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ii
FUNDAMENTALS OF
TELECOMMUNICATION
ENGINEERING
BY
ENGR. (DR.) KAMORU OLUWATOYIN KADIRI
iii
DEDICATION Dedicated to the Most High and all students in Engineering Departments in
Nigeria.
iv
ACKNOWLEGDEMENTS
First and foremost I am grateful to the Almighty God, the beneficent and
the Most Merciful who makes all things possible.
I appreciate my well-wishers for their support physically and morally in
making this text a success. Also I acknowledge the support received from the staff
of Electrical/Electronics Department, the Federal Polytechnic, Offa, the students,
family members and friends.
Engr. (Dr.) K. O. Kadiri
v
FORWARD
The book “Fundamentals of Telecommunication Engineering” is designed to assist
readers, and to provide necessary fundamentals of telecommunication systems. It
will emphasize principles of communication system in Electrical/Electronics
Engineering Students. It was written to meet the basic requirements of the
National Board for Technical Education (NBTE) syllabus for National Diploma (ND)
level in the Polytechnic for Engineering Students.
TABLE OF CONTENTS
DEDICATION .................................................................................................................................... iii
ACKNOWLEGDEMENTS ................................................................................................................... iv
FORWARD ........................................................................................................................................ v
TABLE OF CONTENTS....................................................................................................................... vi
CHAPTER ONE ................................................................................................................................. 1
INTRODUCTION ............................................................................................................................... 1
Brief History of Telecommunication ........................................................................................... 1
Definition of Telecommunication ............................................................................................... 3
Definition of Telecommunication System ................................................................................... 3
Building Block of Telecommunication System ............................................................................ 4
Building Block of Communication System .................................................................................. 4
CHAPTER TWO ................................................................................................................................ 7
TRANSDUCER .................................................................................................................................. 7
Forms of Transducers .................................................................................................................. 7
Electronic Transducers ............................................................................................................ 7
Passive Transducer .................................................................................................................. 7
Active Transducer .................................................................................................................... 8
Electromagnetic Transducers .................................................................................................. 8
Electrochemical Transducer .................................................................................................... 8
Electromechanical Transducer ................................................................................................ 8
Electro Optical (Photoeletric) Transducers ............................................................................. 8
Electro acoustic Transducer .................................................................................................... 9
vii
Thermoelectric Transducer ..................................................................................................... 9
Radio acoustic Transducers ..................................................................................................... 9
Gravinertia Transducer ............................................................................................................ 9
Sensitivity of a Transducer ...................................................................................................... 9
Microphones ............................................................................................................................. 10
Carbon Microphone ............................................................................................................... 10
Condenser Microphone ......................................................................................................... 12
Dynamic Microphone ............................................................................................................ 13
Crystal Microphone ............................................................................................................... 14
Electret microphone .............................................................................................................. 16
Fiber Optic Microphone......................................................................................................... 16
Ribbon Microphone ............................................................................................................... 17
Loudspeakers, Headphones and Earpieces ............................................................................... 18
Symbols for Loudspeakers, Headphones and Earpiece......................................................... 19
CHAPTER THREE ............................................................................................................................ 20
MODULATION ............................................................................................................................... 20
Introduction............................................................................................................................... 20
Type of Modulation ................................................................................................................... 21
Amplitude Modulation .............................................................................................................. 21
International Telecommunication Union (ITU) Designation ................................................. 23
Sideband ................................................................................................................................ 24
Bandwidth .............................................................................................................................. 24
Modulation Index of Amplitude Modulation ........................................................................ 27
CHAPTER FOUR ............................................................................................................................. 29
viii
FREQUENCY MODULATION .......................................................................................................... 29
Introduction............................................................................................................................... 29
Waveform of Frequency Modulation (Frequency Shift Keying) ............................................... 30
Phase Modulation ..................................................................................................................... 30
Carson’s Rule ............................................................................................................................. 31
Types of Signal ........................................................................................................................... 32
Carrier Waveforms .................................................................................................................... 32
CHAPTER FIVE ............................................................................................................................... 34
DEMODULATION ........................................................................................................................... 34
Introduction............................................................................................................................... 34
Actions Performed by Demodulator/Detector ......................................................................... 34
Types of Demodulator Used For Linear Modulation or Angular Modulation........................... 35
AM Detectors ......................................................................................................................... 35
Envelope Detector ................................................................................................................. 35
Product Detector ................................................................................................................... 36
Frequency Detector (Demodulator) ...................................................................................... 36
Quadrature Detector ............................................................................................................. 36
Foster Seeley Discriminator ................................................................................................... 36
Ratio Detector........................................................................................................................ 37
CHAPTER SIX .................................................................................................................................. 38
RADIO AND BLACK/WHITE TRANSMISSION .................................................................................. 38
Amplitude Modulated Radio Transmitter ................................................................................. 38
AM Broadcasting ................................................................................................................... 38
Frequency Modulation Transmitter .......................................................................................... 39
ix
Straight Radio Receiver ............................................................................................................. 40
Super-Heterodyne Radio Receiver ............................................................................................ 41
Principle of Operation ............................................................................................................... 43
Heterodyning ......................................................................................................................... 45
Frequency Modulated Radio Receiver ...................................................................................... 45
Television Basics ........................................................................................................................ 46
Monochrome TV Receiver ......................................................................................................... 48
Automatic Gain Control (AGC) .................................................................................................. 50
Working Principle of AGC ...................................................................................................... 50
CHAPTER SEVEN ............................................................................................................................ 52
TELEPHONE AND TELEGRAPH ....................................................................................................... 52
Introduction............................................................................................................................... 52
The Morse code ......................................................................................................................... 53
The Murray code ....................................................................................................................... 53
Telephone .................................................................................................................................. 54
Simple Telephone Circuits ..................................................................................................... 55
Telephone Keypad ................................................................................................................. 56
Telephone Trunk System ....................................................................................................... 58
Element of Local Telephone Network ................................................................................... 58
Trunk Group and Routing Of Telephone Calls ....................................................................... 59
Telex ....................................................................................................................................... 59
CHAPTER EIGHT ............................................................................................................................. 61
RADIO FREQUENCY BANDS ........................................................................................................... 61
Introduction............................................................................................................................... 61
x
Speed and Wavelength of Radio Waves ................................................................................... 62
Frequency Characteristics of Radio Propagation ...................................................................... 64
Propagation Loss of Radio Waves ............................................................................................. 64
Qualities of Radio Waves .......................................................................................................... 64
Why Communication Errors Occur ........................................................................................ 65
Effect of Radio Waves on Human Lives and Its Environment ................................................... 65
Aerials ........................................................................................................................................ 65
The Dipole .............................................................................................................................. 66
Types of antenna/aerial ......................................................................................................... 68
Measurement of Aerial Gain ................................................................................................. 73
Receiving Antenna ................................................................................................................. 74
Propagation of Radio Waves ................................................................................................. 74
Ground Wave ............................................................................................................................ 74
Space wave ................................................................................................................................ 75
Sky Wave Propagation ........................................................................................................... 76
The Troposphere ................................................................................................................... 77
The Ionosphere ...................................................................................................................... 77
Critical Frequency (CF) ........................................................................................................... 80
Maximum Usage Frequency (MUF) ....................................................................................... 80
Optimum Working Frequency (OWF) .................................................................................... 81
CHAPTER NINE .............................................................................................................................. 82
CABLE AND SATELLITE TV .............................................................................................................. 82
Cable Television CATV ............................................................................................................... 82
Satellite Television .................................................................................................................... 83
xi
Direction Broadcasting by Satellite (DSB) ................................................................................. 83
Geostationary (Synchronous) Satellites .................................................................................... 83
REFERENCES .................................................................................................................................. 85
1
CHAPTER ONE
INTRODUCTION
Brief History of Telecommunication
Telecommunication began in Africa, America and parts of Asia with the use
of smoke signals and drums. Initially, fixed semaphore system emerged in Europe
in the 1790’s, however, it was until the 1830’s that electrical telecommunication
system started to appear. Examples of telecommunication system since the
Middle Ages are semaphore line, telegraph and telephone, submarine
communication cable, conventional telephone, radio and television etc.
Semaphore line otherwise known as first fixed visual telegraphy system was
built in 1792 by a French Engineer named Claude Chappe between Lille and Paris.
Europe’s last commercial semaphore line in Sweden was abandoned in 1880 due
to competition from the electrical telegraph and as the need for skilled operators
and expensive towers at intervals of 10 – 30 kilometers (6 – 20mi) emerged which
was not readily available.
Initially, experiment on communication with electricity was carried out by
some scientists, some of which are Laplace, Ampere and Gauss in 1726 but was
not successful. Another practical electrical telegraph was proposed in January
1837 by William Fothergill Cooke who considered it as an improvement on the
existing “electromagnetic telegraph”, in which an improved five needle, six-wire
system was developed with the assistance of Charles Wheatstone which was
widely used for commercial purposes in 1838. Also, several wires connected to a
number of indicator needles were used for early telegraphs.
Simple version of the electrical telegraph was later developed
independently by business man Samuel F. B. Morse and physicist Joseph Henry of
the United States. Electrical telegraph was successfully demonstrated by Morse
on 2nd of September 1837. Contribution was made technically on electrical
telegraph with the use of simple and highly efficient Morse code which was co-
developed with his associate Alfred Vail. The development was an important
2
advancement over Wheatstone’s more complicated and expensive system and it
required just two wires. As a result of communications efficiency of the Morse
code which preceded that of the Huffman code in digital communications by over
100 years, electrical telegraph was developed by Morse and Vail by the use of
code which was purely empirical and shorter codes for more frequency letter.
Another telecommunication system was first permanent transatlantic
telegraph cable which was successfully completed on 27th July 1866, enabling
transatlantic electrical communication for the first time. It transmitted and
received greeting messages back and forth between President James Buchan of
the United State and Queen Victoria of the United Kingdom.
In 1832, James Lindsay gave a classroom demonstration of wireless
telegraphy through conductive water to his students. While in 1854, James
Lindsay was able to demonstrate a transmission across the firth of Tay from
Dundee, Scotland, to Woodhaven, a distance of about two miles (3km) using
water as the transmission medium. In December 1901, Guylielmo Marconi
established wireless communication between St. John’s New found land and
Poldhu, Cornwall (England), earning him the Noble Prize in physics for 1909, one
which he started with Karl Braun.
Conventional telephone which is currently in use worldwide was first
patented by Alexander Graham Bell in March 1876. After which, the first licensed
given to Bell to sell his inventions on telephone devices and features followed.
Public recognition for the invention of the electric telephone has been constantly
argued about, and new public debate/disagreement about the invention which
arouses strong opinions have arisen from time to time. There were other great
inventions such as radio, television, the light bulb, and digital computer. Also,
some important innovators like Alexander Graham Bell and Gardiner Greene
Hubbard did some experimental work on voice transmission over a wire.
Gardiner Greene Hubbard and Alexandra Graham Bell created the first
telephone company which was popularly known as Bell Telephone Company in
the United States, which later changed into American Telephone and Telegraph
(AT&T), at that time the World’s largest phone company. The first commercial
telephone services were set up in 1878 and 1879 on both sides of the Atlantic in
the cities of New Haven, Connecticut and London, England.
3
Transmission of moving pictures at the Selfrigde’s Department Store in
London, England was demonstrated by John Logie baird of Scotland on 25 March
1925. Baird’s system depended on the fast rotating Nipkowdisk, and hence, its
known as the mechanical television. The system formed the basis of experimental
broadcasts done by the British Broadcasting Corporation, which started 30
September 1929. Although, in the last 20th century, television systems were
designed around the Cathode Ray Tube (CRT), invented by Karl Braun. Moreover,
Philo Farnsworth of the United States produced the first version of electronic
television and it was demonstrated to his family in Idaho on 7 September, 1927.
However, television is not solely a technology that is limited to moving pictures
only.
Definition of Telecommunication
Telecommunication can be defined as the extension of communication over
a distance, under the practical constraints of attenuation, noise and interference
such that something may be lost in the process, hence, the term
“telecommunication covers all forms of distance and/or conversion of the original
communications like radio, telegraphy, television, telephony, data communication
and computer networking.
Telecommunication is now the world’s fastest growing technology. All
telecommunication systems have one thing in common, because the messages
they send are changed into signals that can be transmitted through wires,
interplanetary space and even glass fibres.
Definition of Telecommunication System
A telecommunication system receives and converts some original
information energy (voice, music, video, and data) into an electronic signal at the
destination back to its desired form. Example of telecommunication system
include: telephone networks, data communication networks, computer networks,
broadcast networks (radio and television).
4
Building Block of Telecommunication System
Source source coding channel coding modulation
Channel
Receiver source coding channel coding Demodulation
Telecommunication systems may be line systems, radio systems or satellite
– based systems.
Line system (channel) passes the electronic information signal down a wire,
cable or fibre link (or a combination of wire, cable together) from the transmitter/
source (sender) to the receiver.
Source coding is the process of compressing the data efficiently or the
process of optimizing the length of data.
Source encoder converts analogue information into a pulse code modulated
(PCM) signal before transmission.
Channel coding involves the addition of redundant bits to a message signal
that will make up for the errors. This involves the identification and as well as the
correction of errors, if any. Best example of channel coding is the Hamming code.
Interactive telecommunication systems require two-way or duplex
channels, but broadcast need only simplex or one-way channels.
