0 TABLE OF CONTENT Unit Unit Unit 11 Unit 17 Unit 22 Unit 27 Grammar review .43 UNIT SEMICONDUCTORS Reading Task 1: Read this passage and find a sentence in which is similar in meaning to each of the Reverse voltage Forward current following sentences 6.0 4.0 2.0 1.0 2.0 Reverse current Forward voltage Figure 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 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 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 The positive of the source is connected to the p-side of the diode and the negative to the n-side When a forward voltage is applied across the diode, there is, at first, only a slow rise in current The diode allows current to flow freely If a reverse voltage is applied to the diode, it conducts badly There is almost no change in leakage current until the reverse voltage reaches breakdown point Task 2: Meaning from context Select a word from three alternatives given which is most similar in meaning to the word in italics as it is used in the reading passage: characteristics (line 5) substantially (line 16) a typical behavior a almost b voltage figures b greatly c graph c hardly sharp (line 17) phenomenon (line 20) a slight a voltage b steep b effect c cutting c result fluctuations (line 22) a rises and falls b increases c failures Task 3: Work in-group of three; complete this description of the current-voltage characteristics of silicon diode Use the reading passage and Figure 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 4: Checking facts and ideas Decide if these statements are true or false Quote from the reading passage to support your decisions The first quadrant of the graph shows the characteristics of the diode in forward bias For forward voltages over 600mV, the diode conducts well When the source is connected with the negative to the n-side and the positive to the pside, the diode is reverse biased When a reverse voltage is first applied, a diode conducts badly Zener diodes are never used beyond breakdown point Task 5: Writing description from graphs This graph describes the characteristics of a tunnel diode Use the information it contains to complete the description below: Forward current Reverse voltage P 0.2 0.1 V 0.1 0.2 0.3 0.4 0.5 0.6 Reverse current Forward voltage 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 Further Reading The transistor During and after World War II, electronic equipment became so important and so complex that a substitute for the vacuum tube was required for the continued growth of the field In a sense, the answer to the problem was found in the improvements in the use of crystals – vacuum tubes were replaced by transistors consisting basically of silicon Silicon is one of the substances classes as a semiconductor These substances have a conductivity somewhere between good conductors like silver, copper, and aluminum, and insulators like rubber, porcelain, and glass, which permit almost no movement of electrons at all Modern transistors are small pieces of semiconductor material such as silicon that have been treated with impurities so that they possess free electrons at room temperature Another material used is a metal called germanium Germanium, however, is very rare, whereas silicon is extremely common – it is the basic element in sand Another element, selenium, is used in lightsensitive transistors A number of chemical compounds have also been found suitable for use in different types of transistors The transistor was developed at Bell Laboratories in 1948 by three American scientists, John Bareen, Walter Brattain, and William Shockley The transistors created as a result of their work can perform most of the same tasks as vacuum tubes To make a transistor, controlled impurities are introduced into semiconductor material which has been refined to a high degree of purity Impurities such as arsenic, antimony, or phosphorous add readily available electrons, creating an n type of treated or “doped”, semiconducting material The elements aluminum and boron, on the other hand, create tiny vacant spots, or holes, that tend to capture the free electrons This kind of doped semiconducting material is called the p type When n and p types of semiconductors are placed together, current flowing through them is rectified because the electrons can flow only from n to p In this way, the transistor is similar to the diode vacuum tube, but unlike the diode, it has a three-part sandwich like structure There are three terminals, called the emitter, the collector, and the base The base forms the middle part of the sandwich The first type of transistor developed for widespread use has an npn structure, the base being made of p-type semiconductor The other is called pnp Depending upon how they are connected in a circuit, the can perform a wide variety of functions, including those of amplifiers, switches, and oscillators Transistors overcame most of the disadvantages of the vacuum tube Since they were very small, they could be packed closely together – observe the difference in size between