First Edition, 2007 ISBN 978 81 89940 49 2 All rights reserved. Published by: Global Media 1819, Bhagirath Palace, Chandni Chowk, Delhi-110 006 Email: globalmedia@dkpd.com

Table of Contents

1. General Theories

2. Resistance, Capacitance, Inductances

3. Time Constants

4. Telecommunication

5. Electrical Engineering

6. Power Engineering

7. Photodiode

8. Photomultiplier

9. Digital Circuit

10. Boolean Algebra

11. Logic Analyzer

12. Logic Gate

13. Programmable Logic Device

14. Reconfigurable Computing

15. Analogue Electronics

16. Artificial Intelligence

17. Control System

18. Control Theory

19. Control Engineering

20. Programmable Logic Controller

21. Building Automation

22. HVAC Control System

23. Signal Processing

24. LTI System Theory

25. Fourier Transform

26. Signal (Electrical Engineering)

Notation The library uses the symbol font for some of the notation and formulae. If the symbols for the letters alpha beta delta do not appear here [ ] then the symbol font needs to be installed before all notation and formulae will be displayed correctly. E GI R P

voltage sourceconductance current resistance power

[volts, V] [siemens, S][amps, A] [ohms, ] [watts]

VXYZ

voltage dropreactance admittanceimpedance

[volts, V] [ohms, ] [siemens, S][ohms, ]

Ohms Law When an applied voltage E causes a current I to flow through an impedance Z, the value of the impedance Z is equal to the voltage E divided by the current I. Impedance = Voltage / Current Z = E / I Similarly, when a voltage E is applied across an impedance Z, the resulting current I through the impedance is equal to the voltage E divided by the impedance Z. Current = Voltage / Impedance I = E / Z Similarly, when a current I is passed through an impedance Z, the resulting voltage drop V across the impedance is equal to the current I multiplied by the impedance Z. Voltage = Current * Impedance V = IZ Alternatively, using admittance Y which is the reciprocal of impedance Z:

Voltage = Current / Admittance V = I / Y

Kirchhoffs Laws Kirchhoffs Current Law At any instant the sum of all the currents flowing into any circuit node is equal to the sum of all the currents flowing out of that node: Iin = Iout Similarly, at any instant the algebraic sum of all the currents at any circuit node is zero: I = 0 Kirchhoffs Voltage Law At any instant the sum of all the voltage sources in any closed circuit is equal to the sum of all the voltage drops in that circuit: E = IZ Similarly, at any instant the algebraic sum of all the voltages around any closed circuit is zero: E - IZ = 0

Thvenins Theorem Any linear voltage network which may be viewed from two terminals can be replaced by a voltage-source equivalent circuit comprising a single voltage source E and a single series impedance Z. The voltage E is the open-circuit voltage between the two terminals and the impedance Z is the impedance of the network viewed from the terminals with all voltage sources replaced by their internal impedances.

Nortons Theorem Any linear current network which may be viewed from two terminals can be replaced by a current-source equivalent circuit comprising a single current source I and a single shunt admittance Y. The current I is the short-circuit current between the two terminals and the admittance Y is the admittance of the network viewed from the terminals with all current sources replaced by their internal admittances.

Thvenin and Norton Equivalence The open circuit, short circuit and load conditions of the Thvenin model are: Voc = E Isc = E / Z Vload = E - IloadZ Iload = E / (Z + Zload) The open circuit, short circuit and load conditions of the Norton model are: Voc = I / Y Isc = I Vload = I / (Y + Yload) Iload = I - VloadY

Thvenin model from Norton model Voltage = Current / Admittance Impedance = 1 / Admittance

E = I / Y Z = Y -1

Norton model from Thvenin model

Current = Voltage / Impedance Admittance = 1 / Impedance

I = E / Z Y = Z -1

When performing network reduction for a Thvenin or Norton model, note that: - nodes with zero voltage difference may be short-circuited with no effect on the network current distribution, - branches carrying zero current may be open-circuited with no effect on the network voltage distribution.

Superposition Theorem In a linear network with multiple voltage sources, the current in any branch is the sum of the currents which would flow in that branch due to each voltage source acting alone with all other voltage sources replaced by their internal impedances.

Reciprocity Theorem If a voltage source E acting in one branch of a network causes a current I to flow in another branch of the network, then the same voltage source E acting in the second branch would cause an identical current I to flow in the first branch.

Compensation Theorem If the impedance Z of a branch in a network in which a current I flows is changed by a finite amount Z, then the change in the currents in all other branches of the network may be calculated by inserting a voltage source of -IZ into that branch with all other voltage sources replaced by their internal impedances.

Millmans Theorem (Parallel Generator Theorem) If any number of admittances Y1, Y2, Y3, ... meet at a common point P, and the voltages from another point N to the free ends of these admittances are E1, E2, E3, ... then the voltage between points P and N is: VPN = (E1Y1 + E2Y2 + E3Y3 + ...) / (Y1 + Y2 + Y3 + ...) VPN = EY / Y The short-circuit currents available between points P and N due to each of the voltages E1, E2, E3, ... acting through the respective admitances Y1, Y2, Y3, ... are E1Y1, E2Y2, E3Y3, ... so the voltage between points P and N may be expressed as: VPN = Isc / Y

Joules Law When a current I is passed through a resistance R, the resulting power P dissipated in the resistance is equal to the square of the current I multiplied by the resistance R: P = I2R By substitution using Ohms Law for the corresponding voltage drop V (= IR) across the resistance: P = V2 / R = VI = I2R

Maximum Power Transfer Theorem When the impedance of a load connected to a power source is varied from open-circuit to short-circuit, the power absorbed by the load has a maximum value at a load impedance which is dependent on the impedance of the power source.