Characteristics of line system are: Attenuation and Noise, Modulation (AM,
FM, PM, PCM), twisted pair copper wires, coaxial cable or tube, optical fibre and
transmission line characteristics.
Building Block of Communication System
Source output
message
Figure 1.1: Communication system
Input Transducer Transmitter Channel Receiver
Output Transducer
5
The diagram above comprises of input Transducer which takes message
sent from the source and convert it to suitable form for transmission along the
channel (i.e. electrical signal). Transmitter, channel is the path along which
message is transmitted from the source to the receiver and output Transducer
change the electrical transmitted signal to sound signal for the receiver. An
amplifier can be used to increase the strength of the signal in the Transducer.
(a) INPUT TRANSDUCER: An input Transducer such as microphone converts
sound energy (sound signal) to electrical signal (electrical energy) for
proper communication to take place through channel. The sound signal
(input message) may be digital or analogue must be converted to electrical
signal and processed by an electronic instrument or system.
(b) TRANSMITTER: Transmitters are important component parts of
telecommunication system that enable effective communication from the
information/input message to be sent through channel.
Transmitter is an electronic device with the aid of an antenna produces
radio waves. The transmitter itself generates a radio frequency alternating
current, which is applied to the antenna. When excited by the alternating current,
the radio wave is radiated by the antenna. In the transmitter, modulation of signal
takes place.
(c) CHANNEL: channel is the path through which the transmitted signal from
the source gets to the receiver. Channel may be through ground, through
underground, overhead cables, sky or space.
(d) RECEIVER: Receiver receives the information (message) being sent through
a cable. It usually has an antenna that intercepts the transmitted signal
through the channel. Receiver converts the selected signal to a form
suitable for the output Transducer. Good receiver must be able to select
well desired signal from various signal and reject the unwanted signals.
6
(e) OUTPUT TRANSDUCER: An output Transducer such as speaker is always at
the receivers end, which will convert the output electrical signal being sent
through channel to a form desired by the user (sound). Other forms of
output Transducers are: Analogue Meters, Digital Meter, Oscilloscope,
Cathode Ray Tubes (CRT), Electrical Motor etc.
7
CHAPTER TWO
TRANSDUCER
A Transducer is a sensor that changes energy from one form to another.
More technically, Transducers convert physical parameter to another form.
Transducers can be used at the input or output in a telecommunication system.
Transducers that have input or output to be electrical energy, sound energy will
be considered in this chapter. With the help of electronic measuring system, an
input Transducer usually at the transmitting end [sender] convert sound energy or
mechanical energy to electrical energy and an output Transducer convert the
output electrical energy into a mechanical energy (sound energy). Energy types
are electrical, mechanical, electromagnetic, chemical, acoustic and thermal
energy.
Forms of Transducers
There are different forms of Transducer, some of which are Electronic
Transducers, Passive Transducers and Active Transducers. Others are:
Electromagnetic Transducer, Electrochemical Transducer, Electromechanical
Transducer, Electro-Acoustic, Electro-Optical (Photoelectric Transducer),
Electrostatic, Thermoelectric, Radio Acoustic, Gravinertia.
Electronic Transducers
Electronic Transducers are Transducers which provide output as electrical
signal (voltage, current, capacitance, inductance or a change in resistance).
Passive Transducer
Passive Transducers are Transducers which do not require energy to
operate. An example of passive Transducer is solar cell.
8
Active Transducer
Active Transducers are Transducers that require (need) energy to operate.
Example of active Transducer is photo resistor.
Electromagnetic Transducers
Electromagnetic Transducers are Transducers which convert
electromagnetic waves, magnetic fields to electrical signal (electrical energy).
Examples of electromagnetic Transducers are antenna, magnetic cartridge, tape
head, disk read and write head, Hall Effect sensor etc.
Electrochemical Transducer
Examples of Electrochemical Transducers are electro-galvanic fuel cell,
hydrogen sensor, PH probes
Electromechanical Transducer
Electromechanical Transducers are sometimes referred to as activators
Examples of electromechanical Transducers are electro-active polymers, rotary
motor, linear motor, micro electromechanical systems, galvanometer,
potentiometer (when used for measuring position), load cell (converts force to
millivolt/volt electrical signal using strain gauge), vibration powered generator,
accelerometer, strain gauge, linear variable differential transformer (rotary
variable differential transformer), air flow sensor etc.
Electro Optical (Photoeletric) Transducers
Electro optical Transducer are Transducers that convert electric power
(electric energy) into incoherent light or coherent light or visual signals. Examples
of electro-optical Transducers are light emitting diode laser diode, fluorescent
lamp, incandescent lamp, photo resistor, photo multiplier, photo diode, photo
detector, photo detector, photo transistor, light dependent resistor, cathode ray
tube etc.
9
Electro acoustic Transducer
Examples of electro acoustic Transducers are earphone, microphone,
loudspeaker, tactile Transducer, geophone, piezoelectric crystal, hydrophone,
sonar transponder, gramophone pick up, ultrasonic transceiver etc.
Thermoelectric Transducer
Thermoelectric Transducer converts temperature into an electrical
resistance signal or electrical energy.
Examples of thermoelectric Transducers are: thermocouple, peltier cooler,
RTD (Resistance temperature detector), thermistor etc.
Radio acoustic Transducers
Radio acoustic Transducers are Transducers that convert incident ionizing
radiation to electrical impulse signal (electrical energy).
Examples of radio acoustic Transducers are Geiger – miller rube, receiver,
transmitter etc.
Gravinertia Transducer
Examples of gravinertia Transducer is Woodward effect
Factors to consider when choosing type of Transducer
1. Sensitivity
2. Range
3. Temperature coefficient
4. Linearity
5. Size
6. Cost
Sensitivity of a Transducer
Sensitivity is the ability of a Transducer to indicate correctly output signal of
message of being sent. Example; force Transducer’s output voltage can be
calculated as follows:
U = U0 . C. f/Fnom.
10
Where; U is the output voltage, U0 is the excitation voltage, C is the
sensitivity, F is the applied force and Fnom is the Transducer’s nominal (rated)
force.
Microphones
Microphone (colloquially called a mic or mike) is an acoustic – to- electric
Transducer or sensors that converts sound in air into an electrical signal.
Microphones are used mainly in applications like telephones, tape recorders, FRS
radios, megaphone radio and TV broadcasting and in computers for recording,
speech recognition and for non-acoustic purposes such as ultra sonic checking.
Most microphones use electromagnetic induction, capacitance change or piezo
electric generation to produce electric signal.
Carbon Microphone
Carbon Microphone otherwise known as carbon button microphone,
button microphone or carbon transmitter was invented by David Edward Hughes
in 1870s. Carbon Microphone was the first microphone that enabled proper voice
telephone, referred to as transmitter then. Carbon Microphone was developed
independently by David Edward Hughes in England, Emile Berliner and Thomas
Edison in the US. Although, the first patent awarded in mid-1877 was Edison, but
Hughes has demonstrated his working devices in front of many witnesses some
years earlier, and most historians credit him with the invention.
Carbon Microphone is an example of microphone i.e. a Transducer that
converts sound energy (sound signal) to an electrical audio signal. Carbon
Microphone loosely packed carbon granules. Carbon Microphone consists of two
metal plates separated by granules of carbon. One plate facing outward acts as
diaphragm and is very thin compared to the other plate. When sound wave
strikes the plate acting as diaphragm, the pressure on the granules changes,
which later changes the electrical resistance between the plates. The resistance
reduces when higher pressure is exerted on it and the granules moved closer
together. Modulation of current of the same frequency with the sound wave
occurs if steady direct current is passed between the plates varying resistance.
Resistance of carbon varies proportionally as a result of varying pressure exerted
11
on the granules by the diaphragm from the acoustic waves enabling relatively
accurate electrical reproduction of the sound signal. Hence, the frequency
response of the carbon Microphone is limited to a narrow range and the device
produces significant electrical noise.
Carbon Microphone are widely used in AM radio broadcasting systems, but
later abandoned for use in AM radio broadcasting systems because of high noise
level and low frequency response in 1920s. In some decades after, carbon
microphone are used commonly for low-end public address, military and amateur
radio applications.
It was Hughes who coined the word “Microphone”, he showcased his
apparatus to the Royal Society by enlarging the sound of insects scratching
through a sound box. Different from Edison, Hughes decided not to take out a
patent; instead, he made his invention a gift to the world.
In America, Edison and Berliner fought a prolonged legal battle over the
patent rights. Fortunately, Federal Court awarded Edison full right of the
invention, stating “Edison preceded Berliner in the transmission of speech. The
use of Carbon in a transmitter is beyond the controversy, the invention of Edison,
and Berliner’s patent was ruled invalid.
Carbon Microphones can also be used as amplifiers. They can produce high
level audio signals from very low D.C. voltages, without using any form of
additional amplification or batteries. Carbon Microphones are widely used in
safety critical applications such as mining and chemical manufacturing, where
higher line voltage cannot be used due to risk of sparkling and consequent
explosions. They are also resistant to damage from high voltage transients, such
as those produced by lightning strikes and electromagnetic pulses of the tape
generated by nuclear explosions, and so are still maintained as back up
communication systems in critical military installation.
ELECTRICAL EQUIVALENT CIRCUIT OF A CARBON MICROPHONE CONNECTED IN ITS BIAS
CIRCUIT
Equivalent circuit of Carbon Microphone connected in its bias circuit.
12
If r is made smaller than RL + Ro, then;
if the soundwave is sinusoidal, then Vout is also sinusoidal;
Condenser Microphone
Condenser Microphone were the first design that proved practical for
recording music and these microphones remain work houses in the recording
industry today. Condenser Microphone was inverted by Bell Labs in 1916, the first
practical microphone using amplifiers to record sound.
Description of Condenser Microphone
Condenser Microphone is a very thin plastic film, coasted on one side with
gold or nickel, and mounted very close to a conductive back plate. It consists of a
diaphragm and a back plate separated by a small amount of air forming an
electrical component called a capacitor/condenser. It has an external power
supply which may be battery or phantom power applied on the diaphragm which
is a polarizing voltage.
Condenser Microphone responds very quickly to transients because the
diaphragm of the condenser is not loaded with the mass of the coil. Condenser
capsule can be designed to be very small and have excellent sonic characteristics.
Condenser microphone is widely used in high quality professional microphones in
sound reinforcement, measurement and recording. Condenser Microphone is
widely chosen for recording voices and acoustic instruments.
Working Principle of Condenser Microphone
A thin, conductive diaphragm is mounted parallel a plate which is
electrically charged. The energy stored between these two plates varies as the
13
freely suspended diaphragm is displaced by the sound wave. When air pressure
from the sound source hits the diaphragm, acoustic signal from the thin
difference in space between the diaphragm and plate is converted to electrical
current. The diaphragm moves closer to the back plate and further away from the
back plate when it vibrates in response to the sound. During the process,
electrical charge induces in the back plate changes proportionally and electrical
representation of the fluctuating voltage on the back plate is diaphragm motion.
The electrical current is modified for broadcast or recording. Condenser
Microphones are sensitive to very high audio frequencies because of the nature
of the diaphragm, and the inherent sensitivity of Condenser Microphones which
requires less amplification than dynamics microphones. Large diaphragm
condensers give a quality to vocals that is warm, detailed and full range, while
small diaphragm condenser microphones are the most accurate at capturing
sound.
Dynamic Microphone
Dynamic Microphone known as moving coil microphone was invented in
1877 by Curttris and patented in 1931. It works on the principle of
electromagnetic induction to generate an output signal voltage. Dynamic
microphone is similar to a miniature loudspeaker working in reverse. The
diaphragm of the dynamic microphone is attached to a coil of fine wire. The coil is
placed in the air gap of the magnet in order to freely move to and fro within the
gap. The diaphragm of the dynamic microphone vibrates in response to the
contact made with it by the sound wave. The coil attached to the diaphragm
oscillates to and fro in the field of the magnet. As the coil oscillates through the
lines of magnet force in the gap, a small electric current is induced in the wire, the
magnitude and direction of which is directly related to the motion of the coil and
current which is the electrical form of the sound wave. One of the major
disadvantage of dynamic microphone is the mass of the moving coil. Dynamic
microphone has a very poor transient response and is less sensitive due to the
mass of the moving coil.
14
Crystal Microphone
Crystal Microphone uses the piezoelectric effect of Rochelle salt, quartz, or
other crystal line materials. Rochelle salt is otherwise known as Sodium Potassium
Tartrate. A voltage, ElectroMagnetic Force (EMF) is generated when mechanical
stress is acted on the material. Rochelle salt has the highest voltage output for a
given mechanical stress, which makes it the most widely used crystal in
microphones. Crystal microphone has a high voltage output of about 100mv
mostly in one direction (Omni directional), it is cheaper, although capacitor,
ribbon or dynamic has high performance than crystal microphone, and however,
crystal microphone has very high impedance.
