UNIT 1 Electric Circuit
Task 1: Match the words in column A with the definitions in column B.( Phù hợp
với các từ trong cột A với các định nghĩa trong cột B.)
A
1. cell (pin)
2. circuit(mạch)
3. generator(máy
phát điện)
4. potential(tiềm
năng)
5. relay(chuyển
tiếp)
B
a. component which changes a form of energy
(usually chemical energy) into electrical energy. thành
phần có thể thay đổi một dạng năng lượng (thường là
năng lượng hóa học) thành năng lượng điện.
b. energy of an electric charge expressed in
volts(năng lượng của một dòng điện thể hiện trong
volts)
c. electromechanical switch operated by an
electromagnet(điện chuyển đổi hoạt động bằng nam
châm điện)
d. closed path around which a current can flow(đóng
cửa con đường xung quanh đó một dòng điện có thể)
e. device which produces electrical energy.( thiết bị
sản xuất năng lượng điện)
Task 2: Read the following passage and check your answer in task 1(Nhiệm vụ 2:
Đọc đoạn văn sau và kiểm tra câu trả lời của bạn trong nhiệm vụ 1)
Current moves from a point of high potential energy to one of low potential.
It can only do so if there is a path for it to follow. This path is called an electric
circuit. All circuits contain four elements: a source, a load, a transmission system
and a control.( Hiện tại di chuyển từ một điểm có tiềm năng năng lượng cao với
1
một tiềm năng thấp. Nó chỉ có thể làm như vậy nếu có một con đường cho nó để
làm theo.Con đường này được gọi là một mạch điện. Tất cả các mạch có chứa bốn
yếu tố: một nguồn, tải, một hệ thống truyền tải và kiểm soát)
The source provides the electromotive force. This establishes the difference
in potential, which makes current flow possible. The source can be any device,
which supplies electrical energy. For example, it can be a generator or a battery.(
Nguồn cung cấp lực điện. Điều này thiết lập sự khác biệt về tiềm năng, mà làm cho
dòng điện có thể. Các nguồn có thể được bất kỳ thiết bị, mà nguồn cung cấp năng
lượng điện. Ví dụ, nó có thể là một máy phát điện hoặc pin.)
The load converts the electrical energy from the source into some other form
of energy. For instance, a lamp changes electrical energy into light and heat. The
load can be any electrical device.( Tải chuyển đổi năng lượng điện từ nguồn vào
một số hình thức khác của năng lượng. Ví dụ, một ngọn đèn thay đổi năng lượng
điện thành ánh sáng và nhiệt. Tải có thể là bất kỳ thiết bị điện)
The transmission system conducts the current round the circuit. Any
conductor can be part of the transmission system. Most systems consist of wires. It
is often possible, however, for the metal frame of a unit to be one section of its
transmission system. For example, the metal chassis of many electrical devices are
used to conduct current. Similarly, the body of a car is part of its electrical
transmission system.( Hệ thống truyền dẫn tiến hành vòng đàm phán lần mạch. Bất
kỳ dây dẫn có thể là một phần của hệ thống truyền dẫn.Hầu hết các hệ thống bao
gồm dây. Nó thường có thể, tuy nhiên, cho khung kim loại của một đơn vị là một
phần của hệ thống truyền dẫn của nó.Ví dụ, khung kim loại của nhiều thiết bị điện
được sử dụng để tiến hành hiện nay. Tương tự như vậy, cơ thể của một chiếc xe là
một phần của hệ thống truyền tải điện của nó.)
The control regulates the current flow in the circuit. It may control the
current by limiting it, as does a rheostat, or by interrupting it, as does a switch.(
Kiểm soát quy định dòng điện trong mạch. Nó có thể kiểm soát hiện tại bằng cách
hạn chế nó, cũng như rheostat một, hoặc bằng cách ngắt nó, cũng như chuyển đổi
một)
2Figure 1
0.3 w bulb
Switch
wires
3.0 V
Study Figure 1. In this simple flashlight circuit, the source comprises two 1.5 V
cells in series. The load is a 0.3 W bulb. Part of the transmission system is the metal
body of the flashlight, and the control is a sliding switch.( switchStudy hình 1.
Trong mạch này đèn pin đơn giản, nguồn bao gồm hai 1,5 V tế bào trong loạt. Tải là
một bóng đèn 0,3 W.Một phần của hệ thống truyền tải là cơ thể kim loại của các
đèn pin, và kiểm soát là một chuyển đổi trượt.)
Battery of solar cells: Pin của các tế bào năng lượng mặt trờiRelay : chuyển tiếpTelevision camera : truyền hinh camera
Compare Figure 2. The function of this circuit is to operate a television camera
aboard a space satellite. Here the source is a battery of solar cells. A solar cell is an
electric cell, which converts sunlight into electrical energy. The load is the
television camera. The transmission system is the connecting wires. The control is a
relay actuated by transmissions from ground control. Although the function of this
circuit is much more complex than that of the flashlight, it too consists of the four
basic elements.( So sánh hình 2. Các chức năng của mạch này là để vận hành một
máy ảnh truyền hình trên một vệ tinh không gian. Ở đây nguồn là một pin của các tế
bào năng lượng mặt trời. Một tế bào năng lượng mặt trời là một tế bào điện, chuyển
đổi ánh sáng mặt trời thành năng lượng điện. Tải là máy ảnh truyền hình. Hệ thống
3
Figure 2
Battery of solar cells
Television
camera
Relay
Transmission system
truyền dẫn là dây kết nối. Kiểm soát là một relay khởi động bằng các truyền dẫn từ
kiểm soát mặt đất. Mặc dù chức năng của mạch này là phức tạp hơn nhiều hơn so
với của các đèn pin, nó cũng bao gồm bốn yếu tố cơ bản.)
Task 3: Rewrite the following sentences, replacing the words in italics with
expressions from the passage in task 2 which have a similar meaning.
1. A lamp converts electrical energy into light.
2. The generator provides the circuit with electromotive force.
3. The metal frame of the oscilloscope is part of its transmission system.
4. The rheostat controls the current flow in the circuit.
5. A battery of solar cells supplies power to the circuit.
Task 4: Meaning from context
What do the pronouns in italics in these sentences refer to?
1. Current moves from a point of high potential energy to one of low potential.
(a) current
(b) energy
(c) a point
2. For example, it may be a generator or a battery.
(a) the source
(b) a device
(c) electromotive force
3. It is often possible, however, for the metal frame of a unit to be one section of
its transmission system.
(a) the metal frame's
(b) the unit's
(c) the circuit's
4. Although the function of this circuit is much more complex than that of the
flashlight, it too consists of the four basic of elements.
(a) this circuit
(b) the function
(c) the flashlight
Task 5: Checking facts and ideas
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Decide if these statements are true or false. Quote from the passage to support your
decisions.
1. A difference in potential is required before a current can flow in a circuit.
2. A generator is a source of electromotive force.
3. Loads convert electrical energy into light and heat.
4. Transmission systems must consist of wires.
5. A rheostat may be used as a control.
6. The load in the flashlight circuit is a bulb.
7. The source in the satellite circuit is a solar cell.
8. The current flow in the satellite circuit is regulated by a relay.
9. The flashlight circuit differs basically from the satellite circuit.
Task 6: Further reading
In conventional power generation, fuel such as oil or coal is burned. The
burning fuel heats boilers to produce steam. The steam is used to drive turbo –
alternators. The Magneto hydrodynamic (MHD) process generates electricity
without requiring a boiler or turbine.