Note that power is zero for an open-circuit (zero current) and for a short-circuit (zero voltage).

Voltage Source When a load resistance RT is connected to a voltage source ES with series resistance RS, maximum power transfer to the load occurs when RT is equal to RS. Under maximum power transfer conditions, the load resistance RT, load voltage VT, load current IT and load power PT are: RT = RS VT = ES / 2 IT = VT / RT = ES / 2RS PT = VT2 / RT = ES2 / 4RS Current Source When a load conductance GT is connected to a current source IS with shunt conductance GS, maximum power transfer to the load occurs when GT is equal to GS. Under maximum power transfer conditions, the load conductance GT, load current IT, load voltage VT and load power PT are: GT = GS IT = IS / 2 VT = IT / GT = IS / 2GS PT = IT2 / GT = IS2 / 4GS Complex Impedances When a load impedance ZT (comprising variable resistance RT and variable reactance XT) is connected to an alternating voltage source ES with series impedance ZS (comprising resistance RS and reactance XS), maximum power transfer to the load occurs when ZT is equal to ZS* (the complex conjugate of ZS) such that RT and RS are equal and XT and XS are equal in magnitude but of opposite sign (one inductive and the other capacitive).

When a load impedance ZT (comprising variable resistance RT and constant reactance XT) is connected to an alternating voltage source ES with series impedance ZS (comprising resistance RS and reactance XS), maximum power transfer to the load occurs when RT is equal to the magnitude of the impedance comprising ZS in series with XT: RT = |ZS + XT| = (RS2 + (XS + XT)2) Note that if XT is zero, maximum power transfer occurs when RT is equal to the magnitude of ZS: RT = |ZS| = (RS2 + XS2) When a load impedance ZT with variable magnitude and constant phase angle (constant power factor) is connected to an alternating voltage source ES with series impedance ZS, maximum power transfer to the load occurs when the magnitude of ZT is equal to the

magnitude of ZS: (RT2 + XT2) = |ZT| = |ZS| = (RS2 + XS2)

Kennellys Star-Delta Transformation A star network of three impedances ZAN, ZBN and ZCN connected together at common node N can be transformed into a delta network of three impedances ZAB, ZBC and ZCA by the following equations: ZAB = ZAN + ZBN + (ZANZBN / ZCN) = (ZANZBN + ZBNZCN + ZCNZAN) / ZCN ZBC = ZBN + ZCN + (ZBNZCN / ZAN) = (ZANZBN + ZBNZCN + ZCNZAN) / ZAN ZCA = ZCN + ZAN + (ZCNZAN / ZBN) = (ZANZBN + ZBNZCN + ZCNZAN) / ZBN Similarly, using admittances: YAB = YANYBN / (YAN + YBN + YCN) YBC = YBNYCN / (YAN + YBN + YCN) YCA = YCNYAN / (YAN + YBN + YCN) In general terms: Zdelta = (sum of Zstar pair products) / (opposite Zstar) Ydelta = (adjacent Ystar pair product) / (sum of Ystar)

Kennellys Delta-Star Transformation A delta network of three impedances ZAB, ZBC and ZCA can be transformed into a star network of three impedances ZAN, ZBN and ZCN connected together at common node N by the following equations: ZAN = ZCAZAB / (ZAB + ZBC + ZCA) ZBN = ZABZBC / (ZAB + ZBC + ZCA) ZCN = ZBCZCA / (ZAB + ZBC + ZCA) Similarly, using admittances: YAN = YCA + YAB + (YCAYAB / YBC) = (YABYBC + YBCYCA + YCAYAB) / YBC YBN = YAB + YBC + (YABYBC / YCA) = (YABYBC + YBCYCA + YCAYAB) / YCA YCN = YBC + YCA + (YBCYCA / YAB) = (YABYBC + YBCYCA + YCAYAB) / YAB In general terms: Zstar = (adjacent Zdelta pair product) / (sum of Zdelta) Ystar = (sum of Ydelta pair products) / (opposite Ydelta) Electrical Circuit Formulae

Notation The library uses the symbol font for some of the notation and formulae. If the symbols for the letters alpha beta delta do not appear here [ ] then the symbol font needs to be installed before all notation and formulae will be displayed correctly. C E

capacitance voltage source

[farads, F] [volts, V]

Qq

charge instantaneous Q

[coulombs, C][coulombs, C]

e G I i k L MN P

instantaneous E conductance current instantaneous I coefficient inductance mutual inductance number of turns power

[volts, V] [siemens, S][amps, A] [amps, A] [number] [henrys, H][henrys, H][number] [watts, W]

RTtVvW

resistance time constant instantaneous timevoltage drop instantaneous V energy magnetic flux magnetic linkageinstantaneous

[ohms, ] [seconds, s][seconds, s][volts, V] [volts, V] [joules, J] [webers, Wb][webers, Wb][webers, Wb]

Resistance The resistance R of a circuit is equal to the applied direct voltage E divided by the resulting steady current I: R = E / I

Resistances in Series When resistances R1, R2, R3, ... are connected in series, the total resistance RS is: RS = R1 + R2 + R3 + ...

Voltage Division by Series Resistances When a total voltage ES is applied across series connected resistances R1 and R2, the current IS which flows through the series circuit is: IS = ES / RS = ES / (R1 + R2) The voltages V1 and V2 which appear across the respective resistances R1 and R2 are: V1 = ISR1 = ESR1 / RS = ESR1 / (R1 + R2) V2 = ISR2 = ESR2 / RS = ESR2 / (R1 + R2) In general terms, for resistances R1, R2, R3, ... connected in series: IS = ES / RS = ES / (R1 + R2 + R3 + ...) Vn = ISRn = ESRn / RS = ESRn / (R1 + R2 + R3 + ...) Note that the highest voltage drop appears across the highest resistance.