The figure 2.1 shows a crystal microphone in which the crystal is mounted
so that the sound waves make it directly. Figure 2.2 shows a diaphragm that is
mechanically linked to the crystal so that the sound waves are indirectly coupled
to the crystal. While, figure 2.3 shows the internal structure of crystal microphone
15
Figure 2.1: Directly Actuated Type Crystal Microphone
crystal
Figure 2.2: Diaphragm Type Crystal
MICROPHONE
Metal electrode
output
crystal
diaphragm
Figure 2.3: Crystal Microphone Internal Structure
Crystal Sound-wave
Output
voltage
Electrodes
Diaphr
agm
Sound-wave
Output
voltage
Electrodes
16
Electret microphone
An electret microphone is a type of capacitor microphone. It was invented
by Gerhard Sessler and Jim West at Bell Laboratories in 1962.
Fiber Optic Microphone
Fiber optic microphone converts acoustic waves to electrical signals by
detecting changes in light intensity, instead of detecting changes in capacitance or
magnetic field done by conventional microphones.
Working Principle of Fibre Optic Microphone
During operation, light from a laser source travels through an optical fiber
to illuminate the surface of a reflective diaphragm sound vibrations of the
diaphragm modulated intensity of light reflecting off the diaphragm in specific
direction. The modulated light is then transmitted over a second optical fiber to a
photo detector, which transforms the intensity modulated light into an analog or
digital audio for transmission or recording. Fiber optic microphones do not react
to or influence any electrical magnetic, electrostatic or radioactive fields, the
phenomenon of which is referred to as “EM/RFI immunity”.
Fiber optic microphone possesses high dynamic and frequency range
similar to the best high fidelity conventional microphones. Fiber optic micro
design is therefore ideal for use in arrears where conventional microphone are
ineffective or dangerous, such as inside industrial turbines or in Magnetic
Resonate Imaging (MRI) equipment environment.
Capacitor plate
Capacitor plate 2
Capacitor
Output
17
Fiber optics are resistant to environmental changes in heat and moisture,
they are robust, and can be produced for any directionality or impedance
matching. They are suitable for industrial and surveillance acoustic monitoring.
They are used in specific applications areas such as for infrasound monitoring and
noise cancelling. They are especially useful in medical applications, such as
allowing radiologists, staff and patients within the powerful and noisy magnetic
field to converse normally, inside the MRI suites as well as in remote control
rooms. They are also used for industrial equipment monitoring and sensing audio
calibration and measurement, high fidelity recording and in law enforcement
agency.
Ribbon Microphone
Ribbon microphone was invented by Edmund Lowe. Ribbon Microphone
uses a thin, usually corrugated metal ribbon suspended in a magnetic field. The
ribbon is electrically connected to the microphone’s output and its vibration
within the magnetic field generates the electrical signal.
Ribbon microphones are similar to moving coil microphone (dynamic
microphone) in such a way that both produce sound by means of magnetic
induction. Basic ribbon microphone detect sound in a bi-directional pattern
because the ribbon, which is open to sound both front and back, responds to the
pressure gradient rather than the sound pressure. Though the symmetrical front
and rear pick up can be a nuisance in normal stereo recording, the high side
rejection can be used by positioning a ribbon microphone horizontally. Crossed
figure eight or bloomless pair, stereo recording is gaining in popularity, and figure
eight response of ribbon microphone is ideal for that application.
Other directional patterns are produced by enclosing one side of the ribbon
in an acoustic trap or baffle, allowing sound to reach only one side. The classic
RCA type 77 – DX microphone has several extremely adjustable positions of the
internal baffle, allowing the selection of several response patterns ranging from
“figure 8” to unidirectional. Some older ribbon microphones still provide high
quality sound reproduction, but a good low frequency response could only be
obtained when the ribbon was suspended very loosely, which made them
relatively fragile.
18
New modern ribbon materials including new nonmaterial have been
introduced that eliminate those concerns, and even improve the effective
dynamic range of the ribbon microphone at low frequencies. Protective
windscreens can reduce the range of damaging a vintage ribbon and also reduce
plosive artificial in the recording of properly negligible treble attenuation.
Ribbon microphone does not require phantom power. Some new modern
ribbon microphones are designed to resist damage to the ribbon and transformer
by phantom power. Also, there are new ribbon materials available that are
immune to wind blest and phantom power. Other types of microphone include
Liquid Microphone, Laser Microphone, Microelectronic Chemical System
Microphone (MEMS) etc.
An electronic symbol for microphone is:
Figure 2.4: Circuit Symbol of the microphone
Loudspeakers, Headphones and Earpieces
Loudspeaker converts electrical energy from an amplifier to mechanical
energy or sound waves. Headphones and earpieces also convert electrical energy
from an amplifier into mechanical energy or sound waves. Loudspeaker,
headphones and earpiece are example of output devices. They have three
important properties which make them unique during operation, they are
frequency response, power rating and impedance.
There are different types of loudspeakers, namely: Crystal type
loudspeaker, efficient type loudspeaker, moving coil (dynamic) type loudspeaker
etc. Types of headphones include: dynamic types headphones, magnetic type
headphones etc. While, types of earpiece also are dynamic type earpiece, crystal
type earpiece etc.
19
Symbols for Loudspeakers, Headphones and Earpiece.
(a) Loudspeaker (b) Earpiece (c) Headphone
Figure 2.5: Symbols for Loudspeakers, Headphones and Earpiece
20
CHAPTER THREE
MODULATION
Introduction
In telecommunication system, message sent from the information source is
transmitted through a channel to the receiver. Message of which is transmitted
over a long distance from the information source, before it get to the final
destination. However, there may be some losses along the way due to
interference, which may not make the exact information sent get to the receiver.
Considering this situation, a technique invented called “modulation’’ is
useful when sending multiple channels along the same circuit. Modulation allow
information to be transmitted through the intended medium even when the
information is not suitable for transmission, by modulating it on some kinds of
carrier signal. It also helps in a situation whereby the information is far from the
destination, often an electromagnetic wave and transmitted through the channel.
Modulation also helps to decrease electromagnetic noises around the channel
and interference. Modulation also helps in multiplexing signals.
Modulation can then be defined as the addition of information (or signal) to
an electronic or optical signal carrier. It also means varying some aspect of a
higher frequency continuously wave carrier signal with an information bearing
modulation waveform, such as an audio signal which represent sound or a video
signal which represents images, so that the carrier will transmit the information.
General procedure for any communication system is as shown in Figure 3.1.
21
Figure 3.1: Block Diagram of Communication System
The block diagram above shows signal from the information source being
added to the carrier in the modulator, the modulated signal being sent along a
channel in any medium (cable or radio wave) by the transmitter, and the receiver
which may have an antenna to amplify the signal being sent and select the
desired one before the demodulator will extract information signal for delivery to
the receptor of information.
Type of Modulation
There are four (4) types of modulation, which are: Amplitude Modulation
(AM), Frequency Modulation (FM), Phase Modulation And Pulse Modulation.
Amplitude Modulation
Amplitude modulation was the earliest method of modulation used to
transmit voice by radio. AM was developed during the first 2 decades of the 20th
century beginning with Reginald Fessenden’s (Canadian researcher) made the 1st
AM transmission on 23rd December, 1900 using a spark gap transmitter with a
specially designed high frequency 10KHz interrupter, over a distance of 1mile
(1.6km) at Cobb Island, Maryland, USA. His first transmitted words were “Hello
one, two, three, four. Is it snowing where you are, Mr. Thiessen’’? The words
were barely intelligible above the background buzz of the spark.
Amplitude Modulation (AM) is used in one of the crude pre-vacuum tube
AM transmitters, a Telefunken arc transmitter from 1906. The carrier wave is
generated by six electric arcs in the vertical tubes, connected to a tuned circuit.
Modulator Information
source
Transmitter
Demodulated Receptor or
information
Receiver
Propagating
medium
22
Modulation of which is done by large carbon microphone (cone shape) in the
antenna lead.
AM was also used in one of the first vacuum tube, AM radio transmitters
built by Meissner in 1913 with an early triode tube by “Robert Von Lieben”.
Robert used it in a historic 36km (24km) voice transmission from Berlin to Nauen,
Germany.
Although, AM was used in a few crude experiments in multiplex telegraph
and telephone transmission in the late 1800s, the practical development of
amplitude modulation is similar to the development between 1900 and 1920 of
“radio telephone” transmission, that is, the effort to send sound (audio) by radio
waves.
The first radio transmitters called spark gap transmitters, transmitted
information by wireless telegraphy, using different length pulses of carrier wave
to spell out text messages in Morse code. One of the first detectors able to rectify
and receive AM known as “Liquid Barrette or Electrolytic Detector” was
developed by “Fassenden’’ in 1902. Other detector that could rectify AM signals
are radio detectors invented for wireless telegraphy such as Fleming valve
invented in 1904 and the crystal detector invented in 1906.
Definition of Amplitude Modulation
Amplitude modulation is a modulation technique used in electronic
communication, most commonly for transmitting information through a radio
carrier-wave. Amplitude modulation is the encoding of information in a carrier
wave by variation of its amplitude in accordance with an input signal.
Working Principle of Amplitude Modulation
Amplitude Modulation works on the principle of heterodyning. Amplitude
Modulation works by varying the strength (amplitude) of the carrier in proportion
to the waveform being sent. The waveform may correspond to the sounds to be
reproduced by a loudspeaker or the light intensity of telephone pixels. The
amplitude of the carrier oscillation varies. For example, a sinusoidal carrier wave
has its own AM by an audio waveform before transmission in audio radio
communication.
23
The audio waveform modifies the amplitude of the carrier wave and
determines the envelope of the waveform. In the frequency domain, Amplitude
Modulation produces a signal with power concentrated at the carrier frequency
and two adjacent sidebands. Each sideband being equal in bandwidth to that of
the modulating signal.
All AM techniques have one disadvantage in that, the receiver amplifies
and detects noise and electromagnetic interference in equal proportion to the
signal. AM is not suitable for music and high fidelity broadcasting, but rather for
voice communications and broadcasts (e.g. news) etc.
AM is inefficient in power usage, in that at least two thirds of the power is
concentrated in the carrier signal. The carrier does not contain the original
information being transmitted. The figure 3.2 below shows the waveform of AM
signal
+ =
Figure 3.2: Amplitude Modulation
International Telecommunication Union (ITU) Designation
ITU designated the types of amplitude modulation in 1982 Designation Description
A3E Double-side band, a full carrier – the basic amplitude modulation
scheme
R3E Single-side band and reduced carrier
H3E Single-side band full carrier
J3E Single – sideband suppressed carrier
B8E Independent – sideband emission
C3F Vestigial sideband
Linco pex Linked compressor and expander
24
Sideband
John Renshaw Carson on 1st December, 1915 did the first mathematical
analysis of AM, showing that a signal and carrier frequency combined in a non
linear device would create two side bands on either side of the carrier frequency
and passing the modulated signal via another non linear device would extract the
original baseband signal. Sideband comprises of two: Single Side Band (SSB) and
Double Side Band (DSB).
Bandwidth
Bandwidth is the range of frequencies that a communication channel can
accommodate known as (bandwidth of a channel) and range of frequency a signal
accommodates (bandwidth of a signal)
Bandwidth of a Signal
Bandwidth of a signal can be defined as the range of frequency a signal
accommodates. In Amplitude Modulation, carrier frequency is denoted by Fc and
modulated frequency is denoted by Fm, in which two side frequencies is
produced (upper-side frequency and lower side frequency). Upper side frequency
being (Fc + Fm) and lower side frequency being (Fc – Fm) which is shown below:
Figure 3.3: AM bandwidth
Examples on bandwidth
(1) A standard A.M. broadcasting station is allowed to transmit a modulating
frequency up to 6KHz, if the A.M. station is transmitting on a frequency of
Upper-side
frequency
Lower-side
frequency
Carrier
Fc - Fm Fc Fc + Fm
25
1.2MHz and compute the upper and lower sideband in KHz and the total
bandwidth in kHz
Solution
Upper side band = Fc + Fm
Lower side band = Fc – Fm
Fc =1.2MHz = 1200kHz
Fm = 6kHz
Upper side band = Fc + Fm = 1200kHz + 6kHz = 1206kHz
Lower side band = Fc – Fm = 1200 – 6kHz = 1194kHz
Total band width = (Fc + Fm) – (Fc – Fm) = Fc + Fm- Fc + Fm=2Fm
= 2(6kHz) = 12kHz
(2) If the carrier frequency Fc of a signal is 2MHz and the highest modulating
frequency is 2kHz, compute the upper side band and lower side band in
MHz and total band width in MHz
Solution:
Fc = 2MHz
Fm = 2KHz = 0.002MHz
Upper side band = Fc + Fm = (2 + 0.002) MHz = 2.002MHz
Lower side band = Fc – Fm = (2 - 0.002) MHz = 1.998MHz
Total band width = 2fm
= (0.002)MHz = 0.004MHz
Upper-side
band
Lower-side
band
Carrier
FC - FM FC FC + FM
Bandwidt
hh
26
(3) What is the bandwidth, in percent in the diagram shown below?
Upper side band = Fc + 6.5kHz
= 8kHz + 6.5kHz = 14.5kHz
Lower side band = Fc – Fm
= 8kHz - 6.5kHz = 1.5kHz
Fc = 8kHz
=162.5%
Upper-side
frequency
Lower-side
frequency
Carrier
FC - FM FC FC + FM
FC – 6.5KHZ FC
(8KHZ)
FC + 6.5KHZ
27
Modulation Index of Amplitude Modulation
Modulation index can be defined as the mathematical and practical
measure based on the ratio of the modulation excursions of the RF signal to the
level of the unmodulated carrier. It can be represented mathematically as;
Where M and A are the Modulation Amplitude and Carrier Amplitude
respectively.
