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 3 - Pulse Amplitude Modulator (PAM)
Theory
There are different kinds of modulation technique based on varying the properties of a
pulse train. There is a simple pulse modulation technique called Pulse Amplitude
Modulation (PAM) which is proved to be more power efficient and consumes constant
power for individual pulses. In PAM the amplitude of the individual pulses are varied
according to the amplitude of the modulating signals. The PAM modulator and
demodulator circuits simple compared to other kind of modulation and demodulation
techniques. There are two kinds of PAM, one in which the pulses have the same polarity
and the other in which the pulses can have both positive and negative polarity according
to the amplitude of the modulating signal.
Objective
This experiment demonstrates how to generate a single polarity Pulse Amplitude
Modulation (PAM) wave corresponding to a modulating sine wave with the help of
simplest possible circuitry. To demonstrate the PAM a sine wave signal and square wave
signal generations must be presented. The modulator uses N-Channel Field Effect
Transistor (FET). The PAM modulator circuit has to be constructed according to the
specification provided.
Procedure
The entire circuit should be divided into three blocks:
(1) Sine wave generator.
(2) Square wave generator.
(3) PAM Modulator.
The PAM modulation varies the amplitude of the pulses only at a single side. Simply the
top of the pulses are modulated corresponding to the message signal. In this experiment
the PAM is generated from a pure sine wave modulating signal and a square wave
generator which produce the carrier pulses and a PAM modulator circuit. The block
diagram of the PAM modulation used in this experiment is shown in figure 1.

3. AIMST University Faculty of Engineering and Computer Technology
BEng (Hons) in Electrical and Electronic Engineering Communication Systems 2
PAM modulated signal
Figure1
Specifications
The PAM modulator circuit in this experiment has to modify the amplitude of the pulses
having a frequency of 45 KHz with the modulating sine wave of frequency 1 KHz. The
modulator here is realized using a simple FET and a couple of resistors. The FET used
here is BFW10 which is an N-channel FET and since it is a unipolar device it is suitable
for applications in which the signals having both positive and negative polarities are used.
The resistors are simply used as current limiters where the 10K resistor is used to limit
the current flowing into the base of the FET and the 1K resistor is used to limit the
current flowing through the N-channel. The PAM signal is obtained across the 1K
resistor and the ground. Construct the PAM circuit as shown in figure 2.
Figure 2: PAM modulator
Sine Wave
Square Wave
10 KΩ
1 KΩ
PAM
BFW10
Sine Wave
Generator
Square Wave
Generator
Pulse Amplitude
Modulator
(PAM)

4. AIMST University Faculty of Engineering and Computer Technology
BEng (Hons) in Electrical and Electronic Engineering Communication Systems 3
Report
Write a laboratory report on this experiment.
(1) Explain in detail about the theory, calculations of time period of pulse and
frequency, and its experimental techniques of PAM modulator.
(2) Discuss the obtained of results for different time period in relation to the component
values and variations of sine wave and square wave.