MHD generation works on the principle that when a conductor cuts a
magnetic field a current flows through the conductor. In MHD generation the
conductor is an ionized gas. Small amounts of metal are added to the gas to improve
its conductivity. This is called seeding the gas. The seeded gas is then pumped at a
high temperature and pressure through a strong magnetic field. The electrons in the
gas are collected at an electrode. This movement of electrons constitutes a current
flow.
Tow methods of MHD generation can be used: the open cycle and closed
cycle. In the open cycle method the hot gas is discharged. In the closed cycle
method it is recirculated.
The open cycle method uses gas from burning coal or oil. The gas is seeded
and then passed through a magnetic field to generate current. The seeding elements
are recovered and the gas then be used to drive a turbine before being allowed to
escape.
The closed cycle method uses an inert gas, such as helium, which is heated
indirectly. The gas is circulated continually through the MHD generator.
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MHD generation is still in its early stages but already an efficiency rate of
60% has been reached. This compares with a maximum of 40% from conventional
power station.
Answer the questions which follow
1. How does the MHD process differ from conventional systems?
2. What principle does MHD generation make use of?
3. What form does the conductor take in the MHD process?
4. What happens to the gas?
5. What methods of MHD generation are in use?
6. How do the two methods differ?
Task 7: Describing the position and connection
EX: The turning capacitor is connected across the coil
Now complete each sentence using an appropriate phrase from this list:
1.located within 2.connected across 3.applied to
4.connected to 5.connected between 6.mounted on
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a. The bulbs are _____the battery
core
b. The core is _______the pole pieces
c. The 27pF capacitor is ______the collector and the base
27pF
d. The feedback voltage is______the base of transistor through the C1
rotor
UNIT 2 Electric Energy
1. brush bar and brushes
2. driving shaft
3. terminals (main)
4. commutator
5. induction coil winding
6. main pole
7. wire loops
Figure 1: A typical DC machine with a section removed
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f. The rotor is _______the shafte. The antenna is ______the coil
N
S
1
2
3
4 5
6
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7
Task 1: Match the words in column A with the definitions in column B.
A B
1. armature
2. core
3. electrolysis
4. impedance
5. pole
a. positive or negative terminal
b. chemical decomposition by action of electric current
c. wire - wound core of dynamo or electric motor
d. central solid conductor
e. combined resistance to AC and DC
Task 2: Read this passage.
An electric motor is a machine for converting electrical energy into
mechanical energy. Motors can be designed to run on direct (DC) or alternating
current (AC). The motor shown in Figure 1 is a DC motor. Its most important parts
are the rotor, the stator and the brush gear.
The rotor is the moving part. It contains an armature, which is a set of wire
loops wound on a steel core. When current is fed to the armature, these windings
produce a magnetic field. The armature and the core are mounted on a shaft, which
runs on bearings. It provides a means of transmitting power from the motor.
The rotor also contains a commutator. This consists of a number of copper
segments insulated from one another. The armature windings are connected to these
segments. Carbon brushes are held in contact with the commutator by springs.
These brushes allow current to pass to the armature windings. As the rotor turns, the
commutator acts as a switch making the current in the armature alternate.
The stator does not move. It consists of magnetic and electrical conductors.
The magnetic circuit is made up of the frame and the poles. Wound round the poles
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are the field coils. These form the stator's electric circuit. When current is fed to
them, a magnetic field is set up in the stator.
The motor operates on the principle that when the current- carrying
conductor is placed in a magnetic field, a force is produced on the conductor. The
interaction of the forces produced by the magnetic field of the rotor and the stator
makes the rotor spin.
Task 3: Complete the diagram follow:
Task 4: Meaning from context
Select a word from the three alternatives given which is most similar in meaning to
the word in italics as it is used in the passage in Task 1.
1. provides (line 8)
(a) produces
(b) supplies
(c) allows
3. alternate (line 13)
(a) reverse
(b) change
(c) flow in one direction then
in another
2. segments ( line 9)
(a) sections
(b) pieces
(c) wires
4. interaction (line19)
(a) acting together
(b) operation
(c) result
9
21
Commutator
Brush gear
5 6 Core
3 4 Spring
Electric conductor
Magnetic conductor
87 Pole
DC MOTOR
Task 5: Making definition
Study these two sentences:
The cables were undamaged
The cables were armored
We can link them in two ways using relative clauses
1.The cables which were armored were undamaged
2. The cables, which were armored, were undamaged
-Sentence 1 means that only the armored cable were undamaged. Other cables, for
example PVC coated cables, were damaged. The relative clause is a defining one. It
defines the type of the cables, which was undamaged. It carries essential
information.
- Sentence 2 means that all the cables were undamaged and all the cables were
armored. The relative clause is a non-defining one. It adds some extra information
to the sentence but it is not essential. We can remove it from the sentence still
makes good sense. It is separated the rest of the sentence by the commas.
Study this diagram:
Now make the definition using the information from this table:
A B C
a generator a material converts mechanical energy into electrical energy
an insulator An instrument readily released electrons
an alternating a current flow first in one direction, then in the
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(a) a solar cell
(b) an electric cell
(c) convert sunlight into electrical energy
is
which
currents other
a direct current a device does not readily released electrons
a conductor flows in one direction
Task 6: Describing the cause and effect
When a sentence contains a cause and effect, we can link them as follow:
Cause Effect
Insulation
Break down
causes
short circuitresults in
produces
leads to
We can put the effect first:
Effect Result
Short circuit
is caused by insulation
break downis the result of
is due to
is the effect of
When a cause has several effects or when an effect has a number of possible causes,
we can put can or may before the causative expression
Example:
Sparkling MAY be caused by worn brushes.
Sparkling CAN be caused by worn commutator.
Now match these causes and the effect pairs. The link them using the expressions
given above.
Cause Effect
1. Glare a. Bad join
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2. Eddy current b. Discomfort to the eyes
3. Excessive heat c. Power losses in transformer
4. Faulty soldering d. Damage to the semiconductor
5. Sparking e. Arcing across the point
Task 7: Further reading
The effects of an electric current are thermal, luminous, chemical and
magnetic. When a current flows through a conductor it may heat the conductor. This
heat is sometime undesirable and has to be reduced. For this reason many electric
motors and generators contain a fan. However, domestic appliances, such as electric
cookers and many industrial processes depend on the heating effect of an electric
current.
The passage of a current may produce light. This can happen in a number of
ways. The heat generated by the current may be so great that the conductor become
incandescent. For example the filament of a light bulb emits intense white light
when heated by a current. Light is also produced when a current ionizes a gas. The
color of the light will vary according to the gas used. Mercury vapor lamps give
greenish blue light.
An electric current can separate a chemical compound into its components.
This is called electrolysis. Chloride is generated by the electrolysis of salt water.
Electrolysis can also be used to break down water into hydrogen and oxygen.
Because pure water does not conduct well, sulphuric acid has to be added before the
electrolysis takes place.
A current flowing through a conductor creates a magnet field around it. This
field has three applications. It can magnetize magnetic material and attract them to
the conductor. The electric relay works on this principle. If the magnetic field is cut
by another conductor, an electromotive force will be introduced in that conductor.
For instance, the change in current flowing through the primary of a
transformer will induce a current in the secondary. This principle is also used in
generator. Thirdly, if a current carrying conductor is placed in a magnetic field, a
force will be exerted on it. This effect is utilized in the electric motor.
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UNIT 3
The cathode ray tube
Task 1: Match the words in column A with the definitions in
column B.
A B
1. cathode
2. anode
3. oscilloscope
4. cathode ray tube
5. lens
a. large valve used to produce a display by firing a beam of
electrons at the phosphor coated screen.
b. negative electrode which emits electrons
c. positive electrode which attracts electrons
d. electronic instrument for measuring and displaying
changing signals on a screen using a cathode ray tube
e. piece of glass or other transparent materials with one or
more curved surfaces used to make things appear clearer
larger or smaller when viewed through it.