Modulation Amplitude is the peak (position or negative) change in the RF
amplitude from its unmodulated value. Modulation Index is normally expressed
as a percentage and may be displayed on a meter connected to an AM
transmitter.
Examples
1. For an unmodulated carrier of 2000V and a modulated peak value of
2700V. What is the modulation index?
Solution
= 0.35
2. Modulated peak value of signals is 20v and the unmodulated carried value
is 10v. What is the modulated index h?
3. Modulation index of a signal is 0.6 and the unmodulated carrier voltage is
10v, calculate the value of modulated peak voltage.
Solution:
28
0.6 x 10 = Mv – 10
6 = Mv – 10
Mv = 6 + 10 = 16v (modulated peak voltage)
4. A modulated signal seen on an oscilloscope has a maximum span of 6v and
a minimum of 2v. What is the modulation index?
5. A signal has a minimum span of 12v and a maximum span of 14v. What is
the modulation index?
0.077
6. Modulation index of signals is 100% and the minimum span of the signals
OV, calculate the maximum span?
100 Vmax = Vmax
100% = 1v (Maximum span)
29
CHAPTER FOUR
FREQUENCY MODULATION
Introduction
The modern Frequency Modulation was invented by Edwin Howard
Armstrong. Frequency Modulation is the encoding of information in a carrier
wave by varying the instantaneous frequency of the wave.
The difference between the instantaneous and the base frequency of the
carrier is directly proportional to the instantaneous value of the input signal
amplitude in analog signal applications.
Frequency Modulation is otherwise known as frequency shift keying and is
widely used in modems and fax modems, and can also be used to send Morse
code. Frequency shift keying is also used in radio teletype.
FM is used in radio, telemetry, radar, seismic prospecting and monitoring
newborns for seizures via EEG. FM is widely used for broadcasting music and
speech, magnetic tape recording systems and some video transmission systems.
The information to be transmitted in FM can be represented as xm(t) and
the sinusoidal carrier represented as Xc(t) = Ac Cos(2FC t). Where FC is the
carrier base frequency, Ac is the carrier’s amplitude. Modulator can then combine
the carrier wave within the baseband data signal to get the transmitted signal as:
F(r) is the instantaneous frequency of the oscillator and F is the frequency
deviation which represents the maximum shift away from fc in one direction,
assuming x(m)t is limited to the range
30
Information signal carrier modulated carrier
Figure 4.1: Frequency modulation
Waveform of Frequency Modulation (Frequency Shift Keying)
MODULATION INDEX FOR FM
Modulation index for FM can be represented as’
fm is the highest frequency component present in the modulation signal xm(t)
and is the peak frequency deviation (i.e. maximum deviation of the
instantaneous frequency from the carrier frequency. FM can be classified as
narrowband if the change in the carrier frequency is about the same as the signal
frequency (i.e. h<<1) or wideband if the change in the carrier frequency is much
higher than the signal frequency (i.e. h>1). Modulation index of FM is therefore
the ratio of the maximum frequency deviation frequency to the frequency of the
modulation.
Phase Modulation
Phase Modulation is a method used to digitally represent sampled analog
signals. Pulse Code Modulation (PCM) is the standard form of digital audio in
computers, compact disks, digital telephony and other digital audio applications.
31
In the PCM streams, the amplitude of the analog signal is sampled regularly
at uniform intervals, and each sample is quantized to the nearest value within the
range of digital steps. The first transmission of speech by digital technique was
the signal encryption equipment used for high level allied communications during
World War II in 1943 bell labs.
Carson’s Rule
Carson’s rule otherwise known as Carson’s bandwidth rule (rule of thumb)
states that “nearly all (approximately 98%) of the power of a frequency
modulated signals lies within a bandwidth. Bandwidth BT = 2( , where,
is the peak deviation of the instantaneous frequency f(t) from the center carrier
frequency, fc.
EXAMPLES
1. A system has 200kHz of bandwidth available for a 5kHz modulating signal.
What is the approximate deviation to be used?
Solution
Re-arranging the equation, we have;
975
2. A system uses a deviation of 150kHz and a modulating frequency of 12KHz.
What is the approximate bandwidth needed?
Solution
By Carson’s rule,
BT = 2(
BT = 2(
= 324
32
Types of Signal
A device that generates carrier signal is known as signal generator. There
are 3 types of signal, namely
(a) See – saw signal
(b) Pulse signal
(c) Sinusoidal signal
Figure 4.2: Signal types
Carrier Waveforms
These are five main carrier waveforms, namely:
(a) Sine wave
(b) Square wave
(c) The ramp
(d) Saw tooth
(e) Triangle wave
33
34
CHAPTER FIVE
DEMODULATION
Introduction
Demodulation otherwise known as detection was first used in radio
receivers. In the year 1884 to 1914, transmitter of wireless telegraphy radio
systems did not communicate sound but transfer information in the form of
pulses of radio waves that represented text messages in Morse code, in order for
the receiver to effectively detect the presence or absence of the radio signal and
produce a short sharp sound, done by detectors.
The first demodulators were coherers, simple device that performed as a
switch. The first Amplifier Modulation demodulator was invented by Fessenden in
1904, of which it consists of a small needle inserted into a cup of dilute acid.
In 1904 also, John Ambrose Fleming invented the Fleming vale or
thermionic diode, which could extract an AM signal. Demodulator is an electronic
circuit used to extract information signal from the modulated carrier wave before
being applied to the output Transducer.
Actions Performed by Demodulator/Detector
There are several actions carried out by a demodulator (detector). Some of
which are:
(a) Carrier recovery
(b) Bit slip
(c) Frame synchronization
(d) Clock recovery
(e) Error detection
(f) Correction
(g) Pulse compression
(h) Rake receiver
(i) Error correction
(j) Receive signal strength indication
35
Types of Demodulator Used For Linear Modulation or Angular Modulation
For a signal modulated with a linear modulation, like Amplitude
Modulation, coherent or synchronous detector (demodulator) can be used. For a
signal modulator with an angular modulation, Amplifier Modulation demodulator
cannot be used, but instead, Frequency Modulation (FM) detector (demodulator)
or Phase Modulation (PM) detectors (demodulator) used.
AM Detectors
There are two methods used to demodulate Amplitude Modulation signals
– Envelope Detector and Product Detector.
Envelope Detector is simple and widely used for demodulation on
Amplitude Modulation/Double side band or VSB signals. Synchronous Detectors
are complex in nature and used for demodulation of double side band or single
side band signals.
Envelope Detector
Envelope Detector is a simple method of AM demodulation. It consists of a
rectifier which may be formed of a single diode or a more complex circuitry,
which passes current in one direction only and non-linear that boost one half of
the received signal over the other one and a low pass filter.
Basic circuit of an Envelope Detector consists of a series of connected diode
to a parallel connection of resistor and capacitor, in which the voltage produces
two half cycle (positive half cycle and negative half cycle). During the positive half-
cycle, output voltage is proportional to the input signal voltage while during the
negative half cycle, output voltage maintained by the capacitor discharged
because there is no conduction in the diode.
Two variables, extent of charge and discharge of the capacitor in the circuit
depends on the time constant CR. During the non-conduction period of the diode,
modulation envelope may not be followed because of small time constant CR
which results in considerable fall of the output voltage which will result to the
output being low and to contain high RF ripple.
36
D
V R C O/P
Figure 5.1: Basic circuit of envelope detector
Product Detector
Product detector will diode both AM and SSB by multiplying the incoming
signal of a local oscillator with the same frequency and phase as the carrier of the
incoming signal thereby resulting in original audio signal after filtering of the
unwanted signal. However, if the phase of the carrier cannot be determined, a
more complex setup is required. It is important to note that an AM signal can be
rectified without requiring a coherent demodulator. Signal output from a
demodulator may represent sound (analog audio signal), images (analog video
signal) or binary data.
A fax demodulator is a device used to intercept fax messages by listening
on a telephone line or radio signal.
Frequency Detector (Demodulator)
There are four types of FM detectors: Quadrature Detector, Foster Seeley
Discriminator, Ratio Detector and Slope Detector. The most commonly used are
Foster Seeley Discriminator and Ratio Detector, of which Ratio Detector being the
most appropriate one to use because of its advantage over the foster Seeley
discriminator.
Quadrature Detector
Quadrature detector phase shifts the signal by 90 degree (900) and
multiplies the signal with the un-shifted type, of which the original information
signal is selected and amplified. The signal is fed into a PLL and the error signal is
used as the demodulator signal.
Foster Seeley Discriminator
Foster Seeley discriminator consists of an electronic filter which decreases
the amplitude of some frequencies associated with others, next by an AM
demodulator. The final analog output will be proportional to the input frequency
v
37
as desired if the filter response changes linearly with frequency. Foster Seeley
discriminator lack the ability to be sensitive to amplitude changes in the incoming
FM input.
N. B.: Another method of FM detection uses two AM demodulators, one attached
to the high end of the band and the other attached to the low end of the band,
and the outputs fed into a difference amp.
Ratio Detector
Ratio detector is another variant of the Foster Seeley discriminator. Ratio detector
consist of two diodes which conduct equally to produce equal potentials at two
point A and B which gives zero output. An output voltage is produced if the
current through the two diodes are unequal as a result of input frequency
deviating from the centre frequency.
38
CHAPTER SIX
RADIO AND BLACK/WHITE TRANSMISSION
Amplitude Modulated Radio Transmitter
AM Broadcasting
AM broadcasting is the process of radio broadcasting using amplitude
modulation (AM). AM is widely used today and it was the first method of
amplifying sound of a radio signal. Radio broadcasting was made possible by the
invention of amplifying vacuum tube, the Audion (triode) by Lee de Forest in
1906, which led to the development of inexpensive vacuum tube. AM radio
receivers and transmitters were used during World War II. The period was
referred to “Golden Age of Radio” in which AM broadcast specialized in news,
sports and talk radio.
In Amplitude Modulation two signals enter a modulator: HF signal called
the carrier(or the signal carrier) generated by the High Frequency (HF) oscillator
and amplified in the high frequency amplifier to the required signal level, are the
low frequency (LF) modulating signals coming from the microphone or some
other signal source being amplified in the low frequency amplifier. On
modulator’s output, the amplitude modulated signal is acquired. The signal is
then amplified in the power amplifier and then led to the emission antenna.
Information being transferred is the sound and the first step is converting
sound into electrical signal. This is being accomplished by a Microphone Electrical
‘’image’’ of the sound being transferred which represent low frequency (LF)
voltage at microphone output that is being taken into the transmitter. Amplitude
modulation is being carried out under the effect of low frequency signal and high
frequency (HF) voltage is generated at the output, thus, its amplitude changing
according to the current LF (low frequency) frequency voltage, thereby generating
electromagnetic field around it, the electromagnetic field spreads through the
ambient space which can be symbolically represented by small circles on diagram.
The electromagnetic field travelled to the reception place with speed of light “C”
39
= 3 x 108m/s, thereby inducing the voltage in the reception antenna. Amplification
and detection are then performed in receiver, resulting with the Low Frequency
voltage on its output. The output voltage is then change back to sound using
loudspeaker (output device), the output sound being exactly the same as the
sound transmitted into the microphone.
Figure 6.1: Block diagram of amplitude modulation transmitter using high level
modulation
Frequency Modulation Transmitter
In FM transmitter, information being transmitted is low frequency signal
which is then amplified in the low frequency amplifier and transmitted (sent) into
the high frequency oscillator. The carrier signal being created in the HF oscillator
is a high frequency voltage of constant amplitude, whose frequency is in the
absence of modulating signal, equal to the transmitter’s carrier frequency FS. A
capacitance diode (varicap) is situated in the oscillatory circuit of the high
frequency oscillator. Capacitance diode is a diode whose capacitance depends
solely on the potential difference between its ends, and when exposed to low
frequency voltage, its capacitance changes according to the LF voltage. The
frequency modulation can be derived due to frequency of the oscillator as output
power of the transmission signals are produced as a result of FM signal from the
HF oscillator being sent to the power amplifier. If low frequency signal is
increasing, the current value will also increase and when the low frequency signal
Buffer
amplifier
Driver
amplifier
Final
power
amplifier
Speech
processin
g unit
Driver
amplifier
Modulatin
g amplifier
Audio
amplifier
Antenna
Crystal
Microphone
To generate
frequency (carrier
oscillator)
40
is decreasing, the current value also decreases. The LF signal (information) is
equal to the frequency change of the carrier.
The carrier frequency of the radio diffusion frequency modulation
transmitters are placed in the waveband ranging from 88MHz to 108MHz, the
highest frequency shift of the transmitter (during the modulation) being ± 75KHz
will result to the FM signal be drawn much thicker and in a black square shaped
picture.
Figure 6.2: Block diagram showing a typical FM transmitter using indirect FM with
a phase modulator
Straight Radio Receiver
The different parts of a straight or Tuned Radio Frequency (TRF) receiver
are aerial, radio frequency amplifier, modulated Radio Frequency (RF) carrier,
detector (demodulator) AF amplifier, AF power amplifier loudspeaker. Aerial
selects the wanted signal from varieties of signal and the selected signal is being
transmitted and amplified by the radio frequency (R.F) amplifier. Radio Frequency
(RF) amplifier is a voltage amplifier with a tuned circuit as its load, the RF amplifier
Crystal
(Carrier oscillator)
Buffer Phase
modulation
Frequenc
y
multiplier
Microphone
Speech
processin
g unit
Audio
amplifier
Antenna
Driver
amplifier
Final
power
amplifier
41
should have a bandwidth capable of accepting the side band frequencies (i.e.
