Frequency Shit Keying(FSK) modulation project report

An overview of FSK modulation technique

1. ©Dept. of EEE,KUET EE 3200 Roll: 1503066



Khulna University of Engineering & Technology, Khulna-9203
Department of Electrical & Electronic Engineering
Course Title: Electrical & Electronic Project Design
Course no: EE 3200
Project Name: FSK Modulation Scheme
Submission date: 23-01-2019



Submitted to: Submitted by:
A.N.M Enamul Kabir Md. Rayid Hasan Mojumder
Professor Roll: 1503066
Dept. of EEE, KUET Year: Third
Semester: Second

2. Electrical & Electronic Project Design Project on FSK modulation

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FSK modulation scheme using 555 timer

Author: Md. Rayid Hasan Mojumder
Roll:1503066
Department of Electrical & Electronic Engineering, KUET
E-mail: [email protected]

Abstract

In this project two 555 timer have been used; one to generate digital signal of defined frequency
and another is to modulate it to the required mark-sapce frequencies. BC-547 is used for the
controlling of mark-space functionality as it’s used as a switch working on digital level ‘1’ & ‘0’.
This report involves the background theories to the project steps and analysis of the controlling
procedure.

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Table of Contents
1 Objectives .......................................................................................................... 3
2 Introduction ....................................................................................................... 3
2.1 Background to the project 3
2.2 Theoretical basis 3
2.2.1 The DC power supply .......................................................................................................................... 4
2.2.2 Digital signal generator ....................................................................................................................... 5
2.2.3 FSK modulation ................................................................................................................................... 5
3 Practical implementation of project ................................................................ 7
3.1 Circuit configuration of FSK generator 7
3.1.1 Apparatus used ................................................................................................................................. 8
3.2 Project result 9
3.2.1 Related waveshapes ......................................................................................................................... 9
3.2.2 Frequency shift analysis .................................................................................................................... 10
3.3 Applications of the project 11
4 Discussion ....................................................................................................... 11
5 Conclusions and Further Work ...................................................................... 12
Referencs ................................................................................................................ 12

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1 Objectives

The main objectives of this project involves:
• Implementation of Frequency Shift Keying circuit.
• Making an arbitrary digital input signal FSK modulated.
• Analysis of each part of the circuit’s influence on the output.
• Calculating the shifted frequency and defining the project’s application.

2 Introduction

In computer peripheral and radio (wireless) communication, the binary data or code is
transmitted by means of a carrier frequency that is shifted between two preset frequencies. Since
a carrier frequency is shifted between two preset frequencies, the data transmission is said to use
a frequency shift keying (FSK) technique.

2.1 Background to the Report
The FSK mode was introduced in the year 1900 to use in the mechanical teleprinters. The
standard speed of these machines was 45 baud, equivalent to about 45 bits per second. When
personal computers became general and networks come into being, this the signaling speed was
tedious. Transmission of the large text documents and programs taken hours; image transfer was
unknown. During the 1970s, engineers begin to develop modems that run at faster speeds, and
the search for ever-greater bandwidth has unremitting ever since. Today, a standard telephone
modem operates at the thousands of bits per a second. Cable and wireless modems work at more
than the 1,000,000 bps (one megabit per second or 1 Mbps), and the optical fiber modems
function at lots of Mbps. But the fundamental principle of FSK modulation has not changed in
more than half a century.

2.2 Theoretical basis
This project involves 3-parts :
(i.) DC power supply.
(ii.) Digital signal generator.
(iii.) FSK modulator.

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2.2.1 The dc power supply
The part of equipment that converts ac into dc is called DC power supply. In general at the input
of the power supply there is a power transformer. It is followed by a rectifier (a diode circuit)a
smoothing filter and then by a voltage regulator circuit.


Figure 1 – Block diagram of a DC power supply

Figure 2 – (+15 volts) DC power supply

From the block diagram, the basic power supply is constituted by four elements viz
a transformer, a rectifier, a filter, and a regulator put together.
• Transformer is used to step-up or step-down (usually to step-down) the-supply voltage as
per need of the solid-state electronic devices and circuits to be supplied by the dc power
supply. It can provide isolation from the supply line-an important safety consideration.
• Rectifier is a device which converts the sinusoidal ac voltage into either positive or
negative pulsating dc. The output voltage from a rectifier circuit has a pulsating character
i.e., it contains unwanted ac components (components of supply frequency f and its
harmonics) along with dc component.
• Filter is a device which passes dc component to the load and blocks ac components of the
rectifier output. Filter is typically constructed from reactive circuit I elements such as
capacitors and/or inductors and resistors.