a radio with tubes and a transistor radio They could take rough handling, seldom burned out, did not overheat, required no warm-up time, and used less current There were other electronic advantages – they were inexpensive to manufacture and did not need nearly as much as wiring to connect them Transistors were first placed on boards or sheets of insulting materials and wired into a circuit Since then printed circuits have replaced complicated wiring systems in many cases In printed circuits, transistors are connected by thin lines of copper which aanre boned to the surface of an insulator Task 1: Decide whether the following statements are true or false (T/F) by referring to the information in the text Then make the necessary changes so that the false statements become true 1… Transistor was developed due to the disadvantage of the vacuum tube 2… Silicon is a better conductor than aluminum 3… Arsenic is an impurity which creates an n type of semiconducting material by adding readily available electrons 4… When n and p types of semiconductors are placed together, current can only flow through them in on direction 5… The transistor performs a lot of functions no matter how they are connected in a circuit Task 2: Filling in the spaces of the following sentences with the appropriate words or phrases, basing on the information in the text In the …………………… of treated semiconducting material, there are holes from which electrons have been removed A transistor has three terminals: the one connected to the middle part of the “sandwich” is the …………………… and the two outer parts are the ……………………… and the ………………… …………………….and ……………………… are two of the elements from which transistors are made One arrangement of semiconductors in transistors is ………………… and the other is …………………… …………………… is one of the impurities that can create n type semiconducting material …………………… is one of the impurities that can create p type semiconducting material Transistors perform a variety of functions including …………………… of amplifiers, switches, and oscillators UNIT PROPAGATION OF WAVES Reading 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 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 B A Figure 2a 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 2b A C B Figure 2b 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, 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 angel 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 use 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 1: Answer these questions about the passage List three types of propagation How ground wave travel? What is attenuation? Name two factors, which affect the attenuation of ground waves? What frequencies are propagated by space wave? Name two ways in which space waves travel What range have space wave? What controls sky wave propagation? 9 What is the ionosphere? 10 How 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 3: 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 reading passage varies with ( line ) This ( line 15 ) a is proportional to a the ionosphere b is directly proportional to b the atmosphere c is inversely proportional to c transmission approximately ( line 11 ) bounce ( line 20 ) a nearly a reflection b over b attenuation c around c receiving 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 45 e.g By the end of the next century, every student in every school in the world will have a Pc Yours: ……………… Money/Holidays e.g Most families will have videotext systems, with which they can shop and make financial transactions Cash will disappear Yours: Practice 4: Making prediction Expand these sentences using the future perfect tense In ten years' time/a lot of people/connect their television to the telephone line Portable computers/replace/, desktop computers/in a few years' time With the help of computers/ doctors/find/cure/ AIDS and cancer/by the year 2005 By this time next year/software manufacturers/make hundreds of new programs By 2020/post offices and bookshops/disappear By this time next year/ I/ buy/ pen computer Plans and intentions (future arrangement) Present continuous can be used to express a future arrangement Ex: We’re flying via New York Besides, there are many other kinds of future intentions Ex: I intend to buy ten telephone sets I aim to be there before o’clock We are going to reuse the shuttle about 100 times Details of Visit Leave Stockholm : 10.20 on Monday 17 September Length of stay : days Hotel : Hotel Bristol, Geneva Date of return : 20 September at 19.15 Routine tasks Review previous meeting’s minutes Present actions taken since last meeting Assign tasks to various members of the group 46 Special objectives Discuss frequency distribution in the 11.7 to 12.5 GHz band Have a meeting with the Nordic delegates 10 Present latest details of Tele-X project Practice 5: A Ask Mr Lei about his fixed plans (1 - 4), the routine tasks he’ll be performing (5 - 7) and anything special he intends to (8 - 10) Ex: When are you leaving Stockholm? B Now you are Mr Lei Answer the questions