Task 2: Read this passage and complete the diagram, which follows.
The cathode ray tube (CRT) is used in oscilloscopes, radar receivers and
television sets. The type described here is that used in oscilloscopes. By means of a
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CRT, an oscilloscope not only shows the size of a signal, but although the signal
varies with time. In other words it shows the waveform of the signal.
The CRT operates as follows. First electrons are emitted from a heated
cathode. Then these electrons are accelerated to give them a velocity. Next they are
formed into a beam, which can be deflected vertically and horizontally. Finally they
are made to strike a screen coated on this inner surface with a phosphor.
The CRT comprises an electron gun and a deflection system enclosed in a
glass tube with a phosphor coated screen. The electron gun forms the electrons into
a beam. It contains a cathode, which is heated to produce a stream of electrons. On
the same axis as the cathode is a cylinder known as the grid. By varying the
negative potential on the grid, the intensity of the beam can be varied. A system of
three anodes follows. These accelerate the beam and also operate as a lens to focus
the beam on the screen as a small dot. Varying the potential on the central anode,
a2, allows the focus to be adjusted.
On leaving the electron gun, the beam passes through two sets of plates,
which are at right angle to each other. The first sets of plates are the Y plates. As
these are nearer to the anodes, they have a greater effect on the beam oscilloscope.
Therefore the signal is applied to this set. They control the vertical deflection of the
beam.
Figure 1
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Electric gun
11
2
3
4
5
6
7
8
9
Deflection system
Heater
The second sets are the X plates. On an oscilloscope the output from a
timebase oscillator is applied across these plates as a means of moving the beam
horizontally at regular intervals. Hence the horizontal axis of an oscilloscope is the
time axis. By means of the deflection system, then, the beam can be made to
traverse the screen both horizontally and vertically.
The final element is the phosphors coated screen. When the electron beam
strikes the screen, the phosphor coating fluoresces. Various colors of light are
produced depending on the phosphor used.
Task 3: Meaning from context
Select a word from the three alternatives given which is
most similar in meaning to the words in italics as it is used in
the passage in task 1.
1. emitted (line 5)
a. scattered
b. given off
c. absorbed
2. deflected (line7)
a. moved
b. bent
c. changed
3. intensity (line 12)
a. focus
b. brightness
c. shape
4. adjusted (line 15)
a. reduced
b. varied
c. increased
5. regular (line 21)
a. frequent
b. equally timed
c. varying
6. fluoresces (line 25)
a. lights
b. emits electrons
c. turns green
Task 4: Finding out facts
Answer these questions about the passage in task 2
1. Why is an oscilloscope better than a meter?
2. What is the source of electrons for the electron gun?
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3. What is the function of the electron gun?
4. How is the intensity of the beam controlled?
5. In what way is the system of anodes like a lens?
6. Why is the signal applied to the Y plates?
7. What dose the time base do?
8. Why is the horizontal axis of an oscilloscope called the time axis?
Task 5: Further reading
A stream of electron is emitted from the surface of the cathode (C) when it is
heated by the heated filament. The electron are accelerated towards the screen by a
set of three positively charge cylinder anodes (A1, A2, A3). Each anode has a
higher charge than the one before. As the electrons move toward the anodes, they
pass through a hole in a negatively charged metal disc. This disc is known as the
control grid. By adjusting the intensity control on the oscilloscope, the charge on the
grid can be varied. This allowes the number of electrons reaching the screen, and
therefore the brilliance or brightness of the spot on the screen, to be adjusted.
The three anodes form the electron lens. The oscilloscope focus control
allows the voltage on the second anode (A2) to be varied and causes the stream of
electrons to be formed into a narrow beam. If the oscilloscope has an astigmatism
control, it is used to vary the voltage on the third anode (A3). This allows the shape
of the spot on the screen to be adjusted to make it perfectly round.
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17
UNIT 4
Television
Task 1: Match the words in column A with the definitions in
column B.
A B
1. frame a. complete picture in a video display consisting of two fields
2. raster pattern b. number of times per second that a video frame is display on a screen
3. line scan signal c. scan path of an electron beam going across and down the screen of a television receiver
4. frame scan rate d. part of a video signal which adjusts the timing for the display of a frame on a television screen
5. field sync pulse e. part of a video signal which controls the movement of spot across the television screen.
Task 2: Read this passage and check your answers in task 1.
A television picture is built up gradually by moving a spot of light across and
down a screen in a raster pattern (see Figure 1).
Figure 1
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The video signal causes the brightness of the spot to vary in propagation to
the intensity of light in the original image. The movement of the spot light across
the screen is controlled by the line scan signal. Each time the spot reaches the right
side of the screen it is blanked and moved rapidly back to the left side ready to start
the next line. This rapid movement back to a starting position is known as fly back.
Each complete image or frame require as minimum of 500 lines to give a picture of
acceptable quality. The present European TV system uses 625 lines per frame.
The movement of the spot down the screen is controlled by the field scan
signal. When the spot reaches the bottom of the screen, it is blanked and moves
rapidly back to the top of the screen. The frame must be scanned at least 40 times
per second to prevent the screen from flickering. The European TV system has a
frame scan rate of 50 Hz.
The video signal contains line and field sync pulses to make sure that the TV
receiver starts a new line and new frame at the same time as the TV camera (see
figure 2).
Figure 2
To allow the video signal to be transmitted using a smaller range of
frequencies each frame is in two separate halves, known as field. The first time the
spot travels down the screen it displays the first field, which consist of the odd
numbered frame lines. The second time the spot travels down screen it displays the
second field, which consists of the even numbered frame lines. Combining two
fields in this ways is known as interlacing. Although the fields are displayed one
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Video signal
Line sync pulses(during line fly back)
Field sync pulses(during field fly back)
after the other, it happens so quickly that the human eye sees them as one complete
picture.
Task 3: Answer these questions
1. What controls the movement of the spot light across a television screen?
2. What name is given to the rapid movement of the spot back across the screen to
the start of the next line?
3. How many lines are used to build up a frame in present European television
system?
4. What happens to a screen if the frame is not scanned at least 40 times per second?
5. Why are the sync pulses added to the video signal?
Task 4: Meaning from context
Select a word from the three alternatives given which is most similar in meaning to
the word in italics as it is used in the passage in task 2.
1. Image (line 4)
(a) brightness
(b) picture
(c) video signal
3. flickering (line 14)
(a) displaying
(b) breaking
(c) varying in brightness
2. Rapid (line 7)
(a) quick
(b) repeated
(c) to the left side
4. them (line 23)
(a) two fields
(b) odd number lines
(c) even number lines
Task 5: Read this passage and fill in the table, which follows.
In this passage we will examine briefly how an object in front of a television
camera becomes a picture on a television screen.
A television camera contains a lens system, which is used to focus an image
of the object on to the face of the camera tube. This tube contains a photo-cathode,
which emits electrons in response to light. The brighter the light from the image the
more electrons are emitted by the photo-cathode. In a black and white camera, the
photo-cathode responds only to brightness, hence it is at this point that information
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on the color of the image is lost. The electrons from the cathode are now made to
strike a target electrode causing some of its atoms to become positively charged.
The target electrode is scanned by electron beam. The beam sweeps the
target electrode in a series of closely spaced lines. There are 405 or 625 of these
lines depending on the system used.
When the beam reaches the end of the top scan line, it is brought quickly
back to the beginning of the next line, which is slightly lower. This return is called
fly back and is much quicker than a line scan.