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2.2.2 Digital signal generator
The digital data can be of two level 1 & 0. This can be achieved by using an astable multivibrator
with controllable duty cycle & frequency. The NE555 can be used for this purpose.At this mode
it can generate rectangular/square wave output. In the 555 Oscillator circuit above, pin 2 and pin
6 are connected together allowing the circuit to re-trigger itself on each and every cycle allowing
it to operate as a free running oscillator. During each cycle capacitor, C charges up through both
timing resistors, R1 and R2 but discharges itself only through resistor, R2 as the other side
of R2 is connected to the discharge terminal, pin 7.

Figure 3– Astable multivibrator using 555
Then the capacitor charges up to 2/3Vcc (the upper comparator limit) which is determined by
the 0.693(R1+R2)C combination and discharges itself down to 1/3Vcc (the lower comparator
limit) determined by the 0.693(R2*C) combination. This results in an output waveform whose
voltage level is approximately equal to Vcc – 1.5V and whose output “ON” and “OFF” time
periods are determined by the capacitor and resistors combinations.
The output frequency of oscillations: f =
1
??????
=
1
??????1+??????2
=
1
0.693∗(??????1+2∗??????2)∗??????
=
1.44
(??????1+ 2∗??????2)∗??????


2.2.3 FSK modulation
Frequency-shift keying (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. The
simplest FSK is binary FSK (BFSK). BFSK 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. The time domain of an FSK modulated carrier is illustrated in the
figures to the right

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Figure 4 –Frequency Shift Keying Technique
The Circuit given here illustrates how FSK modulated wave can be generated. It is build using
IC555. Square pulses are given as input to represent bit 1 and bit 0, and as an output IC555
generates FSK modulated wave. To generate square pulses one more IC555 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.
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. The above circuit is capable of
producing an FSK signal with respect to the given i/p signal. The Ra, Rb and C in the circuit
determine the frequency of the FSK modulated signal in the mode of Astable.


Figure 5–FSK modulation circuit using 555 Timer
The o/p frequency of the signal was based on the i/p digital signal given to the base terminal of the
transistor and IC works in the Astable mode. Here the resistors Ra, Rb & Capacitor C was chosen
in such a way to get o/p frequency of 1070Hz. When the i/p was high, then it is written by the
following equation f = 1.45/(Ra + 2Rb) C

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When the i/p binary data are logic 0, the PNP transistor is ON and its connects the Rc resistance
across Ra resistance. The Rc resistor is selected in such a way that the value of 1270Hz.
Here Rc value added in addition to the Ra value, Rb and Contribute to donate the working of the
IC. This makes the charging & discharging faster, resulting in high-frequency waves as
o/p. The resistors and capacitor values were selected in such a way to get an o/p frequency of 1270
Hz. This was given by the following equation.
F = 1.45/(( Ra || Rc ) + 2Rb ) C
Therefore, the output of an FSK will give 1070Hz frequency when i/p is high & 1270 frequencies
when an input is low. So by this technique, the FSK signal was gained using NE555.

3 Practical Implementation of project
3.1 Circuit configuration of FSK generator

Figure 6-FSK modulation scheme with digital data input

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3.1.1 Apparatus used
Table 1 : Required apparatus list for this project

Serial No.

Name of the apparatus

Ratings

Quantity

01

IC-NE555
Vcc=(4.5-16)volts
±200mA,(0-70˚C)

02

02

BC-547
VCBO=50 volts,VCEO=45
volts,VEBO=6
volts,100mA,500mW,
(-65-150)˚C

01

03

IC-7815
V0=(14.5-
15.6)volts,Io=5mA,Po≤15
watts,VI=(18.5-30)volts

01

04

Step-down Transformer
220v/12v,1000mA
01

05

Diode-1N4007
1A,25˚C, 50V,1.0 Silicon
Rectifier

04

06

Resistor
100 Ω , 0.5 Watt
1 KΩ , 0.5 Watt
50 KΩ, 0.5 Watt
100 KΩ(var), 0.5 Watt

01
01
01
03


07

Capacitor
0.01 µF, 25 volts
0.1 µF, 25 volts
1 µF, 25 volts
10 µF,25 volts
100 µF,25 volts
2200µF, 25 volts

02
02
01
01
01
01

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3.2 Project Result
3.2.1 Related waveshapes
Figure 7-Theoretical appearance of FSK modulation.
The output of the project for (1.) the square output at the signal generator end & (2.) the FSK
modulated signal are given below. Theoretically
The practical output after implementation

Figure 8-Practical appearance of FSK modulation.
f1 f2 f1 f2 f1 f2 f1
Square input
Modulated signal
Carrier signal

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3.2.1 Frequency shift analysis
Since here RV1,RV2,RV3 these three variable resistors been used. For convenience we are
assuming at a moment when RV1=10 kΩ, RV2=3.3 kΩ and RV3=47 kΩ
Thus the frequency of the first stage’s digital output (square/rectangular wave frequency):
f =
1.44
(R4 + 2∗RV1)∗C

=
1.44
(1??????+2∗10??????)∗1????????????

