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 2 - Frequency Modulator(FM)
Theory
The Frequency Modulation (FM) is one of the widely used modulation technique in
wireless transmission. They are most commonly used in high quality radio transmission
from FM radio stations. The main advantage of the frequency modulation technique is
the very high signal to noise ratio that can be achieved on the received signals. In the
technique of frequency modulation the frequency of a carrier wave is modulated
according to the variations in the amplitude of the message wave. The receiver tracks for
the change in the frequency of the received signal only and the amplitude variations due
to the noise does not make any changes in the receiver output. Hence quality of the
signal produced at the output of the receiver is very high compared to other modulation
techniques.
Objective
This experiment demonstrates the generation of frequency modulated waveform using the
simplest possible circuit. The carrier signal and message signal used in this experiment
are pure sine waves. The modulating signal is then used to modulate the frequency of
the waves by using a 555 timer integrated chip (IC). The modulator circuit has to be
constructed according to the specifications provided.
Specifications
The 555 IC requires only few components which can be connected to their pins and the
output pulse waveform can be obtained from the pin3 of the 555 IC. The pin number 1
and 8 are used to supply the ground and VCC to the IC respectively. The pin number 2, 6
and 7 are used according to the timing requirements of various pulse generating circuits.
The pin number 4 can be used to reset IC. The pin number 3 provides the output pulse
from the 555 timer IC. The pin number 5 the pin which can change or modulate the pulse
frequency produced at the pin number 3.
In this experiment the 555 must be wired as an astable multivibrator to produce
continuous square waves with a fixed time period. The modulating signal applied to the
pin number 5 of the 555 timer IC to modulate the frequency of the generated pulses.
Since the generated pure sine wave having both the positive and negative half cycles they
should not be directly applied to the 555 which works on positive DC voltage only. The
entire sinusoidal waveform should be clamped to the positive voltage side before it is
applied to the 555 modulator circuit so as to get the output response for both the half
cycles.

3. AIMST University Faculty of Engineering and Computer Technology
BEng (Hons) in Electrical and Electronic Engineering Communication Systems 2
Procedures
The entire circuit should be divided into three blocks:
(1) Sine wave generator
(2) Positive clamper
(3) Frequency modulator using 555
The block diagram of the FM modulation circuit is shown in figure 1.
Figure1
Positive Clamper
A clamper circuit must be used to shift the entire sinusoidal waveform to the positive
voltage side so that the 555 modulator circuit can produce corresponding output response.
The clamping circuit requires only a diode and a capacitor and in the case of positive
clamping the positive end of the diode is grounded and the wave is received at the other
end through a capacitor. The circuit of the positive clamper is shown in figure 2.
Figure 2: Clamper Circuit
The above circuit forms the simplest positive waveform clamper, but it cannot be directly
used in most of the circuits since it cannot be loaded. When a load of very small
resistance is applied at the output end of the circuit it may not work properly. Hence a
buffer (current amplifier) must be added at the output end of the circuit as shown in the
following diagram.
Sine
Wave
Generator
Positive
Clamper
555
Frequency
Modulator
Input Output
0.1 μF
IN4148

4. AIMST University Faculty of Engineering and Computer Technology
BEng (Hons) in Electrical and Electronic Engineering Communication Systems 3
Construct the circuit of the 555 modulator as shown in figure 3.
Figure 3
Report
Write a laboratory report on this experiment.
(1) Explain in detail about the theory and its experimental techniques of FM modulator.
(2) Discuss the analysis of results that obtained for different frequencies and verify
their modulation index.
Sine Wave
1 KΩ
100 KΩ
R1
1 KΩ
R2
FM Output
−5 V
= 𝜋𝑟2
−5 V
= 𝜋𝑟2
IN4148
IN4148 1 MΩ
0.1 μF 1 μF
+5 V
= 𝜋𝑟2
555
8 4
5
3
7
6
2
0.1 μF
C1
Cin
7
4
2
3
1
741
+5 V
= 𝜋𝑟2