The scanning beam loses electrons to the positively charged atoms on the
target electrode and is thus charged or modulated. Its density is thus proportional to
the light intensity of the original image. In this way the camera produces a
continuous waveform, which contains information on the brightness of the original
image. This video waveform has information added to it, sync pulses, to
synchronize the start of each scanning line and frame.
The video signal is transmitted and received in a similar fashion to sound
transmission. After detection and amplification it is fed to the cathode of the CRT in
the television receiver thus controlling the intensity of the electron beam. The sync
pulses ensure that the beam in the CRT is in exactly the same position as the beam
in the television camera. The beam is made to move sideways and progressively
downwards matching line by line the scanning of the television camera. As the
electron beam strikes the television screen, the phosphor coating on the screen emits
light. This light varies in whiteness according to the brightness of the original
image. Because the line by line build up of the picture takes place so quickly, the
eye sees only a complete picture of the object in front of the television.
1. What part of the camera tube is scanned?
2. Why is the color information lost?
3. What two types of information does the video-waveform carry?
4. What is the function of the sync pulses?
Task 6: Further reading
The characters and pictures that we use on the screen are made up of dots,
also called picture elements (pixels). The total number of pixels in which the
21
display is divided both horizontally and vertically is known as the resolution. If the
number of pixels is very large, we obtain a high- resolution display and therefore a
sharp image. If the number of pixels is small, a low resolution is produced. Typical
resolutions are 640 x 480 or 1024 x768 pixels. The diagrams below show how pixel
density affects the image: a large number of pixels give a much clear image.
The cathode ray tube of the monitor is very similar to that of a TV set. Inside
the tube there is an electron beam, which scans the screen and turns on or off the
pixels that make up the image. The beam begins in the top left to right in a
continuous sequence, similar to the movement of our eyes when we read, but much
faster. His sequence is repeated 50, 60 or 75 times per second, depending on the
system. If the rate of this repetition is low, we can perceive a flickering, unsteady
screen, which can cause eye fatigue. However, a fast-moving 75Hz ‘refresh rate’
eliminates this annoying flicker.
What we see on the screen is created and stored in an area of RAM, so that
there is a memory cell allocated to each pixel. This type of display is called bit-
mapped. On monochrome monitors, bits 0 are visualized as white dots, and bits 1 as
black dots.
On color displays, there are three electron guns at the back of the monitor’s
tube. Each electron gun shoots out a beam of electrons; there is one beam for each
of the three primary colors: red, green and blue. These electrons strike the inside of
the screen, which is coated with substances called phosphors that glow when struck
by electrons. Three different phosphor materials are used one each for red, green
and blue. To create different colors, the intensity of each of the three electron beams
is varied.
The monitor is controlled by a separate circuit board, known as the display
adaptor, which plugs into the motherboard driving different types of displays. For
example, the VGA (video graphic array) card has become a standard for color
monitor.
22
Portable computers use a flat liquid-crystal display (LCD) instead of a
picture tube. An LCD uses a grid of crystals block the light in different amounts to
generate the dots in the image.
UNIT 5
Propagation of Waves
Task 1: Match the words in column A with the definitions in
column B.
A B
1. absorption
2. aerial
3. attenuation
4. frequency
5. reflection
a. how often a pattern is repeated a second
b. process of taking in energy
c. device for collecting or sending out signals being
transmitted through free spaced. change of direction of a wave after hitting a surface
e. magnitude reduction of a signal
Task 2: Read this passage and check your answers in task 1.
A signal from a transmitter may be propagated in three ways: by ground
waves, by space waves and by sky waves. Ground waves travel round the surface of
the earth for short distances. As they travel, they lose energy. This loss of power, or
attenuation, depends on the nature of the surface. Attenuation also varies with the
frequency of the signal; the higher the frequency, the greater the ground wave
attenuation. At frequencies above 20 MHz the range is reduced to the line of sight.
Propagation by space waves applies mainly to very high frequencies. Part of
the transmitted signal travels in a direct line from transmitting antenna to receiving
23
antenna. Partly the signal is reflected from the ground. The higher the frequency,
the greater the possible ground wave reflection. The range of the space wave
propagation is restricted to approximately twice the direct optical path.
The range covered by ground waves and space waves is limited. Greater
distances can be achieved using sky waves. Sky wave propagation depends on the
ionosphere.
Figure 1
A signal transmitted from point A would not be received at B because of the
curvature of the earth if it were not for the ionosphere. This consists of a number of
layers of ionized gas in the upper atmosphere. If a transmission is directed towards
these layers, it will be reflected back to the earth as shown in Figure 2.
Figure 2
The wave may then be reflected back from the earth to the ionosphere. Indeed it
may be carried right round the earth by successive reflections although it will lose
power both in the earth and in the ionosphere at each bounce. A receiver at point C,
24
A
B
A BC
which is outside ground wave range yet closer than B, will not receive the
transmission.
The bending effect of the ionosphere depends on the frequency of the signal
and the angle of radiation. The higher the frequency the less the bending. At a
certain frequency, signal will pass straight through the layers and be lost in space.
The smaller the angle of radiation, the greater the distance, which can be covered in
one reflection.
At any time there is a maximum usable frequency for transmissions from a
given site over a particular path. This frequency depends on the state of the
ionosphere, which varies according to many factors including the time of the day
and the season of the year. The lower the frequency of a transmission the greater the
number of reflections needed to cover the required distance and hence the weaker
signal will be. For this reason, it is best to used a frequency as high as possible
without exceeding the maximum usable frequency as this will cover the required
distance with the smallest number of reflections and hence the least attenuation.
Task 3: Answer these questions about the passage in task 2.
1. List three types of propagation.
2. How do ground wave travel?
3. What is attenuation?
4. Name two factors, which affect the attenuation of ground waves?
5. What frequencies are propagated by space wave?
6. Name two days in which space waves travel.
7. What range have space wave?
8. What controls sky wave propagation?
9. What is the ionosphere?
10. How do sky waves cover great distances?
11. Name two factors, which determine the bending effect of the ionosphere.
12. What is the maximum usable frequency?
Task 4: Meaning from context
Select a word from the three alternatives given which is most similar in meaning to
the word in italics as it is used in the passage in task 2.
25
1. varies with ( line 4 )
a. is proportional to
b. is directly proportional to
c. is inversely proportional to
2. approximately ( line 11 )
a. nearly
b. over.
c. around
3. This ( line 15 )
a. the ionosphere
b. the atmosphere
c. transmission
4. bounce ( line 20 )
a. reflection
b. attenuation
c. receiving
Task 5: Further reading
Radio waves from a transmitting aerial can travel in one or more of three
different ways.
Surface or ground waves: This travels along the ground, following the
curvature of the earth's surface. Its range is limited mainly by the extent to which
energy is absorbed from it by the ground. Poor conductors, such as sand, absorb
more strongly than water, and the higher the frequency, the greater the absorption.
The range is about 1500 km at low frequencies (long waves).
Sky waves: It travels skywards and, if it is below a certain critical frequency
(typically 30 MHz), is returned to earth by the ionosphere. This consists of layers of
air molecules stretching from about 80 km above the earth to 500 km. On striking
the earth, the sky wave bounces back to the ionosphere where it is again gradually
refracted and returned earthwards as if by " reflection ". This continues until it is
completely attenuated.
The critical frequency varies with the time of day and the seasons. Sky
waves of high frequencies can travel thousands of kilometers but at VHF and above
they usually pass through the ionosphere into outer space.
Space waves: For VHF, UHF, and microwave signals, only the space wave,
giving line of sight transmission, is effective. A range up to 150 km is possible on
earth if the transmitting aerial is on high ground and there are no intervening
26
obstacles such as hills, buildings, or trees. Space waves are also used for satellite
communications.