Resistances in Parallel When resistances R1, R2, R3, ... are connected in parallel, the total resistance RP is: 1 / RP = 1 / R1 + 1 / R2 + 1 / R3 + ... Alternatively, when conductances G1, G2, G3, ... are connected in parallel, the total conductance GP is: GP = G1 + G2 + G3 + ... where Gn = 1 / Rn For two resistances R1 and R2 connected in parallel, the total resistance RP is: RP = R1R2 / (R1 + R2) RP = product / sum The resistance R2 to be connected in parallel with resistance R1 to give a total resistance RP is: R2 = R1RP / (R1 - RP) R2 = product / difference

Current Division by Parallel Resistances When a total current IP is passed through parallel connected resistances R1 and R2, the voltage VP which appears across the parallel circuit is: VP = IPRP = IPR1R2 / (R1 + R2) The currents I1 and I2 which pass through the respective resistances R1 and R2 are: I1 = VP / R1 = IPRP / R1 = IPR2 / (R1 + R2) I2 = VP / R2 = IPRP / R2 = IPR1 / (R1 + R2) In general terms, for resistances R1, R2, R3, ... (with conductances G1, G2, G3, ...) connected in parallel: VP = IPRP = IP / GP = IP / (G1 + G2 + G3 + ...) In = VP / Rn = VPGn = IPGn / GP = IPGn / (G1 + G2 + G3 + ...) where Gn = 1 / Rn Note that the highest current passes through the highest conductance (with the lowest resistance).

Capacitance When a voltage is applied to a circuit containing capacitance, current flows to accumulate charge in the capacitance: Q = idt = CV Alternatively, by differentiation with respect to time: dq/dt = i = C dv/dt Note that the rate of change of voltage has a polarity which opposes the flow of current.

The capacitance C of a circuit is equal to the charge divided by the voltage: C = Q / V = idt / V Alternatively, the capacitance C of a circuit is equal to the charging current divided by the rate of change of voltage: C = i / dv/dt = dq/dt / dv/dt = dq/dv

Capacitances in Series When capacitances C1, C2, C3, ... are connected in series, the total capacitance CS is: 1 / CS = 1 / C1 + 1 / C2 + 1 / C3 + ... For two capacitances C1 and C2 connected in series, the total capacitance CS is: CS = C1C2 / (C1 + C2) CS = product / sum

Voltage Division by Series Capacitances When a total voltage ES is applied to series connected capacitances C1 and C2, the charge QS which accumulates in the series circuit is: QS = iSdt = ESCS = ESC1C2 / (C1 + C2) The voltages V1 and V2 which appear across the respective capacitances C1 and C2 are: V1 = iSdt / C1 = ESCS / C1 = ESC2 / (C1 + C2) V2 = iSdt / C2 = ESCS / C2 = ESC1 / (C1 + C2) In general terms, for capacitances C1, C2, C3, ... connected in series: QS = iSdt = ESCS = ES / (1 / CS) = ES / (1 / C1 + 1 / C2 + 1 / C3 + ...) Vn = iSdt / Cn = ESCS / Cn = ES / Cn(1 / CS) = ES / Cn(1 / C1 + 1 / C2 + 1 / C3 + ...) Note that the highest voltage appears across the lowest capacitance.

Capacitances in Parallel When capacitances C1, C2, C3, ... are connected in parallel, the total capacitance CP is: CP = C1 + C2 + C3 + ...

Charge Division by Parallel Capacitances When a voltage EP is applied to parallel connected capacitances C1 and C2, the charge QP which accumulates in the parallel circuit is: QP = iPdt = EPCP = EP(C1 + C2) The charges Q1 and Q2 which accumulate in the respective capacitances C1 and C2 are: Q1 = i1dt = EPC1 = QPC1 / CP = QPC1 / (C1 + C2) Q2 = i2dt = EPC2 = QPC2 / CP = QPC2 / (C1 + C2) In general terms, for capacitances C1, C2, C3, ... connected in parallel: QP = iPdt = EPCP = EP(C1 + C2 + C3 + ...) Qn = indt = EPCn = QPCn / CP = QPCn / (C1 + C2 + C3 + ...) Note that the highest charge accumulates in the highest capacitance.

Inductance When the current changes in a circuit containing inductance, the magnetic linkage changes and induces a voltage in the inductance: d/dt = e = L di/dt Note that the induced voltage has a polarity which opposes the rate of change of current.

Alternatively, by integration with respect to time: = edt = LI The inductance L of a circuit is equal to the induced voltage divided by the rate of change of current: L = e / di/dt = d/dt / di/dt = d/di Alternatively, the inductance L of a circuit is equal to the magnetic linkage divided by the current: L = / I Note that the magnetic linkage is equal to the product of the number of turns N and the magnetic flux : = N = LI

Mutual Inductance The mutual inductance M of two coupled inductances L1 and L2 is equal to the mutually induced voltage in one inductance divided by the rate of change of current in the other inductance: M = E2m / (di1/dt) M = E1m / (di2/dt) If the self induced voltages of the inductances L1 and L2 are respectively E1s and E2s for the same rates of change of the current that produced the mutually induced voltages E1m and E2m, then: M = (E2m / E1s)L1 M = (E1m / E2s)L2 Combining these two equations: M = (E1mE2m / E1sE2s) (L1L2) = kM(L1L2) where kM is the mutual coupling coefficient of the two inductances L1 and L2. If the coupling between the two inductances L1 and L2 is perfect, then the mutual inductance M is: M = (L1L2)

Inductances in Series When uncoupled inductances L1, L2, L3, ... are connected in series, the total inductance LS is: LS = L1 + L2 + L3 + ... When two coupled inductances L1 and L2 with mutual inductance M are connected in series, the total inductance LS is: LS = L1 + L2 2M The plus or minus sign indicates that the coupling is either additive or subtractive, depending on the connection polarity.