9kHz) Demodulator separates Radio Frequency (RF) from the AF (Audio
Frequency) and later amplify the Audio Frequency in the Audio Frequency (AF)
power amplifier in order to drive the loudspeaker. The maximum audio frequency
(AF) allowed for the bandwidth of the AF amplifier should not be above 4.5kHz.
Figure 6.3: Block diagram of straight radio receiver
Super-Heterodyne Radio Receiver
Super-heterodyne radio receiver (often abbreviated as superhet) was
invented by US Engineer Major Edwin Armstrong in 1918 in France during World
War 1. Edwin Armstrong invented the superhet in order to overcome the
deficiencies of early vacuum tube triodes used as high frequency amplifiers in
radio direction finding equipment. Direction finders measure the received signal
strength which provides line amplification of the actual carrier wave, while simple
radio communication only needs to make transmitted signals audible.
Super-heterodyne is a contraction of “supersonic heterodyne”, where
“supersonic means frequencies above the range of human hearing. Heterodyne
was derived from the Greek roots hetero – “different” and dyne – “power” for
radio uses. Canadian inventor, Reginald Fessenden, pioneered the term derived
from the “heterodyne detector” in 1905. The proposed his method of producing
Modulated RF
carrier
Loudspeake
r
AF power
amplifier
R F
amplifier
Detector or
demodulato
r
AF
amplifier
Aerial
Amplitude modulated
RF carrier
Amplifier
a. f. a. f.
42
an audible signal from the Morse code transmission of the new continuous wave
transmitters, in which older spark gap transmitters were used. The Morse code
signal consisted of short chirps in the receiver’s headphones, however,
Fessenden’s thought was to run two Alexanderson alternators, one producing a
carrier frequency higher than the other. In the receiver’s detector, the two
carriers would come together to produce a 3kHz tone, hence, in the headphone
beeps. Through this, he coined the term “heterodyne” meaning “generated by a
difference” (in frequency).
Major E. H. Armstrong gave publicity to an indirect method of obtaining
short-wave amplification, called the super heterodyne in December 1919. The
purpose is to decrease the incoming frequency (maybe 1.5 x 106 cycles (200
meters) to suitable audible frequency that can be amplified efficiently and passed
through a radio frequency amplifier, rectifying and proceeding on to one or two
stages of audio frequency amplification.
Armstrong was able to put his innovation into practice, and the technique
was soon adopted by the Military. However, due to super het receiver high cost,
level of technical skill required to operate it, and need for an extra tube (for the
oscillator), it became less popular when commercial radio broadcasting began in
the 1920s. Neutrodyne (tuned radio frequency receivers) were more popular for
early radio because they were cheaper, did not require technical skill person to
own or operate it, and were not costly. Reasons mentioned above led Armstrong
to eventually sold his super heterodyne patent to Westing house, who then sold it
to RCA. RCA monopolized the market for super heterodyne receivers until 1930.
The first commercial super heterodyne receiver, the RCA Radiole AR – 812,
used six diodes, a mixer, local oscillator, 2 if and two audio amplifier stages with
an if of 45KHz. It was a commercial success, with better performance than
competing receivers. In an apparent attempt to prevent competitors from
“reverse engineering it”, the innards were enclosed in solid wax. However,
modern receivers typically use a mixture of ceramic resonator or surface acoustic
wave (saw) resonator as well as traditional tuned inductor LF transformers.
43
Principle of Operation
Principle of operation of super het receiver depends on the use of
“heterodyning or frequency mixing” to convert a received signal to a fixed
intermediate frequency (IF) which can be more conveniently processed than the
original radio carrier frequency. Virtually, all often extra frequency converter
stage, the super heterodyne receiver provides superior selectivity and sensibility
compared with simple designs.
Basic elements common to all super heterodyne radio receivers are: a
receiving antenna, A variable frequency local oscillator, a frequency mixer, a band
pass filter and intermediate frequency (IF) amplifier, a demodulator, and
additional circuit to amplify the actual signal or other transmitted information.
Signal from the antenna is filtered at least to reject the image frequency
and possibly amplified it. Local oscillator in the receiver produces a ‘’sine wave’’,
which mixes with the signal producing a lower intermediate. Intermediate
frequency signal will be filtered, amplified and processed in advanced ways. The
demodulator creates a copy of the original information (such as audio) using the
intermediate frequency signal rather than the original radio frequency. The mixer
produces sum and difference between frequencies signal using non-linear desires
signal. The output of the mixer may contain the original RF signal at FRF, the local
oscillator signal at FLO, and two new heterodyne frequency FRF + FLO (frequency
of the radio frequency + frequency of the local oscillator) and FRF – FLO. The
intermediate frequency bandpass filters the desired if signal at IF and removes all
other signals. The desired IF signal contains the original modulation (transmitted
information) that the received radio signal had at RF.
44
-
Figure 6.4: Block Diagram of Super-Heterodyne Radio Receiver
Superhet receivers have basically replaced all previous receiver designs.
The development of modern semiconductor electronics contributed negatively to
the advantage of designs (such as the regenerative receiver) that used fewer
vacuum tubes. Compared to the Tuned Radio Frequency Receiver (TRF) design,
super het receiver offers superior sensitivity frequency stability and selectivity
because a tunable oscillator is more achieved than a tunable amplifier. Also, ‘IF’
filter can give narrow pass band at the same Q factor than equivalent ‘IF’ filter
because it is operating at a lower frequency. Crystal filter can also be used with
the half of a fixed IF. Regenerative and super regenerative receiver offer a high
sensitivity, but often having stability problem which make them difficult to
operate.
Super heterodyne receiver used in early decades, used IF as low as 20KHz,
often based on the self-resonance of iron – cored transformer which made them
extremely susceptible to image frequency interference. Then, the main objective
of using IF was to achieve better sensitivity rather than selectivity. Small number
of triodes could be made to do the work that previously required dozens of
triodes could do using the technique. However, in the mid 1930s, Superhet were
using much higher intermediate frequencies (440 – 470KHz) with tuned coils
Antenna
RF
amplifie
r
I.F
amplifier
Demodulator
Modulated
RF carrier Mixer Filter
Modulated IF (470 kHz) F0 - Fc
Local
oscillato
r
Audio
amplifier
AF power
amplifier
Loudspeaker
45
similar in construction to the aerial and oscillator coils. Moreover, the name
“Transformer” was retained and is still used today.
Heterodyning
Heterodyning is a radio signal processing technique invented by Canadian
investor (Engineer Reginald Fessenden) in 1901, in which new frequencies were
created combining two frequencies. Heterodyning can be used for frequency
shifting signals into a new frequency range and is also involved in the process of
modulation and demodulation.
Disadvantage of super-het
One major setback (disadvantage) for super-het is the problem of image
frequency. In heterodyne receivers, an image frequency is an undesired input
frequency equal to the station frequency with twice the intermediate frequency.
The image frequency result in two stations is received at the same time, hence,
producing interference. Image frequencies can be eliminated by sufficient
attenuation signal by the RF amplifier filter of the super-het receiver.
Two ways by which sensitivity to the image frequency can be reduced are:
(i) A filter that precedes the mixer
(ii) A more complex mixer circuit that suppress the image.
In most receivers, this can be achieved by a band pass filter in the RF front
end. In many tunable receivers, the band pass filter is tuned in the antenna with
the local oscillator. Image rejection is the direct measured of the ability of a
receiver to reject interfering signals at the image frequency. Image rejection is the
ratio (in decibel) of the output of the receiver from a signal at the received
frequency to its output for an equal strength signal at the image frequency.
Frequency Modulated Radio Receiver
Frequency modulated radio receiver is made up of an oscillator that is
synchronized with the received frequency which works a broadband pre-amplifier
in VHF range. Frequency modulation (FM) is used in VHF (very high frequency
46
radio). It also works based on the super-het principle. Essential part of a
frequency modulated radio receiver are RF amplifier, antenna (aerial),
demodulator, mixer, local oscillator, AF amplifier and loudspeaker.
The first stage of FM radio receiver is RF amplifier which has a fairly wide
bandwidth and helps maintaining proper amplification of the V.H.F carrier. Carrier
frequency will be changed to IF of 10.7MHz by mixer and local oscillator.
Oscillator output will have the same frequency deviation as the received signal
from the FM antenna due to the synchronization. FM tuner comprises of RF
amplifier, mixer and local oscillator. FM radio receiver uses a detector called
“ratio detector” which reduce any changes in amplitude of the carrier due to
unwanted noise being picked up by the antenna and produces a signal that
changes in frequency only. Ratio detectors also contain tuned circuits which
produce an AF voltage that is proportional to the deviation.
Figure 6.5: Block diagram of frequency modulated radio receiver
Television Basics
Television is the act of transmitting and receiving quality broadcast signals
of pictures and sound by very high frequency (VHF) radio waves. Television signal
basically comprises of two modulated carrier wave (video and sound signal).
During transmission, transmitting antenna radiates a single carrier wave
modulated with both visual and audio signals. At the receiving end, receiving
antenna picks up the transmitted wave and amplifies it before demodulating the
Mixer
Aerial
RF
amplifie
r
Mixer AF
amplifier
Demodulator
Radio
amplifier Local
oscillato
r
Loudspeaker Earth
47
audio and components. The two components are separated at the receiving
antenna. The demodulated audio signal will then be passed to an AF amplifier and
later passed to the loudspeaker. The loudspeaker will then re-convert the
electrical signal back to the original sound (audio) signal being sent.
Video amplifier will amplify the demodulated video signal and sent it to
cathode ray tube (CRT) in the TV receiver to reproduce an image of the original
signal.
Video camera Amplification section Antenna
Video signal
Microphone phone sound signal
Figure 6.6: Block diagram of simplified television broadcast system (TV transmitter)
Crystal
master
oscillator
Frequenc
y
multiplier
x2x2x3x3
Classic
amplifier
Power
amplifier
Modulate
d
amplifier
Vestigial
sideband
duplexer
Pre
amplifier
Video
processin
g unit
Video
amplifier
Video
modulate
d
amplifier
Video section
amplifier
Crystal
master
oscillator
r
Automati
c
frequency
control
Pre
amplifier
FM
Modulate
d
amplifier
Power
amplifier
Frequency
multiplier
x2x2x3x3
Power
amplifier
Monitorin
g unit
48
Monochrome TV Receiver
In a black and white TV receiver, number of electrons moving from the
electron gun of a cathode ray tube to its screen is controlled by the incoming
video signal. Early stages of monochrome receiver are similar to those in super-
het radio but frequencies and bandwidth are higher (e.g. the IF is 39.5MHz). Video
and audio signals will later be demodulated and separated from each other and
from the line and sync pulse. Division occurs at the output of the video detector.
At the output, the demodulated AM video signal is amplified by the video
amplifier and applied to the modulator (grid) of the cathode ray tube (CRT) to
control the electron beam. The modulated FM sound signal, being heterodized in
the video detector with the video signal to produce a sound. IF of 6MHz (i.e. the
frequency distinct between the sound and video carriers) is fed into the sound
channel, after amplification and FM detection, it drives the loudspeaker.
The mixed sync pulses are processed in the time base channel by the sync
separator, which produces two sets of different pulses at its two outputs. One set
is derived from the line pulses and triggers the line oscillator. The other set is
obtained from the field pulse and synchronizes the field oscillator. The oscillator
produce the deflecting saw tooth waveforms for the scan coils. The line oscillator
also generates the extra high voltage or tension (e.h.t) of about 15KV required by
the final anode of the CRT (cathode ray rube).
49
Picture tube
Figure 6.7: Diagram showing black and white (monochrome) TV Receiver
Figure 6.8: Diagram showing difference between Sound and video
Sync
separati
on
Field time
base Line time
base
Turner
Antenna
Vision
If amp
Vision
detecto
r
Video
amp
Automatic
frequency
control
(AFC)
6MHz
selector
Sound If
amplifier
FM
detector
AF
amplifier L/S
Adjacent channel
FM sound carrier AM Video carrier
6MHz
Channel
MHz
50
Automatic Gain Control (AGC)
Figure 6.9: Schematic diagram of an AGC used in the analog telephone network, the feedback
from output level to gain is affected via a vactrol resistive opto-isolator
In 1925, Harold Alden Wheeler invented Automatic Volume Control (AVC)
and obtained a patent. In 1928, Kar Kupfmuller published an analysis of AGC
system.
AGC is a closed – loop regulating circuit, the purpose of which is to provide
a controlled signal amplitude at its output, despite variation of the amplitude in
the input signal. AGC is employed for proper functioning of the receiving system
in order to reduce non-constant received signal level, changes in the atmospheric
conditions, variation in movement of the transmitter etc.
The average, peak output signal level is used to dynamically adjust the
input signal levels. Without AGC, the sound emitted from an AM radio receiver
would vary to an extent from a weak to a strong signal, the AGC effectively
reduces the volume if the signal is strong and raises it when the signal is weak.
The attack and delay time of the AGC circuit is designed so that gain changing cuts
at the proper rate and does not release too quickly either.
