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

7

f. The rotor is _______the shafte. The antenna is ______the coil

N

S

1

2

3

4 5

6

24

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

13

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

14

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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