Inductances in Parallel When uncoupled inductances L1, L2, L3, ... are connected in parallel, the total inductance LP is: 1 / LP = 1 / L1 + 1 / L2 + 1 / L3 + ...

Time Constants Capacitance and resistance The time constant of a capacitance C and a resistance R is equal to CR, and represents the time to change the voltage on the capacitance from zero to E at a constant charging current E / R (which produces a rate of change of voltage E / CR across the capacitance). Similarly, the time constant CR represents the time to change the charge on the capacitance from zero to CE at a constant charging current E / R (which produces a rate of change of voltage E / CR across the capacitance). If a voltage E is applied to a series circuit comprising a discharged capacitance C and a resistance R, then after time t the current i, the voltage vR across the resistance, the voltage vC across the capacitance and the charge qC on the capacitance are: i = (E / R)e - t / CR vR = iR = Ee - t / CR vC = E - vR = E(1 - e - t / CR) qC = CvC = CE(1 - e - t / CR) If a capacitance C charged to voltage V is discharged through a resistance R, then after time t the current i, the voltage vR across the resistance, the voltage vC across the capacitance and the charge qC on the capacitance are: i = (V / R)e - t / CR vR = iR = Ve - t / CR vC = vR = Ve - t / CR qC = CvC = CVe - t / CR Inductance and resistance The time constant of an inductance L and a resistance R is equal to L / R, and represents the time to change the current in the inductance from zero to E / R at a constant rate of change of current E / L (which produces an induced voltage E across the inductance). If a voltage E is applied to a series circuit comprising an inductance L and a resistance R, then after time t the current i, the voltage vR across the resistance, the voltage vL across the inductance and the magnetic linkage L in the inductance are: i = (E / R)(1 - e - tR / L) vR = iR = E(1 - e - tR / L) vL = E - vR = Ee - tR / L L = Li = (LE / R)(1 - e - tR / L) If an inductance L carrying a current I is discharged through a resistance R, then after time t the current i, the voltage vR across the resistance, the voltage vL across the inductance and the magnetic linkage L in the inductance are: i = Ie - tR / L vR = iR = IRe - tR / L vL = vR = IRe - tR / L L = Li = LIe - tR / L Rise Time and Fall Time The rise time (or fall time) of a change is defined as the transition time between the 10%

and 90% levels of the total change, so for an exponential rise (or fall) of time constant T, the rise time (or fall time) t10-90 is: t10-90 = (ln0.9 - ln0.1)T 2.2T The half time of a change is defined as the transition time between the initial and 50% levels of the total change, so for an exponential change of time constant T, the half time t50 is : t50 = (ln1.0 - ln0.5)T 0.69T Note that for an exponential change of time constant T: - over time interval T, a rise changes by a factor 1 - e -1 ( 0.63) of the remaining change, - over time interval T, a fall changes by a factor e -1 ( 0.37) of the remaining change, - after time interval 3T, less than 5% of the total change remains, - after time interval 5T, less than 1% of the total change remains.

Telecommunication

Copy of the original phone of Graham Bell at the Muse des Arts et Mtiers in Paris

Telecommunication is the transmission of signals over a distance for the purpose of communication. In modern times, this process almost always involves the sending of electromagnetic waves by electronic transmitters but in earlier years it may have involved the use of smoke signals, drums or semaphore. Today, telecommunication is widespread and devices that assist the process such as the television, radio and telephone are common in many parts of the world. There is also a vast array of networks that connect these devices, including computer networks, public telephone networks, radio networks and television networks. Computer communication across the Internet, such as e-mail and instant messaging, is just one of many examples of telecommunication.

Telecommunication systems are generally designed by telecommunication engineers. Major contributors to the field of telecommunications include Alexander Bell who invented the telephone (as we know it), John Logie Baird who invented the mechanical television and Guglielmo Marconi who first demonstrated transatlantic radio communication. In recent times, optical fibre has radically improved the bandwidth available for intercontential communication helping to facilitate a faster and richer Internet experience and digital television has eliminated effects such as snowy pictures and ghosting. Telecommunication remains an important part of the world economy and the telecommunication industrys revenue has been placed at just under 3% of the gross world product.

Key concepts

The basic elements of a telecommunication system are:

a transmitter that takes information and converts it to a signal for transmission a transmission medium over which the signal is transmitted a receiver that receives and converts the signal back into usable information

For example, consider a radio broadcast. In this case the broadcast tower is the transmitter, the radio is the receiver and the transmission medium is free space. Often

telecommunication systems are two-way and devices act as both a transmitter and receiver or transceiver. For example, a mobile phone is a transceiver. Telecommunication over a phone line is called point-to-point communication because it is between one transmitter and one receiver, telecommunication through radio broadcasts is called broadcast communication because it is between one powerful transmitter and numerous receivers.

Signals can either be analogue or digital. In an analogue signal, the signal is varied continuously with respect to the information. In a digital signal, the information is encoded as a set of discrete values (e.g. 1s and 0s).

A collection of transmitters, receivers or transceivers that communicate with each other is known as a network. Digital networks may consist of one or more routers that route data to the correct user. An analogue network may consist of one or more switches that establish a connection between two or more users. For both types of network, a repeater may be necessary to amplify or recreate the signal when it is being transmitted over long distances. This is to combat attenuation that can render the signal indistinguishable from noise.