Working Principle of AGC
The signal gain controlled (the detector output in radio) goes to a diode and
capacitor which produce a peak of DC voltage. This is fed to the RF gain blocks to
alter their bias, thus altering their gain. Traditionally, all the gain controlled stages
came before the signal detection, but it is also possible to improve gain control by
Vin C1
22n
33k R4 2n
R3
100
k
C2
+
-
51
adding a gain controlled stage after signal detection. An example of a
AGC/compressor for microphone amplification is VOGAD (Voice Operated Gain-
Adjusting Device Or Volume Operated Gain-Adjusting Device) usually used in
radio transmitted to prevent over modulation and to reduce the dynamic range of
the signal which allows increasing average transmitter power.
52
CHAPTER SEVEN
TELEPHONE AND TELEGRAPH
Introduction
Telegraphy (from Greek word tele “at a distance” and graphein “to write’’)
is the long distance transmission of textual/ symbolic (as opposed to verbal or
audio) messages without the physical exchange of an object bearing the message.
Hence, semaphore is a method of telegraphy whereas Pigeon Post is not.
Telegraphy requires that the method used for encoding the message be known to
both sender and receiver.
The word, “Telegraph” was made popular by the French inventor of the
semaphore line, “Claude Chappe” who also coined the word “semaphore”. A
telegraph is a device for transmitting and receiving message over long distance,
i.e. for telegraphy. The word “telegraph” alone now generally refers to an
electrical telegraph.
In 1832, when “pavel schilling” invented one of the earliest electrical
telegraphs. The first telegraphs came in the form of optical telegraph including
the use of smoke signals, beacons or reflected light, which have existed since
ancient times. Early proposals for an optical telegraph system were made to the
royal society by “Robert Hooke in 1684” and were first implemented on an
experimental level by ‘’Sir Richard Lovell Edge worth in 1767’’.
Carl Friedrich Gauss and Wilhelm Weber built the first electromagnetic
telegraph used for regular communication in 1833 in Gottingen, connecting
Gottingen observatory and the Institute of Physics, covering a distance of about
1km. The first commercial electrical telegraph was co-developed by “Sir William
Fothergill Cooke and Charles Wheatstone. In May 1837, they patented the
system.
In 1843, Scottish inventor-Alexander Bain, invented a device that could be
considered the first facsimile machine. He called it “recording telegraphy” and
was able to transmit images by electrical wire.
53
Telegraph can also be defined as the passing of messages by means of a
signaling code. The most commonly used signaling codes are two; Morse code
and Murray code.
The Morse code
Using Morse code, characters of a telegraph signals are represented by
using a combination of DOT signals and DASH signals, the DASH signals having a
period three times that of the DOT signals. Different time durations are employed
to differentiate, spacing between elementary signals of letter and words.
One major disadvantage of the Morse code is that, Morse code is not
convenient for use with automatic printing receiving equipment because the
number of signal elements needed to indicate a character is not the same for all
elements and there are signal elements of different lengths.
Figure 7.1: Morse code
The Murray code
In Murray code, each character is represented by a combination of five
signal element that may be either mark or space and all characters have exactly
the same number of signal elements and the signal elements are of constant
length.
A space is represented by a positive potential in the absence of a tone and
a mark is represented by a negative potential or the presence of a tone.
Letter spacing
(duration = 1 dash)
Elementary signal
space (duration = 1
dot)
Time
54
Murray code is used for all teleprinter systems and its speed is measure in
terms of a unit known as the BAUD. Reciprocal of time duration of the shortest
signal element employed is the band speed of a telegraph signal. Hence, the
bandwidth required for the transmission of signal in MORSE CODE depends upon
the number of words, which generally lies within the range of 100 – 1000Hz.
Bandwidth required to transmit a teleprinter signal depends upon the
character sent, but the maximum bandwidth is required when alternate marks
and spaces are transmitted. That is, letter R and Y. Teleprinters are normally
operated at a telegraph speed of 50 bauds and this means that the time duration
for a mark or space is 1/50 second or 20ms.
M
Y
s
Figure 7.2: Diagram showing period waveforms for R and Y
Telephone
Speaker’s voice in form of mechanical energy is converted into electrical
energy (signals) directly in the hand held phone set, which therefore acts as the
analogue source for the link in a telephone system. The voice signal is then
transmitted through wire/cable to a central telephone exchange where it is
modulated and relayed through a complicated switching procedure in the central
exchange to the intending listener; exchange is determined by the dialing code
set by the speaker, which acts as synchronization information interconnecting the
speaker and the listener. The modulated carrier may simply be cable connected to
it if the listener’s exchange is in a nearby locality.
Electromagnetic space propagation may be used if the listener’s exchange
is at a distance location. At the listener’s exchange, the voice signal is
demodulated and passed through telephone lines to the listeners’ phone. For
long distance, a satellite relay to the listener’s phone may be used. The voice
Periodic time of 1 cycle of the
waveforms for R and Y is 40ms,
and so the fundamental
frequency of the waveform is
1000/40 or 25Hz.
55
modulated carrier will be transmitted to a satellite where it is received, amplified
and retransmitted to a receiver exchange at a distance location on the ground.
Figure 7.3: Block diagram showing mode of telephone exchange
Simple Telephone Circuits
Speech Circuits
a. Basic circuit: Basic circuit of a simple telephone speech system consists of a
microphone connected by wires to an earpiece and a battery of a few volts
which drives a current d.c. round the circuits. Resistance of a microphone
varies in response to the sound and causes corresponding changes in the
current when spoken into (i.e. the mouthpiece), the changes later
reproduce the sound in the earpiece.
Total resistance of the circuit will be high and it will only change
slightly when the microphone resistance varies if the line is long. The
current changes are therefore small and the sound in the earpiece slightly
fades. (Very faint)
Figure 7.4: Diagram of simple telephone basic circuit
Line Earpiece Microphon
e
Supply
d.c.
Telephone
line
Central exchange Receiver
exchange
Multiplex Modulated Demodulate
d
Demultiplexer
56
b. Two way circuit: Speech between two telephones several miles apart is
possible if the battery current does not flow in the line but only in the
microphone and the primary side of the transformer. Varying d. c. will
emanate if the resistance of the microphone in the low resistance local
circuit varies.
A.C current will be induced in the secondary winding of the transmitter
which will flow in the line and through the earpiece at the distance end, In
which communication can occur in both end (directions). The microphone
and earpiece at each end is combined in a single unit, the handset.
Nowadays, most telephones are connected to an exchange with a central battery
or higher voltage of 60v which replaces the local battery at each telephone. The
system requires some arrangement of the circuit and contains a capacitor and a
specially designed transformer.
Figure 7.5: Diagram showing two way circuit of telephone
Telephone Keypad
Electronic equivalent of the dial on a telephone press button is the keypad.
When a caller press a number, it is an integrated circuit (IC) in code or as tones of
Long line
Earpiec
e
Transformer Handset Handset
Micro
ph
on
e
57
different frequencies called multi frequency signaling depending on the type of
numbers of loop-disconnect pulses to be sent to the exchange.
Signaling in an Automatic System
Basically, signaling in an automatic system is achieved by a mechanical dial. A
circular finger-plate rotate with 10 finger holes is fixed to another plate with ten
projections. When the finger plate is turned in clockwise direction, the projections
will pass the pivoted lever A without disturbing the normally close contract B. But,
if the plate assembly returns in an anticlockwise direction to its rest position
under the influence of a spring, the contacts B will be forced openly by every
projection which passes the lever A.
Figure 7.6: Diagram showing mode of signaling in automatic system (dialer)
Pivoted layer A
B
Finger stop
58
Telephone Trunk System
A single trunk wire pair carries voice only in one direction; a trunk is almost
four line path, so that a complete talking circuit requires two pairs (4 wires). In
the central office, transition from two-wire to four-wire and from 4-wire to 2 wire
is made to connect each phone loop. If the caller wants to do another exchange,
the central office is responsible for connecting to a special link between
exchanges called a trunk. A trunk can be a physical wire, a fiber optic cable, a
radio link, or a satellite connection. Calls between adjacent towns usually involve
only a single trunk between two exchanges, but a longer-distance call can be
routed through several which will form a network of interconnected nodes.
For long distance paths, there will be a presence of super trunks that collate
many trunks lines at regional switching centers and combined them together,
often using multiplexing and then go directly to distance end.
telephone telephone
Figure 7.7: Diagram showing elements of local telephone network
Element of Local Telephone Network
a. Subscriber Loop: Subscriber loop is a circuit or line connecting the
customer’s telephone set to the local switch. It mostly consists of one
twisted pair of 26 - 24 – or 22 – gauge copper wire.
Tandem
switch
Local switch
Capital office Trunk
Subscriber loop
59
b. Central Office: Central office houses the local switches that serve the
telephone of its community. All subscriber loops in the office and
interoffice line (called trunks) connect the central offices local switch to
local switches in other central offices and to tandem switch.
c. Tandem Switch: Tandem switches are used together on concentrated long
distance traffic from many local switches so that telephones from a wide
area may share few trunks to distance destination.
Tandem switch can also provide alternate routes for calls between local
switches when all direct trunks between the local switches are in use. It
may also provide automatic billing functions and advanced services to
customers connected to older switching system.
d. Trunk Group: Trunk group is the combination of interoffice trunks, used to
carry the telephone traffic between two switch systems.
Trunk Group and Routing Of Telephone Calls
Trunk group helps in linking from one subscriber loop to another using inter office
trunks.
Figure 7.8: Trunk group
Telex
A device which transmits message typed by an operator at the sender’s end
on the typewriter – style keyboard of a teleprinter, translate letters, figures, signs
and punctuation marks into code is referred to as telex. Telex is useful mainly in
B C
A D
Final route
for A to D Inter office
trunk groups
Telephone
central office
Subscribers
loop
High usage
group
60
firms. In modern telex terminal, the visual display unit permits editing and printer
gives a permanent copy of message if it is required. The receiving teleprinter
decodes the message and types it automatically on to a sheet of paper.
Modern day replacement for the telex is facsimile and it allows a document
to be sent over the telephone system in form of electronic mail. When the
document is inserted in the sender’s facsimile machine, the “optical eye” scan
produces a string of electrical pulses which on reaching the receiver’s machine,
makes it print out the same document which is then available for discussion by
both parties if required in which an A4 page of text can be transmitted and
printed instantly.
61
CHAPTER EIGHT
RADIO FREQUENCY BANDS
Introduction
Radio waves can be defined according to the radio law as “electromagnetic
waves with a frequency of less than 3,000GHz (3THz). Radio waves are
electromagnetic waves. Radio wave has longer wavelength than infrared rays. In
free space (space in which there nothing to obstruct the process of radio waves),
their propagation velocity is the same as that of light at approximately 300,
000Km in 1 second. The distance from the earth to the moon is about 390,000km,
so from moon, a signal would arrive in about 1.3 seconds.
Radio frequency (RF) wave that have frequencies above 30 KHz are grouped
into bands, examples are as follows:
(a) Extremely Low Frequency – E. L. F.
(b) Very Low Frequency – V.L.F.
(c) Low Frequency – L. F.
(d) Medium Frequency – M. F.
(e) High Frequency – H. F.
(f) Very High frequency – V. H. F.
(g) Ultra High Frequency – U. H. F.
(h) Super High Frequency – S. H. F.
(i) Extremely High Frequency – E. H. F.
Some waves can be reverberated from the ionosphere, others are pass
through it. Low frequency waves (below 500KHz) can bend themselves following
earth’s curvature, while the high frequency waves are moving in streamlines, just
as light.
Radio waves can even be transmitted in a vacuum. In fact, radio waves are
transmitted from communication satellite. Radio waves are kinds of transverse
wave (waves that vibrate at right angles to their direction of propagation.
62
Speed and Wavelength of Radio Waves
Propagation speed of radio waves is the same as that of light,
approximately 300,000km, in free space so they would arrive in about 1.3
seconds. The speeds fall slightly when passing through a conductor such as
antenna or cable. The wavelength of radio waves can be calculated as follows,
provided the frequency of the radio wave and speed of the radio wave are given,
using the formula;
C = f
Where, C = speed of radio wave in vacuum 3 x 108 m/s
f = frequency of the radio wave
= wavelength in meter of the radio wave.
Hence;
According to properties of their outspread, radio waves can be classified
into several groups or ranges: Long, mid, short and ultra-short. Limits between
the wavebands are not precise, with the raise of their frequency; the waves are
gradually losing some features, while gaining some other features.
Table 8.1 shows ranges of radio waves, band, frequency range, their
wavelength and their uses.
Table 8.1: ranges of radio waves, band, frequency range, their wavelength
and their uses
S/N RANGES BAND FREQ. RANGE IN (HZ)
WAVELENGTH IN (M)
USE
1. Extremely Low Frequency (E.L.F.)
3– 30KHz 100,000-10km Principal application vessel/airplane beacon.
2. Long waves
Low Frequency
30 -300KHz
10Km – 1km Principal application/airplane
63
(L.F.) beacon, long wave radio and long distance communication.
3. Mid waves Medium Frequency (M.F.)
300 – 3000KHz
100m – 10m A.M. radio, marine radio, amateur radio.
4. Short waves Ultra short waves:
High frequency (H.F.)
3 – 30MHz
100m – 10m Shortwave broadcasting, marine/air radio, amateur radio.