in Practice 2, A Ex: I’m leaving Stockholm (at 10.20) on Monday 17 September Review 2: The use of modals Look at the following examples: a The CPU can be located anywhere in the network b LANs ought to be designed on a fiber optic system c LANs cannot be tapped d Fibre optics should offer higher bit rates Practice 1: Decide what ideas the words on bold type express Use the table below Ideas expressed Possibility Probability Permission Obligation Advice/recommendation Ability Modal verb Can/could/may/might Should/ought to May/can Must/have to Should/ ought to Can/could/be able to Practice 2: Complete the sentences below with appropriate modal verbs e The report …….be finished by o’clock (Obligation) f Your telephone …… be installed by Friday (Probability) g This equipment …… be exported to certain countries (Permission) h You …… wear protective clothing at all times (Advice) 47 i We … not deliver until we receive the parts (Ability) Practice 3: Continue completing the sentences below Also indicate the idea expressed a A LAN …….be installed in most modern offices (… ) b She doesn’t look very well She …… stay in bed (……) c Optical fibre …… come down in price as production expands.(….) d You …… only export this electronic equipment if you get a speacial licence.(… ) e According to the timetable, the plane …… land in 20 minutes (… ) Forecasting with modal auxiliaries When we talk about the future, we indicate the relative probability of events happening as follows: Certainty More advanced systems will be introduced It won’t (will not) work out more than the existing telex You will be getting a lot of new facilities Probability All the signs indicate that… It should be possible… The cost is likely to be… We ought to be able to install…… It’ll probably be or weeks… Possibility You might be interested… That could be useful…… Practice 1: Now use the table below to complete sentences d to j; a to c have been done for you as examples Installation Schedule for Extension Telephones Type of telephone Special fast-rate DIAVOX AMBASSADOR MICKEY MOUSE 3-5 days* 3-5 days 10-15 days Business customers Residential customers 2-4 weeks 5-10 weeks 3-5 weeks 4-6 weeks …… 10-12 weeks 48 REGENCY 3-5 days 4-6 weeks 5-7 weeks * in all cases: first figure (3) represents a minimum waiting time for installation large figure (5) represents a maximum waiting time for installation Possibility a If you need one in a hurry, we could deliver a Diavox in days, but you will have to pay £15 extra Probability b For a business customer we should/ought to be able to deliver an Ambassador in weeks Certainty c For a residential customer, we will install a Mickey Mouse extension phone in 12 weeks d If you need an Ambassador soon, we………deliver one in days e For a business customer, we … deliver a Regency extension in weeks f It’s …….be weeks before we can install a Regency extension for a residential subscriber g We…….be able to deliver a Mickey Mouse extension set before 10 weeks h All…… that delivery time will be around or weeks for the Diavox if you are a residential subscriber i We…… be able to install the Mickey Mouse in your house in 11 weeks time j You………receive the new Ambassador before weeks if you are a residential customer Review 3: Comparison The comparative and superlative of one-syllable a adjectives ending in “y” are formed like this: Adjectives comparative superlative Great Greater The greatest Noisy Noisier The noisiest With most adjectives of two or more syllables, we use more and most: Profitable More The most profitable Profitable Note the following important irregular comparatives and superlatives: Good Better The best 49 Bad Far Little Many /much Worse Further Less more The worse The furthest The least The most Note: how we use as ……as… He’s not as stupid as he looks We have as much time as we need Practice 1: Now fill in the blanks in the following sentences a In my opinion, Bill is … inventor of all time (great) b A Strowger exchange is …… a crossbar central office (noisy) c A Strowger exchange is not …… crossbar (reliable) d Your English accent is … hers (bad) e Bell made a lot of inventions, but the telephone was ……(important) f After inventing the automatic exchange, Strowger found that his business became ……(profitable) g A digital exchange works … his brother (fast) h He speaks English …… his brother (good) Review 4: Quantity and Amount When we talk about quantity and amount, it is important to know if what we are talking about is countable or uncountable Look at the table below Countable Uncountable Calls News Companies Equipment People Information Telephone sets Knowledge Use the words below to express quantity and amount: Approx Countable 100% Every/all 85% Most Many/a lot of Some Several A few Uncountable All Most Much/a lot of Some -a little 50 5% 0% Few No/none little no/none If you need to count something which is uncountable, you must add a countable noun Four pieces of equipment One type of information Practice: Use the table to help you fill in the blanks in the sentences below: Year Calls