A channel is a division in a transmission medium so that it can be used to send multiple independent streams of data. For example, a radio station may broadcast at 96 MHz while another radio station may broadcast at 94.5 MHz. In this case the medium has been divided by frequency and each channel received a separate frequency to broadcast on. Alternatively one could allocate each channel a recurring segment of time over which to broadcast.

The shaping of a signal to convey information is known as modulation. Modulation is a key concept in telecommunications and is frequently used to impose the information of one signal on another. Modulation is used to represent a digital message as an analogue waveform. This is known as keying and several keying techniques exist these include phase-shift keying, amplitude-shift keying and minimum-shift keying. Bluetooth, for example, uses phase-shift keying for exchanges between devices (see note).

However, more relevant to earlier discussion, modulation is also used to boost the frequency of analogue signals. This is because a raw signal is often not suitable for transmission over long distances of free space due to its low frequencies. Hence its information must be superimposed on a higher frequency signal (known as a carrier wave) before transmission. There are several different modulation schemes available to achieve this some of the most basic being amplitude modulation and frequency modulation. An example of this process is a DJs voice being superimposed on a 96 MHz carrier wave using frequency modulation (the voice would then be received on a radio as the channel 96 FM).

Society and telecommunication

Telecommunication is an important part of many modern societies. In 2006, estimates place the telecommunication industrys revenue at $1.2 trillion or just under 3% of the gross world product.Good telecommunication infrastructure is widely acknowledged as important for economic success in the modern world both on a micro and macroeconomic scale.And, for this reason, there is increasing worry about the digital divide.

This stems from the fact that access to telecommunication systems is not equally shared amongst the worlds population. A 2003 survey by the International Telecommunication Union (ITU) revealed that roughly one-third of countries have less than 1 mobile subscription for every 20 people and one-third of countries have less than 1 fixed line subscription for every 20 people. In terms of Internet access, roughly half of countries have less than 1 in 20 people with Internet access. From this information as well as educational data the ITU was able to compile a Digital Access Index that measures the overall ability of citizens to access and use information and communication technologies. Using this measure, countries such as Sweden, Denmark and Iceland receive the highest ranking while African countries such as Niger, Burkina Faso and Mali receive the lowest.Further discussion of the social impact of telecommunication is often considered part of communication theory.

History

A replica of one of Chappes semaphore towers.

Early telecommunications

Early forms of telecommunication include smoke signals and drums. Drums were used by natives in Africa, New Guinea and South America whereas smoke signals were used

by natives in North America and China. Contrary to what one might think, these systems were often used to do more than merely announce the presence of a camp.

In 1792, a French engineer, Claude Chappe built the first fixed visual telegraphy (or semaphore) system between Lille and Paris. However semaphore as a communication system suffered from the need for skilled operators and expensive towers often at intervals of only ten to thirty kilometres (six to nineteen miles). As a result, the last commercial line was abandoned in 1880.

Telegraph and telephone

The first commercial electrical telegraph was constructed by Sir Charles Wheatstone and Sir William Fothergill Cooke and opened on 9 April 1839. Both Wheatstone and Cooke viewed their device as an improvement to the [existing] electromagnetic telegraph not as a new device.

On the other side of the Atlantic Ocean, Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837. Soon after he was joined by Alfred Vail who developed the register a telegraph terminal that integrated a logging device for recording messages to paper tape. This was demonstrated successfully on 6 January 1838. The first transatlantic telegraph cable was successfully completed on 27 July 1866, allowing transatlantic telecommunication for the first time.

The conventional telephone was invented by Alexander Bell in 1876. Although in 1849 Antonio Meucci invented a device that allowed the electrical transmission of voice over a line. Meuccis device depended upon the electrophonic effect and was of little practical value because it required users to place the receiver in their mouth to hear what was being said. The first commercial telephone services were set-up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven and London.

Radio and television

In 1832, James Lindsay gave a classroom demonstration of wireless telegraphy to his students. By 1854 he was able to demonstrate a transmission across the Firth of Tay from Dundee to Woodhaven, a distance of two miles, using water as the transmission medium. In December 1901, Guglielmo Marconi established wireless communication between Britain and the United States earning him the Nobel Prize in physics in 1909 (which he shared with Karl Braun).

On March 25, 1925, John Logie Baird was able to demonstrate the transmission of moving pictures at the London department store Selfridges. Bairds device relied upon

the Nipkow disk and thus became known as the mechanical television. It formed the basis of experimental broadcasts done by the British Broadcasting Corporation beginning September 30, 1929. However for most of the twentieth century televisions depended upon the cathode ray tube invented by Karl Braun. The first version of such a television to show promise was produced by Philo Farnsworth and demonstrated to his family on September 7, 1927.

Computer networks and the Internet

On September 11, 1940 George Stibitz was able to transmit problems using teletype to his Complex Number Calculator in New York and receive the computed results back at Dartmouth College in New Hampshire. This configuration of a centralized computer or mainframe with remote dumb terminals remained popular throughout the 1950s. However it was not until the 1960s that researchers started to investigate packet switching a technology that would allow chunks of data to be sent to different computers without first passing through a centralized mainframe. A four-node network emerged on December 5, 1969; this network would become ARPANET, which by 1981 would consist of 213 nodes.

ARPANETs development centred around the Request for Comment process and on April 7, 1969, RFC 1 was published. This process is important because ARPANET would eventually merge with other networks to form the Internet and many of the protocols the Internet relies upon today were specified through this process. In September 1981, RFC 791 introduced the Internet Protocol v4 (IPv4) and RFC 793 introduced the Transmission Control Protocol (TCP) thus creating the TCP/IP protocol that much of the Internet relies upon today.

