
circuit built around BFW10 which can act as an AM modulator is shown in the following
image.
AM Modulator
Sine Wave
Generator
Carrier
Frequency
Sine Wave
Generator
Message
Frequency

4. AIMST University Faculty of Engineering and Computer Technology
BEng (Hons) in Electrical and Electronic Engineering Communication Systems 3
Figure 2
Report
Write a laboratory report on this experiment.
(1) Explain in detail about the theory and its experimental techniques of AM
modulator.
(2) Discuss the analysis of results that obtained for various frequencies and verify their
modulation index.
Carrier Wave Message Wave
AM Wave
1 KΩ 10 KΩ
R1
R2
R3 10 KΩ
BFW10
S G
D