Fill in the table, which follow
Surface waves Sky waves Space waves
Frequencies
Travels
Range
Difficulties
.............................. .
.............................
..............................
..............................
.............................. ..
.............................
...............................
...............................
.............................. .
.............................
.............................
..............................
27
UNIT 6
Transmission
Task 1: Match the words in column A with the definitions in
column B.
A B
1. amplitude
2. broadcasting
3. distortion
4. inductance
5. modulation
a. unwanted change of shape of a signal
b. shaping of a carrier wave by combining it with a signal
to be carried
c. size of a wave at any given timed. resistance to AC
e. transmitting radio or TV signals
Task 2: Read the passage
We can only communicate information by radio waves by changing the wave in
some ways. This change is known as modulation. The simplest form of modulation
is to turn the wave on or off. This method was used in the early days of radio for
telegraphic signals. The wave was stopped and started to present the dots and
dashes of the Morse code by means of a telegraph key.
Speech and music produce audio frequencies, which cannot be transmitted
directly. But they can be used to modulate radio waves. The modulated radio wave is
then transmitted. When it is received, the wave is demodulated and the original
audio- frequency signal is recovered. The high frequency radio acts only to carry the
audio - frequency signal and is called the carrier wave. The audio - frequency signal
is termed the modulating signal.
The wave has three quantities: amplitude, frequency and phase. Any of these
quantities can be modulated. The two commonest methods of modulation are
amplitude modulation (AM) and frequency modulation, (FM).
In amplitude modulation, the amplitude of the carrier wave is changed
according to the amplitude of the modulating signal. The frequency of the carrier is
kept constant. Figure 1 presents part of an audio - frequency signal, which might be
28
generated by a microphone. Figure 2 represents a radio wave of much higher
frequency. Figure 3 shows the same radio frequency wave after it has been
modulated by the audio - frequency signal in Figure 1.
Figure 1
Figure 2
Figure 3
In frequency modulation, the amplitude of the carrier wave is kept constant,
but the frequency is varied in proportion to the amplitude of the modulating signal.
Frequency modulation has several advantages over amplitude modulation. The most
notable is that reception is less likely to be disturbed. This is because atmospheric
disturbances and " noise " generated in the receiver itself result in a change in the
amplitude of the received signal. However, changes in only the frequency would
distort the modulated information.
29
In FM sound broadcasting, the limit of modulation is usually 75 kHz above
and below the frequency of the unmodulated carrier wave. In other words, FM
broadcasts spread 75kHz either side of the carrier frequency. This is one reason why
FM stations broadcasts in the VHF band (30- 300 MHz) where stations can be
spaced more than several hundred kilohertz apart unlike the medium frequency
bands where spacing of only 9 to 10 kHz are common.
Task 3: Find the answers to these questions in the passage, which follows.
(Work as quickly as you can. Try to ignore information, which will not help you to
answer these questions)
1. What is modulation?
2. Which three quantities of a wave can be modulated?
3. Why is frequency modulation better than amplitude modulation?
Task 4: Meaning from context
Select a word from the three alternatives given which is most similar in
meaning to the word in italics as it is used in the passage in task 2.
1. form (line 2)
a. change
b. communication
c. method
2. termed (line 10)
a. called
b. modulated
c. used
3. these (line 11)
a. three quantities
b. frequencies
c. methods
4. constant (line 15)
a. changed
b. unchanged
c. varied
Task 5 : Checking facts and ideas
Decide if the statements are true or false. Quote from the passage in task 2 to
support your decisions.
1. The first application of radio wave modulation was for telegraphic signals.
2. Phase modulation is commonly used.
3. In amplitude modulation, the frequency of the carrier wave is proportional to
the amplitude of the modulating signal.
4. FM reception is less likely to be disturbed than AM.
30
5. FM stations broadcast in the VHF band.
Task 6: Now read the passage carefully. Each of these sentences summarizes part
of the passage. Identify the lines summarized.
1. In FM the frequency of the carrier wave is modulated according to the
amplitude of the modulating signal. (lines...)
2. Audio frequencies cannot be transmitted. (line...)
3. The amplitude, frequency and phase of a wave can be modulated. (line...)
4. FM broadcasts are in the VHF band partly because FM stations require
greater spacing. (line...)
5. The carrier wave is demodulated by the receiver and the audio - frequency
signal recovered. (line...)
6. No information can be communicated by radio waves without modulating
them. (line... )
7. In AM, the amplitude of the carrier wave is modulated according to the
amplitude of the modulating signal. (line...)
8. Stopping and starting the wave is the simplest method of modulation. (line...)
9. FM is better than AM because there is less interference. (line... )
10. Audio frequencies can be used to modulate high frequency radio waves,
which can then act as carriers of the audio - frequency signal.
Task 7: Further reading
Radio frequency (AF) waves are used to carry audio frequency (AF) waves over
long distances through the air. The audio signals can be combined with the RF
carrier wave in such a way that it varies the amplitude of the carrier. This gives an
amplitude - modulated (AM) carrier wave (see Figure 4).
31
RF carrier
AF signal Modulated RF carrier
FIGURE 4
If frequency-modulated (FM) wave, the audio signal is combined with the RF
carrier wave to vary the frequency of the carrier (see Figure 5).
FIGURE 5
The block diagram of a radio is shown in Figure 6 below. The tuner selects the
required RF wave from those picked up by the aerial. The selected RF wave is
amplified and passed to the detector, which separates the audio modulation from the
RM carrier wave. The audio frequency amplifier amplifies the audio signal to make
it strong enough to drive the loudspeaker.
FIGURE 6
32
AF signal
RF carrier
FM carrier
RF tuner
RFAmplifier
AFamplifie
r
Detector ordemodulator
AF power
amplifier
aerial
ModulatedRF carrier
AmplifiedModulatedRF carrier
AF
Amplified AF
Loud speaker
TunerA typical radio tuner circuit consists of an inductor and capacitor connected in
parallel (see Figure 7). The size of the aerial inductance coil can be kept small by
winding it on a ferrite rod core.
FIGURE 7
The RF waves fed to the tuner cause the circuit to oscillate. The impedance of the
circuit is smallest and the oscillation is greatest at a particular frequency known as
the resonant frequency. This frequency is determined by the values of the
inductance and the capacitance. By using a variable capacitor, the circuit can be
tuned to the required radio frequency, and selected RF wave passed on to the RF
amplifier.
33
C
Aerial
UNIT 7Semiconductors
Task 1: Match the words in column A with the definitions in column B
A
1. bias
2. crystal
3. induction
4. leakage current
5. metal detector
B
a. clear transparent colorless mineral
b. electronic device for indicating the presence of
objects under the ground
c. unwanted current in a transistor
d. production of an electric or magnetic effect at a disk
e. apply a Dc voltage to a component to control its
operating point
Task 2: Read this passage and find a sentence in which is similar in meaning to
each of the following sentences
Figure 1
If two crystals of semiconductor material, one of p-type and one of n-type are
joined together a pn junction is formed. This junction can be used as a rectifier and
is known as a pn junction diode.
Figure 1 illustrates what happens when a voltage is applied across a silicon
pn junction diode. The first quadrant of the graph shows the characteristics of the
34
Reverse voltage 6.0 4.0
2.0 1.0 2.0 Forward
voltage
Forw
ard
cu
rren
t
Reverse
cu
rren
t
diode when the source is connected with the positive to the p-side of the junction
and the negative to the n-side. In the other words, the diode is forward biased. With
forward bias, the current at first increased slowly. When the applied voltage reaches
about 600 mV, the current rises rapidly. The diode is then a good conductor. The
current will continue to rise with increased voltage but eventually a point will be
reached where the diode is destroyed by heat.