However not all important developments were made through the Request for Comment process. Two popular link protocols for local area networks (LANs) also appeared in the 1970s. A patent for the Token Ring protocol was filed by Olof Soderblom on October 29, 1974. And a paper on the Ethernet protocol was published by Robert Metcalfe and David Boggs in the July 1976 issue of Communications of the ACM. These protocols are discussed in more detail in the next section.

Modern operation

Telephone

Optic fibres are revolutionizing long-distance communication

In a conventional telephone system, the caller is connected to the person they want to talk to by the switches at various exchanges. The switches form an electrical connection between the two users and the setting of these switches is determined electronically when the caller dials the number based upon either pulses or tones made by the callers telephone. Once the connection is made, the callers voice is transformed to an electrical signal using a small microphone in the telephones receiver. This electrical signal is then sent through various switches in the network to the user at the other end where it transformed back into sound waves by a speaker for that person to hear. This person also has a separate electrical connection between him and the caller which allows him to talk back.[28] Today, the fixed-line telephone systems in most residential homes are analogue that is the speakers voice directly determines the amplitude of the signals voltage. However although short-distance calls may be handled from end-to-end as analogue signals, increasingly telephone service providers are transparently converting signals to digital before converting them back to analogue for reception.

Mobile phones have had a dramatic impact on telephone service providers. Mobile phone subscriptions now outnumber fixed line subscriptions in many markets. Sales of mobile phones in 2005 totalled 816.6 million with that figure being almost equally shared amongst the markets of Asia/Pacific (204 m), Western Europe (164 m), CEMEA (Central Europe, the Middle East and Africa) (153.5 m), North America (148 m) and Latin America (102 m). In terms of new subscriptions over the five years from 1999, Africa has outpaced other markets with 58.2% growth compared to the next largest market, Asia, which boasted 34.3% growth.[30] Increasingly these phones are being serviced by digital systems such as GSM or W-CDMA with many markets choosing to depreciate analogue

systems such as AMPS.[31] By digital it is meant the handsets themselves transmit digital not analogue signals.

However there have been equally drastic changes in telephone communication behind the scenes. Starting with the operation of TAT-8 in 1988, the 1990s saw the widespread adoption of systems based upon optic fibres. The benefit of communicating with optic fibres is that they offer a drastic increase in data capacity. TAT-8 itself was able to carry 10 times as many telephone calls as the last copper cable laid at that time and todays optic fibre cables are able to carry 25 times as many telephone calls as TAT-8.[32] This drastic increase in data capacity is due to several factors. First, optic fibres are physically much smaller than competing technologies. Second, they do not suffer from crosstalk which means several hundred of them can be easily bundled together in a single cable.[33] Lastly, improvements in multiplexing have lead to an exponential growth in the data capacity of a single fibre. This is due to technologies such as dense wavelength-division multiplexing, which at its most basic level is building multiple channels based upon frequency division as discussed in the Key concepts section.[34] However despite the advances of technologies such as dense wavelength-division multiplexing, technologies based around building multiple channels based upon time division such as synchronous optical networking and synchronous digital hierarchy remain dominant.[35]

Assisting communication across these networks is a protocol known as Asynchronous Transfer Mode (ATM). As a technology, ATM arose in the 1980s and was envisioned to be part of the Broadband Integrated Services Digital Network. The network ultimately failed but the technology gave birth to the ATM Forum which in 1992 published its first standard.[36] Today, despite competitors such as Multiprotocol Label Switching, ATM remains the protocol of choice for most major long-distance optical networks. The importance of the ATM protocol was chiefly in its notion of establishing pathways for data through the network and associating a traffic contract with these pathways. The traffic contract was essentially an agreement between the client and the network about how the network was to handle the data, if the network could not meet the conditions of the traffic contract it would not accept the connection. This was important because telephone calls could negotiate a contract so as to guarantee themselves a constant bit rate, something that was essential to ensure a call could take place without the callers voice being delayed in parts or cut-off completely.[37]

Radio and television

Digital television standards and their adoption worldwide.

The broadcast media industry is at a critical turning point in its development, with many countries starting to move from analogue to digital broadcasts. The chief advantage of digital broadcasts is that they prevent a number of complaints with traditional analogue broadcasts. For television, this includes the elimination of problems such as snowy pictures, ghosting and other distortion. These occur because of the nature of analogue transmission, which means that perturbations due to noise will be evident in the final output. Digital transmission overcomes this problem because digital signals are reduced to binary data upon reception and hence small perturbations do not affect the final output. In a simplified example, if a binary message 1011 was transmitted with signal amplitudes [1.0 0.0 1.0 1.0] and received with signal amplitudes [0.9 0.2 1.1 0.9] it would still decode to the binary message 1011 a perfect reproduction of what was sent. From this example, a problem with digital transmissions can also be seen in that if the noise is great enough it can significantly alter the decoded message. Using forward error correction a receiver can correct a handful of bit errors in the resulting message but too much noise will lead to incomprehensible output and hence a breakdown of the transmission.[38]

In digital television broadcasting, there are three competing standards that are likely to be adopted worldwide. These are the ATSC, DVB and ISDB standards and the adoption of these standards thus far is presented in the captioned map. All three standards use MPEG-2 for video compression. ATSC uses Dolby Digital AC-3 for audio compression, ISDB uses Advanced Audio Coding (MPEG-2 Part 7) and DVB has no standard for audio compression but typically uses MPEG-1 Part 3 Layer 2.[39][40] The choice of modulation also varies between the schemes. Both DVB and ISDB use orthogonal frequency-division multiplexing (OFDM) for terrestrial broadcasts (as opposed to satellite or cable broadcasts) where as ATSC uses vestigial sideband modulation (VSB). OFDM should offer better resistance to multipath interference and the Doppler effect (which would impact reception using moving receivers).[41] However controversial tests conducted by the United States National Association of Broadcasters have shown that there is little difference between the two for stationary receivers.[42]

