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
Mini Project 2 - Frequency Shift Keying (FSK) Modulator and
Demodulator
Theory
FSK is the frequency modulation system in which digital information is transmitted
through the discrete frequency change of a carrier wave. The technology is used in
communication systems such as amateur radio, caller ID and urgent situation broadcasts.
FSK plays a vital role in a wide range of applications in the communication field and it
was treated as an efficient one for wireless modems in data transmission. FSK uses a pair
of discrete frequencies to transmit binary (0s and 1s) information. With this scheme, the
“1” is called the mark frequency and the “0” is called the space frequency.
Objectives
The objective of this project is to demonstrate how to generate FSK modulated signal and
also the demodulation process using the modulator and demodulator circuit as shown in
figure 1 and 2 respectively. The schematic layout of FSK modulator and demodulator
circuit has to be printed on a same printed circuit board. Here the FSK demodulator uses
Phase Locked Loop (PLL) 565 and a Comparator 710. The complete FSK modulator and
demodulator circuits are to be constructed with the help of the specifications provided.
Specifications
Modulator
2 Resistors 50 KΩ
1 Resistor 47 KΩ
1 Resistor 10 KΩ
2 Capacitors 10 nF
1 Timers IC 555
1 NPN Transistor
Demodulator
2 Resistors 600 Ω
1 Resistor 5 KΩ
3 Resistors 10 KΩ
1 Capacitor 0.05 μF
1 Capacitor 0.2 μF
3 Capacitors 0.02 μF
1 Capacitor 0.01 μF
1 Comparator 710
1 PLL 565
Power Supply ±5 V

3. AIMST University Faculty of Engineering and Computer Technology
BEng (Hons) in Electrical and Electronic Engineering Communication Systems 2
Procedure
The Circuit given here in figure 1 illustrates how FSK modulated wave can be generated.
It has to be build using IC 555. Square pulses are given as input to represent bit 1 and bit
0, and as an output IC 555 generates FSK modulated wave. To generate square pulses
one more IC 555 is used. The working of this circuit was very simple to understand as
the output frequency of the signal was based on the digital input given to the base of the
transistor. The Ra, Rb and C in the circuit determine the frequency of the FSK
modulated signal in the mode of Astable. The resistors and capacitor values were
selected in such a way to get an output frequency of 1270 Hz. Therefore, the output of an
FSK will give 1070 Hz frequency when input is high and 1270 frequencies when an input
is low.
FSK demodulator is a very beneficial application of the PLL 565. An FSK signal
demodulator can be made as shown in the figure 2. The demodulator gets a signal at one
of the two separate carrier frequencies, representing the RS-232 C logic levels of mark or
space, respectively. The capacitive connection is used as the input to eliminate a DC
level. As the signal seems at the input of PLL 565, this locks to the input frequency and
the paths it between the two probable frequencies with an equivalent DC shift at the
output. Resistor and capacitor control the free-running frequency of the VCO. Here, C2
capacitor is a loop filter capacitor that founds the energetic characteristics of the
demodulator. This capacitor is selected slighter than the normal one to remove overshoot
on the output pulse. A 3-stage RC ladder filter is used to remove the sum frequency
component from the output. The VCO frequency is familiar with a resistor. This gives
the level of DC voltage at the output pin-7 is the same as that at pin-6. An input at 1,070
Hz frequency makes the demodulator output voltage to a more positive voltage
level, driving the digital output to the high level. An input at 1270 Hz similarly drives
the 565 DC output less positive with the digital output, which than falling to lower levels.
Report
Write a laboratory report on this project.
(1) Explain in detail about the theory of FSK modulator and demodulator for the
designed circuit and its operations.
(2) Discuss the method of productions of printed circuit board layout, fabrications,
assembly of components and the test results.
(3) Prepare slides for presentation and demonstration of this project.

4. AIMST University Faculty of Engineering and Computer Technology
BEng (Hons) in Electrical and Electronic Engineering Communication Systems 3
Figure 1: FSK modulator
Vcc
5V
FSK
Output
Ra
Rs
Rc
Rb
10 KΩ
50 KΩ
50 KΩ
47 KΩ
Q
C2
10 nF
10 nF
C1
Vcc
GND
OUT
RST
DIS
THR
TRI
CON
+
555
Input
Digital
Data at
150 Hz

5. AIMST University Faculty of Engineering and Computer Technology
BEng (Hons) in Electrical and Electronic Engineering Communication Systems 4
Figure 2: FSK demodulator
PLL
565
710
+
−
5 KΩ
600 KΩ
600
KΩ
10 KΩ 10 KΩ 10 KΩ
0.1 μF
0.01 μF
0.2 μF 0.02 μF 0.02 μF 0.02 μF
0.05 μF
+5V
−5V
+14V
Output
Digital
Data
Comparator
Input
1070 Hz
or
1270 Hz