5. Meter range
Very High Frequency (V.H.F.)
30 – 300MHz
10m – 1m TV, FM, fire radio, police radio, disaster radio network.
6. Decimeter range
Ultra High Frequency (U.H.F)
300 – 3000MHz
100cm – 1cm Low power radio, mobile phone, taxi radio, amateur radio, TV., wireless LAN
7. Centimeter range
Super High Frequency
3 – 30GHz
10cm – 1cm Satellite broadcasting, radio
8. Millimeter range
Extremely High Frequency (E.H.F)
30 – 300GHz
10mm – 1mm Satellite broadcasting, radio astronomy, radar.
N.B- Wave with wavelength smaller than 30cm is also called the
microwaves.
Wavelength is the distance that the wave passes moving at the speed of
light (3 x 108m/s), during this period, that is equal to its oscillating period (T),
knowing the wave frequency is f =1/T.
64
Frequency Characteristics of Radio Propagation
Line of sight waves – approximately above 1GHz electromagnetic waves
behave much like light propagating through a Glass, reasonably uniform
atmosphere. When originating from a point source, they propagate in all
directions and the area of the wave front spreads out spherically, the intensity
decreases roughly as the square of the distance from the source.
Surface waves – approximately below 500 KHz electromagnetic waves tend
to follow the earth curvature, guided between the earth and the ionized layers of
the upper atmosphere (i.e. the ionosphere).
Sky waves - In the 1920s, it was discovered that H.F. waves (3 – 30MHz) are
reflected by the ionosphere and, hence, they are also usable, for long range
communication. 500 KHz to 3Mz and 30MHz to 1GHz are transition bands where
the propagation characteristics are more complex.
Propagation Loss of Radio Waves
Radio wave varies proportionally at the square of the distance, and in
inverse proportion to the square of the wavelength of the radio waves in free
space.
Fading is a phenomenon experienced by radio waves emitted by the
transmitter arriving at the receiver by a variety of paths, and at the same time the
received field strength varies due to the effects of the different routes taken and
differences in distance. There are different types of fading depending on the
causes, but representative kind is ‘’multipath fading’’.
Multipath fading can be interpreted as the radio waves that reach the
receiver by various paths, and the aggregate radio waves received by the antenna
which may experience interference and may fluctuate widely. If the signals are
equal (in phase) 90O to each other, the field strength is high, but when they are
out of phase, it gets weak. The wave length of microwaves is relatively short so
the impact of multipath is especially keen.
Qualities of Radio Waves
Radio waves energy is connected in one direction, and they are said to have
strong directivity. The shorter their wavelength becomes, the more they take on
65
the qualities of light, and the more and greater their straightness becomes. The
higher the frequency, the more keen is the attenuation of the wave’s energy.
Generally, radio waves are considered to propagate in a straight line. Radio
waves can penetrate through glass and ceramics, but are reflected by metal and
concrete. Frequency higher than multiple GHz are scattered and absorbed by rain,
snow, fog and their power tends to attenuate.
Why Communication Errors Occur
Communication errors occurs due to noise which may cause changes in
images and sound being transmitted, and desired signal may not reach the
receiver correctly.
Other means by which communication errors occur are:
(a) Noise from the environment
(b) Noise from the natural world (e.g. cosmic noise, movement of the earth’s
crust etc.)
(c) Noise from the device itself (e.g. due to proximity to noise emanating
around the power supply, the CPU and other components).
(d) Interference from the radio equipment
(e) Causes due to the physical properties of radio waves (mountains, building,
walls, people etc.)
Effect of Radio Waves on Human Lives and Its Environment
Radios waves as an electromagnetic waves emit dangerous radiation
substance which may aggravate or cause harm to human lives and its environs.
Examples of effect of radio waves are:
(a) Direct impact of radio waves on the human body which may lead to cancer.
(b) Effect on medical equipment etc.
Aerials
Aerial/antenna is a piece of equipment made of wire or long straight pieces
of metal for receiving/sending television or radio signal (waves). In radio
communication system, audio signal is erected in a particular part of the
frequency spectrum using same form of modulation. The modulated wave is then
66
radiated into space, in form of an electromagnetic wave by a transmitting
aerial/antenna. In order for the transmitted signal to be received at a distant
point, the electromagnetic wave must be intercepted by a receiving aerial.
Properties of any aerial must be the same whether it is used for transmitting or
receiving. Examples of such properties are efficiency and radiation pattern.
Practically, transmitting antenna has large radiation power unlike receiving
aerial which have less. Transmitting aerial may radiate several kilowatts of power,
while a receiving antenna may have few mill watts power squandered in it.
(a) Transmitting Aerials
Transmitting aerials/antenna is a circuit element that provides radiated
electromagnetic wave when alternating current (a.c) from transmitter flows
in transmitting aerial/radio waves of the same frequency f, as the a.c are
emitted and if the length of the aerial is comparable with the wavelength
of the waves.
Examples – if the frequency of a transmitting antenna is 200MHz and the
speed of radio waves is 3 x 108m/s, calculate the wavelength.
Solution –
C = f
C = 3 x 108m/s; f = 200MHz = 2 x 108Hz, =?
If aerials are not to be too long, they must be supplied with r. f. currents
from the transmitter.
The Dipole
A physical dipole consists of two equal and opposite point charges. Literally,
two poles. Dipole antenna (aerial) consists of two vertical or horizontal
conducting rods or wires. Each of the length is one quarter of the wavelength of
the wave to be emitted, and center fed.
Half wave dipole antenna is a type of antenna having half wave length of
the applied frequency.
67
Basic dipole is a hypothetic antenna with half of the wavelength of the
radiating antenna.
Isotropic radiator is a vertical dipole that emits/radiate energy equally in all
horizontal directions, but not all vertical direction.
Beam width of an antenna is the angle subtended by two points when the
maximum power is dropped to its half.
b) Diagram of isotropic radiator
(a) Diagram of dipole aerial
(d)
(c)
(d) Diagram of horizontal half wave dipole
(c) Diagram of vertical half wave dipole
Figure 8.1: Dipole antenna
Insulatio
n Feeder
cable /2
90o
Maximum
power 90o Antenna
Pmax
½
= beam
width
(e) Diagram showing beam-width of an antenna
68
Dipole aerial behaves like a series LC circuit whose resonant frequency depends
on its length, hence this determines its inductances L and its capacitance C.
Types of antenna/aerial
a. Yagi – Uda antenna
b. Rhombic antenna
c. Parabolic antenna
d. Reflective array antenna
e. Turnstile antenna
f. Batwing antenna
g. Patch antenna
h. Satellite and missile/rocket antenna
i. Franklin antenna
j. Cage aerial
k. Bi conical antenna
l. Ground plane antenna
m. Helical antenna
Yagi – Uda Antenna
Yagi Uda antenna commonly known as Yagi antenna was invented in 1921
by Japanist – Shintaro Uda to Tohoku Imperial University, Japan with a lesser role
played by his colleague Hidetsugu Yagi. Yagi – uda antenna is a directional
antenna consisting of multiple parallel dipole elements in a line, usually made of
metal rods. It consists of a single driven element, called reflector and one or more
directors. The reflector elements is slightly longer than the driven dipole, whereas
the directors are little shorter.
Yagi – uda, otherwise called a “beam antenna”. Yagi antenna is widely used
as a high gain antenna on the HF, VHF and UHF bands. It has moderate gain which
depends on the number of elements used, typically limited to about 17dBi, linear
polarization, unidirectional beam pattern with high front-to back ratio of up to
20dB and is lightweight, inexpensive and simple to construct. The bandwidth is
narrow, a few percent of the center frequency and decrease with increasing gain
which makes it useful in fixed frequency applications.
69
Parasitic elements of a yagi-uda antenna are not connected electrically to
the receiver or transmitter, but serves as resonator, reradiating the radio waves
to modify the radiation pattern.
Yagi was first widely used during world war II for airborne radar sets,
because of its simplicity and directionality.
Figure 8.2: Diagram showing Yagi - Uda antenna
Parabolic Antenna
First world’s parabolic reflector antenna was constructed by German
Physicist Heinrich hertz in 1888. The antenna was a cylindrical parabolic reflector
made of zinc sheet metal supported by a wooden frame and had a spark gap
excited dipole as a feed antenna along the focal line. Its aperture was 2 meters
high by 1.2 meters wide, with a focal length of 0.12 meters, and was used at an
operating frequency of about 450MHz.
Italian Radio Pioneer Guglielmo Marconi used a parabolic reflector during
the 1930s in investigations of UHF transmission from his boat in the
Mediterranean. In 1931, a 1.7GHz microwave relay telephone linking across the
English Channel using 10ft (about 3m) diameter dishes was demonstrated. The
first large parabolic antenna, a 9m dish, was built in 1937 by pioneering Radio
Astronomer Grote Reber in his backyard and the sky survey performed with it was
one of the events that founded the field of Radio Astronomy.
10dB
Reflecto
r
Dipole
Lope
Director
Direction of
transmissio
n
Radiation
pattern
-2dB
0.60 0.50
0.30 0.30
Director
Reflecto
r
/2
driven
element
e
le
ment
70
During the 1960s, dish antenna becomes widely used in terrestrial
microwave relay communication network, which carried telephone calls and
television programs across continents. The first parabolic antenna used for
satellite communications was constructed in 1962 at Goonhilly in Cornwall,
England to communicate with the Telstar satellite. The Cassegrain Antenna was
developed in Japan in 1963 by NTT, KDDI and MitSubishi Electric.
Parabolic Antenna is an antenna that uses a parabolic reflector, a curved
surface with the cross sectional shape of a parabola, to direct the radio waves.
The most common is shaped like a dish and is popularly called a dish antenna or
parabolic dish. Parabolic antenna has high directivity. It direct radio waves in a
narrow beam or receive radio waves from one particular direction only. Parabolic
antenna has some of the highest gains and can produce the narrowest beam
widths of any antenna type.
Parabolic antennas are used as high gain antennas for point-to-point
communications, in applications such as microwave relay links that transmit
telephone and television signals between nearby cities , wireless WAN/LAN links
for data communications, satellite, communications and space craft
communication antenna. They are also used in radio telescopes. They are used for
radar antennas.
A practical parabolic antenna consists of a metal parabolic reflector with a
small feed antenna suspended in front of the reflector at its focus, pointed back
toward the reflector. Parabolic reflector can be sheet metal, metal screen or wire
grill construction and it can be circular dish or various types in beam shapes.
Parabolic antenna is of various shapes like: Paraboloidal or dish, shrouded
dish, cylindrical, shaped beam antennas, shaped reflectors, orange peel antenna,
Arrays of feeds. Parabolic antenna can be classified according to the type of feed
(how the radio waves are supplied to the antenna) are: axial or front feed, off-axis
or offset feed, Cassegrain, Gregorian. Gain of a parabolic antenna is the ratio of
power received by the parabolic antenna from a source along its beam axis to the
power received by a hypothetical isotropic antenna. The gain increases with the
square of the ratio of aperture width to wavelength, so large parabolic antennas,
such as those used for space craft communication and radio telescopes, have
extremely high gain.
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Aperture efficiency is the variable which accounts for various losses that
reduce gain of the antenna from the maximum that could be achieved with the
given aperture. Major factors reducing aperture efficiency in parabolic antennas
are; feed spillover, feed illumination taper, aperture blockage, shape errors etc.
Beam width of parabolic antenna is the angular separation between the
points on the antenna radiation pattern at which the power drops to one-half (-
3dB) its of maximum valve.
Figure 8.3: Diagram showing parabolic dish aerial (Antenna)
Rhombic Antenna
Rhombic antenna consists of a pair of wires in the form of horizontal
rhombus supported on poles when one end is energized, the other is terminated
in a resistor.
The arrangement may be regarded as transmission lines which has been
opened out to allow the system to radiate. The value of the terminating
resistance is such as to effectively match the line, so that current distribution
along the wire is approximately to travelling wave. The radiation pattern of
rhombic antenna can be obtained by finding the resultants of a radiation pattern
of four elementary dipoles, each dipole producing its own lobe.
Rhombic antenna is widely used for TV transmission and can be cascaded
either in series or in parallel. It is used for Trans- Atlantic telephone system. It is
used as multi sharable antenna.
Metal dish
Feeder cable
Small dipole
at focus of
dish
Parallel beam
of radio
waves
72
Figure 8.4: Diagram showing rhombic antenna
Turnstile Antenna
Turnstile antenna was invented by George Brown in 1935 and described in
scholarship in 1936. Turnstile antenna is a radio antenna consisting of two dipole
antennas aligned at right angles to each other with current of equal magnitude
and in phase quadrature. It is often referred to as crossed dipoles. Turnstile
antenna can be used in two different modes: Axial mode and normal mode.
Normal mode is the original configuration of the turnstile antenna in which
the orthogonal set of dipoles are each parallel and above the ground. It radiates
Omni-directional, horizontal – polarized radio waves in all azimuth directions. It is
often used in George Brown turnstile antenna.
Axial mode antenna radiates circularly – polarized (CP) radio waves along
the axis. In axial mode, each dipole orients perpendicular to the line of
communication. It is often used for satellite communication because, being
circularly polarized, the polarized of the signal does not rotate when the satellite
rotates.