going via satellite Calls going via sub-ocean cable a In 1960 1980 2000 2050 2100 I… ii… iii… iv… V…… b In 1960 1980 2000 2050 2100 I… ii… iii… iv… V…… 1960 0% 100% 1980 10% 90% 2000 70% 30% 2050 90% 10% 2100 100% 0% Calls Were Will be Transmitted by satellite Information Was Will be Transmitted by satellite c Now choose the correct answer: i, By the year 2000 a lot of / much digital equipment will be used ii, Little / Few information was given about that much / piece of equipment iii, The switchboard received a little / a few calls about the future of satellites iv, A few /Alittle / Some good weather can normally be expected in England during August v, Many /Much people think that a few / a little knowledge is a good thing vi, Many / A piece of news came in concerning a lot of / much /many engineers 51 Review 5: Prepositions of location The following diagrams will help to make clear which preposition you should use: to at Out of across from in into past on Look at the map and then fill in the blanks in the exercise that follows Greenwood Bear Inn Slingdon PN’s car A Golf course Thurby Nutley b c d e f g h i He’s ….the A217 He’s coming… Nultey He’s going ….Greenwood He’ll go ….to Thurby He’s just gone … the windmill When he’s…….Slingdon, he’ll turn right… Thurby Just after he’s gone … Slingdon, he’ll see the Bear Inn When he comes … Slingdon, he’ll see the golf course … his right towards 52 Review 6: Questions Direct questions vs polite questions Notice the difference between direct questions and polite questions in the examples below: Direct: Is she coming tomorrow? When is she coming? Polite: Could you tell me when she’s coming, please? Would you mind telling me what the number is? Now read the following dialogue and put in suitable questions The information after each blank will help you decide what the question should be In the dialogue, Chric Bradley (CB) is talking on the phone to Tom Davies (TD) about Bradfield Electronics’ radio-paging system CB: Good morning Bradfield Electronics Research Division TD: 1………… CB: Chris Bradley speaking TD: Hello Chris It’s Tom Davies from Swanford Tools here CB: 2…… TD: I’m very well thanks I’m phoning about your latest radio-paging system I’d like a bit more information CB: 3…… TD: There’s quite a lot I’d like to know, actually CB: OK, off you go! TD: 4…… CB: No, very little Just a small device called “a pager” or “bleeper” TD: 5…… CB: In his pocket, because it’s very small TD: 6…… CB: Oh, about the size of a packet of cigarettes TD: 7…… CB: He’s simply telephoned in the normal way and the pager bleeps TD: 8…… CB: Nothing It’s free of charge TD: 9…… CB: No, it doesn’t The user must remain at or above ground lever TD: 10…… CB: Not at all The quarterly rental’s only about £30 TD: 11…… CB: No problem at all We’ve got a large amount in stock 53 TD: 12……… CB: No, fine We could organize a demonstration next week Would next Thursday at 2p.m be alright for you? TD: 13……… CB: i, Good ii, (14)…….? TD: No, there’ll be several other people with me, mainly from the Sales Department CB: 15…… TD: of course It’s 53260 Extension 208 CB: Good I look forward to seeing you next Thursday at p.m TD: 16……… CB: Yes Come straight up My office is on the second floor TD: Fine Goodbye Chris CB: Bye Tom Tag questions Structure: Tag questions: Statement, + tag? Classification: Tag question is a short question, added to the end of the positive or negative statements Tag question is divided into two types, depending on the purposes If the tag is not a real question it has a flat or falling intonation (A) If the tag is a real question it has a rising intonation (B) Ex: A The E 10 was one of the earliest TDM systems, wasn’t it? B I don’t suppose the subscriber’s line uses PCM, does it? •The verb in the statement should be the same tense as the verb in the tag Normally a positive statement is followed by a negative tag, and a negative statement is followed by a positive tag Ex: It must be an advantage to have greater capacity, mustn’t it? They didn’t call us yesterday, did they? • If a modal (can, could, will, should, etc.) is used in the statement, then the same modal is used in the tag part If the statement doesn't use an auxiliary verb, then the auxiliary is used in the tag part Ex: Those exchanges seem to be rather expensive, don’t they? Practice: End the following sentences with the correct form of tag A new electronic exchange has just been installed,…………………… ? 2.The processor is the central component of a computer system,………………….? Computers have changed a lot our working conditions,……………………… ? 54 The PAM sampling unit samples the signals, ………………………? 5.A Strowger exchange isn’t as noisy as a crossbar central office,………………? The telephone company handling this function is called the local exchange carrier,………………? In days of old there was only one long-distance carrier -AT&T, ………………? A wide-area network (WAN) links metropolitan or local networks, usually over common carrier facilities,……………………….? A synchronous optical network (SONET) usually embodies a fiber-optic ring that will permit transmission in both directions,…………………………? 