The third quadrant shows the characteristics when the source is connected
with the positive the n- side and the negative to the p-side. When the diode is
reverse biased, there is almost no current flow. The junction is therefore a good
rectifier it conducts well in one direction and not all in the other. However, there is
a small reverse leakage current, this leakage current remains substantially constant
until what is known as breakdown voltage (Vb) is reached. At this point, there is a
sharp increase in the reverse current. This sudden increase in current is called the
Zener effect.
Normal diodes are never operated in the breakdown region but Zener diode is
designed to make use of the breakdown phenomenon. Because any slight increase in
voltage beyond the breakdown point causes a large increase in current. Zener diodes
are often used as a kind of overspill to protect sensitive circuit from fluctuations in
the power supply.
1. The positive of the source is connected to the p-side of the diode and the
negative to the n-side.
2. When a forward voltage is applied across the diode, there is, at first, only a
slow rise in current.
3. The diode allows current to flow freely
4. If a reverse voltage is applied to the diode, it conducts badly
5. There is almost no change in leakage current until the reverse voltage
reaches breakdown point.
Task 3: Meaning from contextSelect a word from three alternatives given which is most similar in meaning to
the word in italics as it is used in the passage in task 2:
1. characteristics (line 5)
a. typical behavior
4. phenomenon (line 20)
a. voltage
35
b. voltage figures
c. graph
b. effect
c. result
3. sharp (line 17)
a. slight
b. steep
c. cutting
5. fluctuations (line 22)
a. rises and falls
b. increases
c. failures
2. substantially (line 16)
a. almost
b. greatly
c. hardly
Task 4: Complete this description of the current-voltage characteristics of
silicon diode. Use the passage in task 2 and Figure 1 to help you.
At first, when a forward voltage is applied, …………………. When the
forward voltage has reached about 600 mV, …………………….. If the forward
voltage is further increased,…………………… . …….………… only a very
small leakage current flows. When the breakdown voltage is reached
………………… After the breakdown point, any further increase in reverse
voltage causes ………………………
Task 5: Checking facts and ideas
Decide if these statements are true or false. Quote from the passage in task 2 to
support your decisions.
1. The first quadrant of the graph shows the characteristics of the diode in
forward bias.
2. For forward voltages over 600mV, the diode conducts well.
3. When the source is connected with the negative to the n-side and the positive
to the p-side, the diode is reverse biased.
4. When a reverse voltage is first applied, a diode conducts badly.
5. Zener diodes are never used beyond breakdown point.
36
Task 6: Writing description from graphs
This graph describes the characteristics of a tunnel diode. Use the information it
contains to complete the description below:
As the forward voltage is increased, the current (1).......................until point
P is reached. P is as known as the peak point. The peak voltage for a germanium
tunnel diode is about (2) ............................ After P the current (3) ................. until
V. V is known as the valley point. From P to V the diode has a negative resistance.
The forward voltage at V is about (4) ....…........ After the volley point, the current
(5) ............…........... with increased voltage and the diode behaves like a normal
diode. When a reverse voltage is applied however, the reverse current
(6)........................... unlike normal diodes.
Task 7: Making predictions.
A prediction is a statement about a particular subject in which we say what we
think will happen in the future. Predictions are not always absolute, but can be
expressed with different levels of certainly, according to the context in which they
are made.
1. Certainty can be expressed by:
will (definitely, certainly)
certain, sure
without doubt, without question
37
Reverse voltage
0.2 0.1
0.1 0.2 0.3 0.4 0.5 0.6
Forward voltage
Forw
ard
cu
rren
t
Reverse
cu
rren
t
1.21.00.80.60.40.2
0.20.40.60.8
2. Probability can be expressed by:
probable, probably, likely
most/highly probable, most probably
most/highly likely
3. Probability can be expressed by:
may (not), might (not), can, could
possible, possibly, perhaps
4. Improbability can be expressed by:
improbable, unlikely
doubtful, questionable
probably not
most/ highly improbable/ unlikely
most/ highly doubtful, questionable
most probably not
5. Impossibility probably not
present or future past
can not, could not could not
not possible, impossible not possible, impossible
These expressions are used in sentences in different ways:
EXAMPLES
1. Computers will definitely be cheaper in the coming time.
2. It is (highly) probable / likely that computers will be cheaper in the coming time.
3. Computers MAY/ MIGHT be cheaper in the coming time.
4. PERHAPS, computers will be cheaper in the coming time.
5. It is unlikely/ doubtful that computers will be cheaper in the coming
time.
6. Computers will most probably not be cheaper in the coming time.
7. Computers will definitely not be cheaper in the coming time.
8. It is impossible that computers will be cheaper in the coming time.
Sometimes predictions are made subject to certain conditions. In such cases,
sentences typically have to parts: the if- clause and the main clause.
38
EXAMPLES
1. If 120 V AC is applied across the transformer primary, a voltage of 12 V will be
induced in the secondary.
2. If the switch is closed, the lamp will light.
When the if- clause comes second, there is no comma between the two clauses.
EXAMPLES
1. The bell will ring if the switch is pressed.
2. The motor will break down if we exceed the motor rating.
39
UNIT 8
Logic Circuit
Task 1: Match the words in column A with the definitions in
column B
A B
1. integrated circuit
2. memory
3. microprocessor
4. silicon chip
5. transistor
a. semiconductor component with three electrodes
(emitted base and collector) used for switching or
amplifying an electronic signal.
b. electronic circuit for storing information
c. very small piece of silicon carrying a complex electric
circuit.
d. electronic circuit containing many components on a
single silicon chip.
e. chip for controlling the device.
Task 2: Read this passage and answer the questions, which follow:
Logic gates are electronic switching devices. Figure 1 represents in simple
terms the function of one type of logic gate, the OR gate.
FIGURE 1
If switch A is closed, the output Z will equal the input. Similarly if B is
closed, or if both A and B are closed, the output and input will be equal. Any of
these three conditions will permit an output Z to flow.
40
B
A
Input
Output Z
Logic gates contain semiconductors, not mechanical switches, which can be
opened and closed. But they have only two levels of input and out put: a high level
and a low level. These correspond to the closed and open states of the switches in
Figure 1. The high level is presented by 1 and the low level by 0. All information in
digital systems is transmitted in terms of these two levels.
We can make a table to represent the out put value of an OR gate for all
possible combinations of inputs. Such a table is called a truth table. A truth table
can be made for any logic gate.
input output
A B Z
0
1
1
1
0
0
1
1
0
1
1
1
Figure 2
We can summarize this table by the formula Z = A+B where the symbol “+”
stands for OR
Other common digital devices are AND, NOR and NAND gates, and
inverters. AND gates will have an output of 1 only if 1 is present on all inputs. An
inverter is a device, which inverts its input. Thus, an input of 0 will have an output
of 4 and vice-versa. Complex circuits are made by combining these basic devices.
Their circuit symbols are as follows:
Figure 3
1. In what ways are logic gates like mechanical switches?
2. This circuit represents the function of an AND gate:
41
inverter AND NAND OR NOR
Complete this truth table for an AND gate:
A B Z
0 0 0
0 1 0
1 0 0
1 1 1
3.What will be the output of this circuit for an input of 1 at B and 0 at A and C
Task 3: Meaning from context
Select a word from the three alternatives give which is the
most similar in meaning to the word in italics as it is used in
the passage in task 2.
1. terms (line 1)
(a) ways
(b) methods
(c) words
2. levels (line 7)
(a) values
(b) classes
(c) grades
3. stands for (line 14)
(a) represents
(b) means
(c) symbolizes
4. vice-versa (line 17)
(a) opposite
(b) against
(c) inversely
42
input
Output Z
ABC
Z
Task 4: Describing the operation of a burglar alarm. Start your description like this:
The relay is set by closing the main switch and press the reset button. This allows
the current from the battery…
Task 5: Further reading
The decision - making circuits used in modern computers are mainly
composed of combinations of digital switching circuits known as logic gates.