In digital audio broadcasting, standards are much more unified with practically all countries (including Canada) choosing to adopt the Digital Audio Broadcasting standard (also known as the Eureka 147 standard). The exception being the United States which has chosen to adopt HD Radio. HD Radio, unlike Eureka 147, is based upon a transmission method known as in-band on-channel transmission this allows digital information to piggyback on normal AM or FM analogue transmissions. Hence avoiding the bandwidth allocation issues of Eureka 147 and therefore being strongly advocated National Association of Broadcasters who felt there was a lack of new

spectrum to allocate for the Eureka 147 standard.[43] In the United States the Federal Communications Commission has chosen to leave licensing of the standard in the hands of a commercial corporation called iBiquity.[44] An open in-band on-channel standard exists in the form of Digital Radio Mondiale (DRM) however adoption of this standard is mostly limited to a handful of shortwave broadcasts. Despite the different names all standards rely upon OFDM for modulation. In terms of audio compression, DRM typically uses Advanced Audio Coding (MPEG-4 Part 3), DAB like DVB can use a variety of codecs but typically uses MPEG-1 Part 3 Layer 2 and HD Radio uses High-Definition Coding.

However, despite the pending switch to digital, analogue receivers still remain widespread. Analogue television is still transmitted in practically all countries. The United States had hoped to end analogue broadcasts by December 31, 2006 however this was recently pushed back to February 17, 2009.[45] For analogue, there are three standards in use (see a map on adoption here). These are known as PAL, NTSC and SECAM. The basics of PAL and NTSC are very similar; a quadrature amplitude modulated subcarrier carrying the chrominance information is added to the luminance video signal to form a composite video baseband signal (CVBS). On the other hand, the SECAM system uses a frequency modulation scheme on its colour subcarrier. The PAL system differs from NTSC in that the phase of the video signals colour components is reversed with each line helping to correct phase errors in the transmission. For analogue radio, the switch to digital is made more difficult by the fact that analogue receivers cost a fraction of the cost of digital receivers. For example while you can get a good analogue receiver for under $20 USD[46] a digital receiver will set you back at least $75 USD.[47] The choice of modulation for analogue radio is typically between amplitude modulation (AM) or frequency modulation (FM). To achieve stereo playback, an amplitude modulated subcarrier is used for stereo FM and quadrature amplitude modulation is used for stereo AM or C-QUAM (see each of the linked articles for more details).

The Internet

The OSI reference model

Today an estimated 15.7% of the world population has access to the Internet with the highest concentration in North America (68.6%), Oceania/Australia (52.6%) and Europe (36.1%).[48] In terms of broadband access, countries such as Iceland (26.7%), South Korea (25.4%) and the Netherlands (25.3%) lead the world.[49]

The nature of computer network communication lends itself to a layered approach where individual protocols in the protocol stack run largely independently of other protocols. This allows lower-level protocols to be customized for the network situation while not changing the way higher-level protocols operate. A practical example of why this important is because it allows an Internet browser to run the same code regardless of whether the computer it is running on is connected to the Internet through an Ethernet or Wi-Fi connection. Protocols are often talked about in terms of their place in the OSI reference model a model that emerged in 1983 as the first step in a doomed attempt to build a universally adopted networking protocol suite.[50] The model itself is outlined in the picture to the right. It is important to note that the Internets protocol suite, like many modern protocol suites, does not rigidly follow this model but can still be talked about in the context of this model.

For the Internet, the physical medium and data link protocol can vary several times as packets travel between client nodes. Though it is likely that the majority of the distance travelled will be using the Asynchronous Transfer Mode (ATM) data link protocol across optical fibre this is in no way guaranteed. A connection may also encounter data link protocols such as Ethernet, Wi-Fi and the Point-to-Point Protocol (PPP) and physical media such as twisted-pair cables and free space.

At the network layer things become standardized with the Internet Protocol (IP) being adopted for logical addressing. For the world wide web, these IP addresses are derived from the human readable form (e.g. 72.14.207.99 ) using the Domain Name System. At the moment the most widely used version of the Internet Protocol is version four but a move to version six is imminent. The main advantage of the new version is that it supports 3.40 1038 addresses compared to 4.29 109 addresses. The new version also adds support for enhanced security through IPSec as well as support for QoS identifiers.[51] At the transport layer most communication adopts either the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). With TCP, packets are retransmitted if they are lost and placed in order before they are presented to higher layers (this ordering also allows duplicate packets to be eliminated). With UDP, packets are not ordered or retransmitted if lost. Both TCP and UDP packets carry port numbers with them to specify what application or process the packet should be handed to on the clients computer.[52] Because certain application-level protocols use certain ports, network administrators can restrict Internet access by blocking or throttling traffic destined for a particular port.

Above the transport layer there are certain protocols that loosely fit in the session and presentation layers and are sometimes adopted, most notably the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols. These protocols ensure that the data transferred between two parties remains completely confidential and one or the other is in use when a padlock appears at the bottom of your web browser. Security is generally based upon the principle that eavesdroppers cannot factorize very large numbers that are the composite of two primes without knowing one of the primes. Another protocol that loosely fits in the session and presentation layers is the Real-time Transport Protocol (RTP) most notably used to stream QuickTime.[53] Finally at the application layer are many of the protocols Internet users would be familiar with such as HTTP (web browsing), POP3 (e-mail), FTP (file transfer) and IRC (Internet chat) but also less common protocols such as BitTorrent (file sharing) and ICQ (instant messaging).

sLocal area networks

A local area network.