10 The common channel signaling network works closely with the PSTN, … ……………? Review 7: Classification The network The local network is composed of consists of comprises is made up of parts can be /divided into can be /is broken down into can be /separated into parts A + B There are two exchanges Both are traditional electromechanical exchanges A B Two SPC exchanges are installed; one to extend the central area, the other to serve the new area A 70% of traffic is routed direct from one GSC to another GSC + B 55 Use the map to complete the sentences below Northern Area Sisley Central Area Southern Area Fig 1: The Newtown local telephone area a) b) c) d) The Newtown telephone network ……….2 secondary centres and……… The Area ………….into sub-areas; North………… There ……… secondary centres………are connected to the DSC in Sisley by radio link There ……junction routes between the secondary centres……….direct …… via a local exchange e) Routing from……local exchange to …….can be direct or via ……exchanges f) …… Northern and Southern areas are served by …… local exchanges Review 8: Process description Look at the following short descriptions of processes: - The height of the pulse is measured - and (it is) given a binary code - each code is then transmitted as the train of pulses - First, by selecting suitable modulation frequencies - the conversations can be placed next to each other - Then, they are transmitted as a group to the receive side - Where a similar station demodulates the signals - And extracts the various conversations - Before they are transmitted to another exchange Notice two features: 56 the use of the present simple (active or passive) Ex: is measured, are transmitted, extracts the use of sequence markers Ex: first, then, before etc Describe the following processes in a similar way: a) - A subscriber A/lift/the handset Wait for /the dialing tone Dial /the number Hear /the ring tone Subscriber B /pick up/the receiver Conversation /take place The two subscribers /replace the handsets - The handset /lift a signal /send /the exchange The exchange /send /a dialing tone The number /dial Subscriber B number /select /in the exchange Subscriber A number /connect / subscriber B number Conversation /take place Handsets /replace Connection /break b) Review 9: Explaining relationships The greater the number of channels, the larger the number of repeaters No of channels No of repeaters 57 The greater the diameter of the parabola in the relation to the ware length, the higher the degree of directivity Diameter or parabola Degree of directivity Note that when taking about “degree” or “frequency” we normally use “higher” or “lower” Frequency No of channels Number of samples/sec Cost of transmission a… b… Degree of automation Degree of network digitalisation No of manual operators No of exchanges d… c… Now, make similar sentences for the following relationships: 4000 Frequency in Hz 3000 2000 2000 4000 6000 8000 Sampling rate /sec 58 Ratio 2:1 The sampling rate is double two times twice Ratio 1:2 The frequency is A half of half Ratio Treble 3:1 Three times 1:3 One/a third of 4:1 Four times One/a quarter of 1:4 A fourth (Am.) 5:1 Five times 1:5 One/ a fifth of The frequency The sampling rate Now make similar sentences for the following relationships: b Return on investment $20,000: original investment $5000 c New system 48 channels: old system 24 channels d Cable costs 1985 $1500 per kilometre: cable costs 1980 $300 per kilometre e New distance between repeaters km: old distance between repeaters km Review 10: Definition Look at the way we can put together a definition of a product or service Name Teletex Classification International service Characteristics/Function It enables subscribers to exchange correspondence on an automatic memory to memory basis 59 Teletex is an international service enabling subscriber to exchange correspondence on an automatic memory to memory basis Use the table below to put together similar definitions: Name a Videotex Classification Means of transmission b Lasers Service c PCM Circuit switched network d Electronic mail e Telefax New service f CCS Packet switched network g SPS Electronic system h Datex Light sources Method of signaling i Remote New service concentrators Remote part of an exchange j Nordic Public Data network Characteristics/Function It provides a means of synchronous data transmission It uses one pair for performing all signaling functions of one route It involves the sampling and coding of analogue signals They enable PCM transmission to be used further out in the local network They produce light of a closely defined wavelength It enables a subscriber to obtain alphanumeric and/or graphic information over the PSTN It uses data processing and computer techniques for switching It enables videotex subscribers to send messages directly and instantaneously to one another It provides a means of asynchronous data transmission It provides for the transmission and reproduction of still pictures and printed matter