The output of each gate depends on the combination of its inputs. This is
known as combinational logic. The output for all possible inputs is shown using a
truth table. The truth tables show that the output of an AND gate is only high (i.e.
logic level 1) when all its inputs are high. The output of a NOT gate (also known as
an inverter) is always the opposite of its input.
43
Bell
Contacts on closed door and windows
Resetbutton Main
switch
1314 12 11 10 9 8
21 3 4 5 6 7
GND
Vcc 4B 4A 4Y 3Y 3B 3A
Quad 2 input NAND gates TTL 7400 (CMOS 4011
Computers use ICs, which contain a number of logic gates on one chip. An
IC pin-out diagram shows the arrangement of the gates and the function of each pin
on the chip.
The number of ICs used in a computer, i.e. the chip count, can be reduced by
connecting NAND gates together to form other types of gates as follow:
Its scale of integration as shown in Table1 indicates the number of
components in an IC. The IC shown there is an SSI device but microprocessors used
in computers are VLSI or SLSI devices
Scale of integration Abbreviation No. of activecomponents
Small - scale integration
Medium - scale integration
Large - scale integration
Very large - scale integration
Super large - scale integration
SSI
MSI
LSI
VLSI
SLSI
1 to 10
10 to 102
102 to 103
103 to 10 4
104 to 105
TABLE 1
There are two common families of logic ICs used in computers. TTL
(transistor - transistor logic) ICs use bipolar transistors to form each gate whereas
CMOS (complementary metal oxide semiconductor) ICs use field effect transistors
(FETs). The different characteristics of each family determine which will be used in
a particular computer (see Table 2). For example, TTL ICs are used in large, high-
44
=
=
=
speed computers and CMOS ICs are better for battery - powered portable
computers.
Properties TTL CMOS
Supply voltage
Supply current
Power dissipation
Switching speed
Input impedance
+5V+0.25%
mA
mW
fast
low
+3 V to + 15 V
A
W
relatively slow
high
TABLE 2
1. What terms are used in the passage for each of the followings?
a. a digital switching circuit
b. the output of each gate depending on the combination of its input
c. the number of ICs used in a computer
d. an indication of the number of components used in an IC
2. What is shown by
a. a truth table? b. a pin - out diagram?
3. What is another name for a NOT gate?
4. What are the two common families of logic ICs?
5. What do these abbreviations stand for?
a. TTL b. VLSI c. CMOS d. MSI
6. Which of these statements are true for CMOS ICs?
a. They contain bipolar transistors.
b. They contain field effect transistors.
c. They are particularly suitable for use in battery - operated portable
computers.
d. They are particularly suitable for use in large, high - speed computers.
45
46
UNIT 9Transmission Lines
Task 1: Match the words in column A with the definitions in column B.
A B
1. cable
2. exchange
3. interference
4. laser
5. light emitting
diode
a. unwanted signals.
b. semiconductor which converts electrical energy into light
c. device giving strong beam of radiation in one direction.
d. switching center for switching signals from one telephone
line to another.
e. insulated wire or set of wires used for carrying electrical
current or signals.
Task 2: Read this passage and answer the questions, which follow.
Telecommunications involves the transmission of information, including
voice, data, TV, and radio over long distances. The transmission medium can be
free space (ground, space, and sky waves), or the information can be guided
between transmitters and receivers using transmission line cables of various kinds.
These include:
Parallel wires: This is the simplest type of transmission line consisting of a pair of
insulated copper wires running side- by-side and covered by a plastic sheath. It is
47
Parallel wire copper conductor
Wire insulator
prone to interference and is only used to carry information over small distances such
as telephone connections with a building.
Twisted pair: Two insulated copper wires are twisted together to reduce
interference effects and are enclosed in an insulating polyethylene sheath Because
the wires are twisted, unwanted stray signals picked up by one tend to be cancelled
by similar signals picked up by other. They are used for communications over
longer distances, for example to connect telephones to their local exchange.
Coaxial cable (coax)
Flexible coax has a copper wire core surrounded by copper braid. The core and
braid are insulated from each other by a dielectric material such as polyethylene and
covered by a PVC sheath.
The braid helps to screen the signals from interference. Coax can carry a
large number of signals over long distances at frequencies up to 1000MHz. It is
used to connect telephone exchanges and for cable television.
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Insulator sheath
insulator covering wire
Coppering wire conductor
DielectricPVC sheath
Inner conductor
Copper braid
Waveguide
Microwaves can be guided along rectangular copper ducts by a series of reflections
from the inner walls. The exact dimensions of the ducts are determined by the
frequency to be transmitted. Suitable frequencies are between 1 GHz and 300 GHz.
Waveguides are used to carry microwave signals between dish receivers.
Optical fibres
An inner core made from the very pure silica fibre is surrounded by a similar glass
sheath, known as cladding. This is covered by a protective plastic sheath. Non-
visible light from lasers or LEDs (light emitting diodes) can travel along the fibre
by reflection from the surface where the core and the cladding meet
Although the optical fibre has a smaller diameter than a human hair, it can be no
interference from other signals. Optical fibre cable can be used in corrosive
environment, and light, flexible and cheap. This type of cable is gradually replacing
conventional copper wire for connecting telephones and computer networks.
1. Why are wires sometimes twisted together in transmission lines?
2. What are the purposes of the dielectric material in coaxial cable?
3. What are the advantages of optical cable?
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Connecting flange
Rectangle wave guide
Power flow
Sidewall of waveguide
Sidewall of waveguide
Task 3: Meaning from context
Select a word from the three alternatives given which is the most similar in meaning
to the word in italics as it is used in the passage in task 2.
1. medium (line2)
b. line
c. means
d. relative
2. side by side (line 6)
a. together
b. close
c. in parallel
3. sheath (line 16)
a. coating
b. housing
c. cover
4. dimensions (line 22)
a. length
b. width
c. size
Task 4: Complete this table using information from the passage in task 2
Transmission line Component materials Examples of use
Telephone connections within buildings
twisted pairCopper wire, plastic insulation
Copper wire, copper braid, polythylene sheath
Trunk telephone lines
Copper ducts
Opical fibres
Task5: Further reading
Telephones, connected by a network of cables, are commonly used for two-
way transmission of speech. The signals are switched from one line to another at
switching center known as telephone exchanges. Lines in a small area are switched
by local exchanges are connected through trunk exchanges, and trunk exchanges are
connected to other countries by international exchanges. Such a system is called a
Public Switching telephone networks (PSTN)
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Modern digital telephone networks can use videophones to transmit video
images as well as speech. The telephone network is used by video-conferencing
services to interconnect small television studios. In this way, business people can
hold conferences at a distance.
Public telephone networks are used by many other data communications
services. One of the oldest is the telex system. This enables messages, typed on
Teletype terminals, to be automatically printed by distant teleprinters. Telex can
only transmit simple text containing capital letters and punctuation marks. It is also
slow, about 100 words per minute
A newer, more advances telex system, known as teletex, is also available.
This uses VDU (visual display unit) terminals to transmit a variety of text and
graphics characters. High quality formats can be used and it is much faster than
telex, operating at speeds up to 2600 words per minute
A facsimile (FAX) system allows the transmission of text, graphics and
photographic images.
Contacting people on the move is possible using radiopaging service. By carrying a
small radio receiver called radiopaper, people can be contacted wherever they are.
Keying their number in on a telephone causes the paper to beep. The person then
goes to the nearest telephone to get in touch with the caller.