= 68 Hz
The ‘mark’ frequency from the 2
nd
stage (FSK modulated level ‘1’) :
f1=
1.45
(50k||3.3k + 2∗RV3)∗10000pF

=
1.45
(50k||3.3k + 2∗47k)∗10000pF

=1.483 KHz
The ‘space’ frequency from the 2
nd
stage (FSK modulated level ‘0’):
f2=
1.45
(50k+ 2∗RV3)∗10000pF

=
1.45
(50k + 2∗47k)∗10000pF

=1.0 KHz
Therefore, the output of an FSK will give 1483Hz frequency when i/p is high & 1000Hz
frequencies when an input is low. So by this technique, the FSK signal was gained using NE555.
If we select RV2=50k and RV3=47k then the mark frequency results in 1270Hz and space
frequency results in 1070Hz.
If we use high frequency pnp(BC-548) transistors in the place of npn(BC-547) we gets mark-
space frequency of 1270Hz-1070Hz. When transmitting teletypewriter information using a
modulator-demodulator (modem for short), this (mark-space) pair represents the originated
signal; while a 2025 Hz - 2225 Hz (mark-space) pair represents the answer signal.Which can be
generated by applying further modifications.

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3.3 Applications of the project
The appearance of FSK modulation in the practical communication schemes involves the
followings
• Early telephone-line modems used audio frequency-shift keying (AFSK) to send and
receive data at rates up to about 1200 bits per second. The Bell 103 and Bell 202 modems
used this technique.
• Early microcomputers used a specific form of AFSK modulation, the Kansas City
standard, to store data on audio cassettes.
• AFSK is used in the United States' Emergency Alert System to transmit warning
information.
• It is used at higher bit rates for Weathercopy used on Weatheradio by NOAA in the U.S.
• The CHU shortwave radio station in Ottawa, Ontario, Canada broadcasts an exclusive
digital time signal encoded using AFSK modulation.
• Frequency-shift keying (FSK) is commonly used over telephone lines for Caller
ID (displaying callers' numbers) and remote metering applications.
• Usually used in voice lines, in high frequency radio transmission .
• It is used on voice grade lines for data rates upto 1200 bps.
• It is used for high frequency radio transmission from 3 to 30 MHz.
• It is also used in coaxial cable based LAN (Local Area Network) at higher frequencies.

4 Discussion

Though FSK modulation technique although is a trivial now-a-days ,it employs an important role
in the basic digital signal transmission. In computer peripheral and radio (wireless)
communication, the binary data or code is transmitted by means of a carrier frequency that is
shifted between two preset frequencies. Since a carrier frequency is shifted between two preset
frequencies, the data transmission is said to use a frequency shift keying (FSK) technique. It has
lower probability of error, provides high SNR, higher immunity to noise due to constant
envelope ,moreover FSK transmitter and FSK receiver implementations are simple for low data
rate application. This projects aim had been to focus on the FSK modulation for a input bit
sequence. For the convenience here astable multivibrator had been introduced. The mark and
space frequency had been set by varying the resistances value. The mark-space frequency can be
set by hardware configuration for the required range. For modem it uses 1070Hz-1270Hz
represents the transmitting signal range and 2025Hz-2225Hz for answer signal.This project
enables to control this mark-space frequency i.e; controlling the bandwidth of the
communication channel & shifting a input signal to two closer high frequencies.

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5 Conclusion and future work

FSK modulated wave generated through this experiment can be demodulated by using FSK
demodulator or detector circuit. This part then can separate the digital bit sequence from the received
modulated wave in the receiver end. FSK needs higher bandwidth than ASK modulation but its noise
immunity is higher. Thus it can be implemented where less level of transmission rate is requied such
as such as amateur radio, caller ID, and urgent situation broadcasts.

References

[1] Modern Digital and Analog Communication Systems – B.P Lathi,4
td
edition.
[2] Digital Communications with Emphasis on Data Modems: Theory, Analysis, Design,
Simulation, Testing, and Applications - Richard W. Middlestead,chapter-5.
[3] Digital Modulation in Communications Systems – An Introduction Application Note
1298,content-3.3,Hewlett Packard]
[4] Digital And Analog Communication Systems - LEON W. COUCH, II, 8
th
content-5.9