Terminatin
g resistor
Insulator
Input
Insulator
Resistor Input
Lobe
Resistor Input
Lobe
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Measurement of Aerial Gain
Aerial gain can be defined as how much more in decibel (db) an aerial will
radiate by an isotropic point source. The amount of power received by an antenna
through free space can be predicated by the following formula:
Where, Pr = power received (watt), pt = power transmitted (W), Gt-
transmitting antenna gain (not in dB), Gr (receiving antenna gain (not in db), -
wave length (m), d = distance between the antenna.
Example: Two half wave dipole are separated by 80km. They are aligned for
optimum reception; if the gain of the dipole is 3.12 db and the transmitter fixed
the antenna with 20W at 120MHz. Calculate the power received.
Solution;
80Km
C = f, =c/f
Where C = 3 x 108m/s, F = 120MHz, d = 80km= 80 x 1013m, G = 3.12db;
10 log10 K = 3.12
Log10 K= 0.312
K = 100.312, K = 2.05(this will be the gain)
Pt = 20W, F=120MHz = 120 x 106Hz, Pr =?
Using the formula;
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Receiving Antenna
Receiving antenna transfer energy, i.e. electromagnetic wave from the
atmosphere to its terminal, with the same efficiency with which it transfers
energy from the transmitter into the atmosphere.
There are four major receiving antennas used in modern communication
system, these are:
i. Yagi aerial
ii. Rhombic aerial
iii. Log – periodic aerial
iv. The parabolic reflector
Propagation of Radio Waves
There are four basic methods for obtaining radio wave from the
transmitting end get to the receiving end. These are:
i. Ground wave
ii. Space wave
iii. Sky wave
iv. Satellite communication wave
Figure 8.5: Diagram showing propagation of radio waves
Ground Wave
Ground wave, also known as surface wave, is a radio wave that travels
along the ground, following the curvature of the earth’s surface. The ground wave
Communication
satellite
Ionosphere Spacewave
Sky wave
Surfacewav
e
EARTH
Rx antenna
(receiving
antenna)
Tx (transmitting
antenna)
Spacewav
e
75
must be vertically polarized (electric field). Its range is limited mainly by the
extent to which energy is absorbed from it by the ground. Poor conductors e.g.
sand absorb more strongly than wave and the higher the frequency the greater
the absorption.
The wave must be vertically polarized because the earth will be horizontally
polarized. If the earth surface is highly conductive, the absorption of wave energy
will take place and it is referred to as “attenuation loss”. Attenuation factor
depends on the frequency of the wave, the permittivity and conductivity of the
earth, types of ground over which the wave travels.
Ground wave propagation is much better over water (salt water, ocean, sub
marine etc.) than a very poor conductivity (desert terrain). Ground waves are not
very effective at frequency above 2MHz but they are very reliable in
communication link than sky wave propagation. Ground wave is the only known
means to communicate into the ocean.
Figure 8.6: Ground wave
Space wave
Space wave is the propagation means, giving line of sight transmission,
effective for V.H.F., U.H.F., and microwave signals. Space wave propagation
occurs in the region of about 16km above the earth surface. There are to types of
space wave namely:
i. The direct wave
ii. The ground reflected wave
GROUND
Rx antenna Tx
antenna E
76
The direct wave is by far the widely used for mode of antenna
communication. The propagated wave is direct from the transmitting to the
receiving antenna and it does not travel along the ground and therefore do not
attenuate rapidly. Direct space wave has one severe limitation and this limitation
is called, “line of sight (LOS” transmission distance, antenna height, and curvature
of the earth).
Ground reflected wave can cause reception problem, if the two received
component are not in phase, the segment will fade out, this is as a result of”
direct and ground wave”, which when occurred, refers to as “ghosting in TV”.
In space wave propagation, there are no intervening obstacles such as hills,
buildings or trees.
Figure 8.7: Diagram of space wave Propagation
Sky Wave Propagation
Sky wave propagation is one of the most frequently used methods used for
long transmission. Sky waves are those waves radiated from the transmitting
antenna in a direction that produces a large angle with reference to the earth
surface. The sky wave has the ability to strike the “ionosphere”, be refracted back
towards the ionosphere until it is completely attenuated. sky wave of low,
medium and high frequencies can travel thousands of kilometers but at VHF and
above, they usually pass through the ionosphere into outer space.
Direct wave
Ground reflected
wave
Ground
Rx
antenna Tx
antenna
77
Transmitted wave shown below, leaves the antenna at point A is refracted
from A to the ionosphere at point B and refracted back to the ground, it continues
in that manner until it finally gets to the receiving antenna at point F.
Ionosphere
Figure 8.8: Diagram of Sky Wave Propagation
The Troposphere
The troposphere is the layer before the stratosphere and the layer just
above the ionosphere
The Ionosphere
Ionosphere is the atmospheric layers that produce the refractive effect on
radiated signals consisting of free ions and electrons in the upper atmosphere
region (60miles above earth surface and above). The ionosphere is the layer of
partially ionized gas that is above the oxygen – rich layer we live in. The ionization
is caused by ultraviolet radiation from the sun. The ionosphere is composed of 3
layers designed from the lowest level to the highest level as D, E and F
respectively. The major different between the layers is the distance “3 layer on
ionosphere”
The amount of ionization depends on many factors, amount of sunlight
season of the year, sunspots, weather conditions and local terrain.
A B
D F
C E
Tx
antenna Rx
antenna
Ground
40km
F –
Layer E –
Layer
78
Figure 8.9: Diagram showing layers of the ionosphere
The D-Layer
D – Layer ranges from about 40km to about 88km above the earth surface.
Ionization in the D – layer is low because it is the lowest region of the ionosphere
and farthest from the sun. D layer only exists during the daytime. This layer has
ability to refract signals of lower frequencies. Higher frequencies pass right
through it but are partially attenuated. D layer can be used for signals up to
several megahertz. After sunset, the D-layer ceases to exist (disappear) because
of the rapid recombination of its ions.
The E-Layer
The E – Layer is approximately ranging from 88km to 144km above the
earth surface. The rate of ionic recombination in the layer is rather rapid after
sunset and is almost complete by midnight. This layer has the ability to refract
signals of a higher frequency than those refracted by D-layer. The E layer can
refract signal with frequency as high as 20MHz.
The F-Layer
The F – layer exists from about 144km to 400km. During the daytime, the F-
layer separates into two layers during daylight hours namely: (i) the F1 – layer and
(ii) the F2 layer.
Ionization level in this layer is quite high and varies widely during the cause
of the day. At noon, this portion of atmosphere is closest to the sun and the
degree of ionization is maximum. A fairly constant ionized layer is present at all
times. The F – layer is responsible for high frequency, long distance transmission
due to refraction for frequencies up to 30MHz. in the night, F1 and F2 layers
merge.
D –
Layer
79
Figure 8.10: Diagram showing layers in the ionosphere
D – layer : 40km – 88km
F1 – layer : 88km – 144km
F2 – layer : 144km – 248km
With the disappearance of the D and E layers, at night, signals normally
refracted by these layers are refracted by much higher layer resulting in greater
skip distances at night.
The layers that form ionosphere undergo considerable variation in altitude,
ionization density and the thickness due to primarily varying degrees of solar
activities. The unit of ionization is electron per meter cube.
Effect of Ionosphere on the Sky Wave
The ability of the ionosphere to return a radio wave to the earth depends
on the ionization density, the frequency of the radio wave and the angle of the
transmission. The refractive ability of the ionosphere increases with the degree of
ionization.
Earth
F -
Cyc
le
F – Layer
Radiation
from the
sun.
F2 – Layer
F1 – Layer
E – Layer
D – Layer
80
Ionosphere
Figure 8.11: Diagram showing effect of ionosphere on the sky wave
Critical Frequency (CF)
Critical Frequency is the highest frequency that will be returned to earth
when transmitted vertically under a given ionosphere condition. If the frequency
of a radio wave being transmitted vertically is gradually increased, a point is
reached where the wave is not refracted adequately to curve its path back to
earth, instead these waves continue upward to the next layer where refraction
continues. If the frequency is adequately high, the wave penetrates all layers of
the ionosphere and continues out into space.
Maximum Usage Frequency (MUF)
MUF is the highest frequency that is returned to earth at a given distance.
There is a best frequency for optimum communication between any point at any
specific condition of the ionosphere. Since absorption of signal energy by the
ionosphere is much less at higher frequencies, best result occur when the MUF is
used rather than lower frequencies.
Rx antenna Tx antenna
Earth
81
Ø critical angle
Maximum usable
frequency
Figure 8.12: Diagram showing relationship between critical frequency and MUF
Optimum Working Frequency (OWF)
Optimum working frequency can be defined as 85% of the maximum usable
frequency and the best one that provides the most consistent communication.
20MH
z
21MH
z
Earth
82
CHAPTER NINE
CABLE AND SATELLITE TV
Cable Television CATV
Cable Television is a system of delivering television programmes to paying
subscribers through Radio Frequency (RF) signals transmitted through coaxial
cables or light pulses through fiber – optic cables. In order to receive cable
television at a given location, cable distribution lines must be available on the
local utility poles or underground utility lines. Coaxial cable brings the signal to
the customer’s building through a service drop, an overhead or underground
cable. There are two standards for cable television, older analog cable and newer
digital cable which are capable of carrying high definition signals used by newer
digital HDTV televisions.
Cable system using modern coaxial cables or optical fibers can carry
simultaneously several TV channels, ordinary telephone links and other cables are
laid in wide bandwidth. Multiple television channels are distributed to subscriber
residents through a coaxial cable, which comes from a trunk line supported on
utility poles, originating at the cable company’s local distribution facility, called
the head end. Multiple channels are transmitted through the cable by technique
called frequency division multiplexing. At the head end, each television channel is
translated to a different frequency. Coaxial cables are capable of bi-directional
carriage of signals as well as the transmission of large amounts of data. Each
home has its own junction box and control, which would enable the subscriber to
send information back to the cable station, which is called “Interactive system and
opens the way for the television set to provide additional facilities like: Viewing
TV programme beamed live via satellite from across the world, reading of
domestic meters remotely, two-way teaching, home banking in which bank
accounts could be debited automatically at the push of a button etc.
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Satellite Television
Satellite television is a system of supplying television programming using
broadcast signals relayed from communication satellites. The signals are receiving
through an outdoor parabolic reflection antenna usually referred to as a satellite
dish and a low – noise block down converter (LNB). A satellite receiver then
decodes the desired television programme for viewing on a television set.
Satellite television provides a wide range of channels and services, especially to
geographic areas without terrestrial television or cable television.
There are three primary types of satellite television usage: reception direct
by the viewer, reception by local television affiliate, or reception by hand ends for
distribution across terrestrial cable systems. Direct to the viewer reception
includes direct broadcast satellite (or DSB) and television receive only (or TVRO),
both used for homes and business.
Direction Broadcasting by Satellite (DSB)
DSB also known as “Direct-To-Home” refer to communications satellite that
delivers DBS service or actual television service. Most of the DBS systems use the
DVB-S standard for transmission. DBS enable homes in any part of a country to
become low cost earth stations and receive TV programme from the national
geostationary satellite if they have a small roof-top dish aerial, pointing towards
the satellite. Frequency converter in the base of the aerial will convert the 12GHz
signals from the satellite down to around 1GHz before going indoors to the
receiver. Also, a sound converter must reduce the frequency to UHF Satellite
transmitters use FM to carry the programme (because of the limited power
available). The signal will later be change from FM to AM. Sound signals
accompanying the TV signals are digitally encoded and converted to analogue.
Geostationary (Synchronous) Satellites
In 1945, British Science Fiction writer/ space science writer Arthur C. Clarke
proposed a worldwide communication system which would function by means of
three satellites equally spaced apart in earth orbit. It was published in October
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1945 issue of the Wireless World Magazine and won him the Franklin institute’s
start Ballantine Medal in 1963.
The first satellite television signals from Europe to North America were
relayed through the Telstar satellite over the Atlantic Ocean on 23rd July, 1962.
The signals were received and broadcast in North America and European
countries and watched by over 100million. ‘’Relay 1’’ satellite was the first
satellite to transmit television signals from the US to Japan in 1962. The first
Geosynchronous Communication Satellite, “syncom2’’ was launched on 26th July,
1963
“Intelsat 1”, world’s first commercial communications satellite, nicknamed
“early bird” was launched into Geosynchronous orbit on April 6, 1965. The first
national network of television satellites, called “orbita” was created by the Soviet
Union in October 1967 and was based on the principle of using the highly elliptical
Molniya satellite for rebroadcasting and delivering of television signals to ground
down link stations. The first commercial North America satellite to carry television
transmissions was launched on 9th November, 1972. The world first experimental
educational and Direct Broadcast Satellite (DBS), “ATS – 6” was launched on 30th
May 1974. The first in a series of Soviet Geostationary Satellite to carry Direct-To-
Home television, “Ekran 1” was launched on 26 October 1976. The system is
managed by 114 members International Telecommunications Satellite
Organization (INTELSAT). Satellites are launched for different countries either by
the American space shuttle or by the European rocket Arianne. The first INTELSAT
V satellite, with its power generating solar panels, was launched into
geostationary orbit in 1980. Currently, there are 11 INTELSAT V type satellite in
orbit. The latest of these can handle two TV channels plus 15000-telephone circuit
using microwave frequencies of 4, 6, 11 and 14 GHz.
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