Telephone network can be used to connect personal computers to an
electronic mail ( e-mail ) system. Messages sent from a personal computer are
stored on a central computer. User can read and reply to these messages using their
own computer.
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Local Trunk Trunk LocalInternational
exchangeInternational
exchange
Videotex or viewdata systems transmit papers of text and graphics through the
PSTN to be displayed on a viewdata terminal or a television screen. The data comes
from a central computer. It is an interactive system, allowing the user to send
messages back to the computer using a keyboard. The user can perform various
tasks from home such as ordering goods and controlling bank accounts.
A similar data communications service, known as teletext, uses the television
broadcasting system rather than the PSTN. Text and graphics are transmitted as part
of the television video signal. The user can switch between pages on the screen
using special keys on a remote control unit. Unlike viewdata, teletext is not
interactive but does provide a similarly wide variety of useful information, such as
news and travel information. Figure 9 shows a teletext screen.
An Integrated Services Digital Network (ISDN) is gradually being developed
which uses telephone networks with microwave links and satellite communications
to interconnect all types of data communications services throughout the world.
Task 6: Match the services with the given type of signal
Communications service Type of signal transmitted1. telephone
2. teletex
3. viewdata
4. radiopaging
5. fax
6. videophone
7. e-mail
8. teletext
9. telex
a. high - quality text, graphics characters
b. video, speech
c. simple text, punctuation
d. messages
e. radio signal beep
f. text, graphics, photographic images
g. speech
h. interactive information, e.g. travel,
shopping, banking.
i. general information, e.g. news, sports
results.
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UNIT 10Internet Ptotocol Version6 - Ipv6
Reading passage
1. Introduction
IPv6 (Internet Protocol version 6) is the latest revision of the Internet
Protocol (IP), the primary communications protocol upon which the entire Internet
is built. It is intended to replace the older IPv4, which is still employed for the vast
majority of Internet traffic as of 2012. IPv6 was developed by the Internet
Engineering Task Force (IETF) to deal with the long-anticipated problem of IPv4
running out of addresses.
Each device on the Internet, such as a computer or mobile telephone, must be
assigned an IP address, a number with a certain number of binary digits (each
presented by one bit), in order to communicate with other devices. With the ever-
increasing number of new devices being connected to the Internet, there is a need
for more addresses than IPv4 can accommodate. IPv6 uses 128-bit addresses,
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allowing for 2128, or approximately 3.4×1038 addresses - more than 7.9×1028 times as
many as IPv4, which uses 32-bit addresses. IPv4 allows for only 4,294,967,296
unique addresses worldwide (or fewer than one address per person alive in 2012),
but IPv6 allows for around 4.8×1028 addresses per person - a number unlikely to
ever run out. However, this means the two protocols are not compatible,
complicating the transition to IPv6. IPv6 addresses, as commonly displayed to
users, consist of eight groups of four hexadecimal digits separated by colons, for
example 2001:0db8:85a3:0042:1000:8a2e:0370:7334.
The deployment of IPv6 is accelerating, with a symbolic World IPv6 Launch
having taken place on 6 June 2012, in which major internet service providers,
especially in countries that had been lagging in IPv6 adoption, deployed IPv6
addresses to portions of their users. Data from Arbor Networks showed a peak of
0.2% of Internet traffic on IPv6 during the launch. As of late November 2012, IPv6
traffic share was reported to be approaching 1%.
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IPv4 addreses and IP v6 addresses
The main advantage of IPv6 over IPv4 is its larger address space. The length
of an IPv6 address is 128 bits, compared to 32 bits in IPv4. The address space
therefore has 2128 or approximately 3.4×1038 addresses. By comparison, this amounts
to approximately 4.8×1028 addresses for each of the seven billion people alive in
2011. In addition, the IPv4 address space is poorly allocated, with approximately
14% of all available addresses utilized. While these numbers are large, it wasn't the
intent of the designers of the IPv6 address space to assure geographical saturation
with usable addresses. Rather, the longer addresses simplify allocation of addresses,
enable efficient route aggregation, and allow implementation of special addressing
features. In IPv4, complex Classless Inter-Domain Routing (CIDR) methods were
developed to make the best use of the small address space. The standard size of a
subnet in IPv6 is 264 addresses, the square of the size of the entire IPv4 address
space. Thus, actual address space utilization rates will be small in IPv6, but network
management and routing efficiency is improved by the large subnet space and
hierarchical route aggregation.
Renumbering an existing network for a new connectivity provider with
different routing prefixes is a major effort with IPv4.[24][25] With IPv6, however,
changing the prefix announced by a few routers can in principle renumber an entire
network, since the host identifiers (the least-significant 64 bits of an address) can be
independently self-configured by a host.
2. Multicasting
Multicasting, the transmission of a packet to multiple destinations in a single
send operation, is part of the base specification in IPv6. In IPv4 this is an optional
although commonly implemented feature. IPv6 multicast addressing shares
common features and protocols with IPv4 multicast, but also provides changes and
improvements by eliminating the need for certain protocols. IPv6 does not
implement traditional IP broadcast, i.e. the transmission of a packet to all hosts on
the attached link using a special broadcast address, and therefore does not define
broadcast addresses. In IPv6, the same result can be achieved by sending a packet to
the link-local all nodes multicast group at address ff02::1, which is analogous to
IPv4 multicast to address 224.0.0.1. IPv6 also provides for new multicast
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implementations, including embedding rendezvous point addresses in an IPv6
multicast group address, which simplifies the deployment of inter-domain solutions.
In IPv4 it is very difficult for an organization to get even one globally
routable multicast group assignment, and the implementation of inter-domain
solutions is very arcane. Unicast address assignments by a local Internet registry
for IPv6 have at least a 64-bit routing prefix, yielding the smallest subnet size
available in IPv6 (also 64 bits). With such an assignment it is possible to embed the
unicast address prefix into the IPv6 multicast address format, while still providing a
32-bit block, the least significant bits of the address, or approximately 4.2 billion
multicast group identifiers. Thus each user of an IPv6 subnet automatically has
available a set of globally routable source-specific multicast groups for multicast
applications.
3. IPv4-mapped IPv6 addresses
Hybrid dual-stack IPv6/IPv4 implementations recognize a special class of
addresses, the IPv4-mapped IPv6 addresses. In these addresses, the first 80 bits are
zero, the next 16 bits are one, and the remaining 32 bits are the IPv4 address. You
may see these addresses with the first 96 bits written in the standard IPv6 format,
and the remaining 32 bits written in the customary dot-decimal notation of IPv4.
For example, ::ffff:192.0.2.128 represents the IPv4 address 192.0.2.128. A
deprecated format for IPv4-compatible IPv6 addresses was ::192.0.2.128.
Because of the significant internal differences between IPv4 and IPv6, some
of the lower-level functionality available to programmers in the IPv6 stack does not
work identically with IPv4-mapped addresses. Some common IPv6 stacks do not
implement the IPv4-mapped address feature, either because the IPv6 and IPv4
stacks are separate implementations (e.g., Microsoft Windows 2000, XP, and Server
2003), or because of security concerns (OpenBSD). On these operating systems, a
program must open a separate socket for each IP protocol it uses. On some systems,
e.g., the Linux kernel, NetBSD, and FreeBSD, this feature is controlled by the
socket option IPV6_V6ONLY, as specified in RFC 3493.
Task 1 Answer the following questions
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1. What is Ipv6?
2. What is the advantage of Ipv6 over Ipv4?
3. What is multicasting?
4. What are IPv4-mapped IPv6 addresses?
Task 2. Underline all the noun phrases in the reading passage and translate them
into Vietnamese.
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