Despite the growth of the Internet, the characteristics of local area networks (computer networks that run over at most a few kilometres) remain distinct.

In the mid-1980s, several protocol suites emerged to fill the gap between the data link and applications layer of the OSI reference model. These were Appletalk, IPX and NetBIOS with the dominant protocol suite during the early 90s being IPX due to its popularity with MS-DOS users. TCP/IP existed at this point but was typically only used by large government and research facilities.[54] However as the Internet grew in popularity and a larger percentage of local area network traffic became Internet-related, LANs gradually moved towards TCP/IP and today networks mostly dedicated to TCP/IP traffic are common. The move to TCP/IP was helped by technologies such as DHCP introduced in RFC 2131 that allowed TCP/IP clients to discover their own network address a functionality that came standard with the AppleTalk/IPX/NetBIOS protocol suites.

However it is at the data link layer that modern local area networks diverge from the Internet. Where as Asynchronous Transfer Mode (ATM) or Multiprotocol Label Switching (MPLS) are typical data link protocols for larger networks, Ethernet and

Token Ring are typical data link protocols for local area networks. The latter LAN protocols differ from the former protocols in that they are simpler (e.g. they omit features such as Quality of Service guarantees) and offer collision prevention. Both of these differences allow for more economic set-ups. For example, omitting Quality of Service guarantees simplifies routers and the guarantees are not really necessary for local area networks because they tend not to carry real time communication (such as voice communication). Including collision prevention allows multiple clients (as opposed to just two) to share the same cable again reducing costs. Though both Ethernet and Token Ring have different frame formats, it is in terms of collision prevention that the two present the greatest difference. With Token Ring a token circulates the network and clients only transmit when they have the token. The token must be managed to ensure it is not lost or duplicated. With Ethernet any client can transmit if it thinks the medium is idle, but clients listen for collisions and if one is detected suspend communication for a random amount of time.[55]

Despite Token Rings modest popularity in the 80s and 90s, with the advent of the twenty-first century, the majority of local area networks have now settled on Ethernet. At the physical layer most Ethernet implementations use copper twisted-pair cables (including the common 10BASE-T networks). Some early implementations used coaxial cables. And some implementations (especially high speed ones) use optical fibres. Optical fibres are also likely to feature prominently in the forthcoming 10-gigabit Ethernet implementations.[56] Where optical fibre is used, the distinction must be made between multi-mode fibre and single-mode fibre. Multi-mode fibre can be thought of as thicker optical fibre that is cheaper to manufacture but that suffers from less usable bandwidth and greater attenuation.

Electrical engineering

Electrical Engineers design power systems

and complex electronic circuits.

Electrical engineering (sometimes referred to as electrical and electronic engineering) is a professional engineering discipline that deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century with the commercialization of the electric telegraph and electrical power supply. The field now covers a range of sub-disciplines including those that deal with power, optoelectronics, digital electronics, analog electronics, artificial intelligence, control systems, electronics, signal processing and telecommunications.

The term electrical engineering may or may not encompass electronic engineering. Where a distinction is made, electrical engineering is considered to deal with the problems associated with large-scale electrical systems such as power transmission and motor control, whereas electronic engineering deals with the study of small-scale electronic systems including computers and integrated circuits.Another way of looking at

the distinction is that electrical engineers are usually concerned with using electricity to transmit energy, while electronics engineers are concerned with using electricity to transmit information.

History History of electrical engineering

Early developments

Electricity has been a subject of scientific interest since at least the 17th century, but it was not until the 19th century that research into the subject started to intensify. Notable developments in this century include the work of Georg Ohm, who in 1827 quantified the relationship between the electric current and potential difference in a conductor, Michael Faraday, the discoverer of electromagnetic induction in 1831, and James Clerk Maxwell, who in 1873 published a unified theory of electricity and magnetism in his treatise on Electricity and Magnetism.

During these years, the study of electricity was largely considered to be a subfield of physics. It was not until the late 19th century that universities started to offer degrees in electrical engineering. The Darmstadt University of Technology founded the first chair and the first faculty of electrical engineering worldwide in 1882. In 1883 Darmstadt University of Technology and Cornell University introduced the worlds first courses of study in electrical engineering and in 1885 the University College London founded the first chair of electrical engineering in the United Kingdom.The University of Missouri subsequently established the first department of electrical engineering in the United States in 1886.

Thomas Edison built the worlds first large-scale electrical supply network

During this period, the work concerning electrical engineering increased dramatically. In 1882, Edison switched on the worlds first large-scale electrical supply network that provided 110 volts direct current to fifty-nine customers in lower Manhattan. In 1887, Nikola Tesla filed a number of patents related to a competing form of power distribution known as alternating current. In the following years a bitter rivalry between Tesla and Edison, known as the War of Currents, took place over the preferred method of distribution. AC eventually replaced DC for generation and power distribution, enormously extending the range and improving the safety and efficiency of power distribution.

Nikola Tesla made long-distance electrical transmission networks possible.

The efforts of the two did much to further electrical engineeringTeslas work on induction motors and polyphase systems influenced the field for years to come, while Edisons work on telegraphy and his development of the stock ticker proved lucrative for his company, which ultimately became General Electric. However, by the end of the 19th century, other key figures in the progress of electrical engineering were beginning to emerge.

Modern developments

Emergence of radio and electronics

During the development of radio, many scientists and inventors contributed to radio technology and electronics. In his classic UHF experiments of 1888, Heinrich Hertz transmitted (via a spark-gap transmitter) and detected radio waves using electrical equipment. In 1895, Nikola Tesla was able to detect signals from the transmissions of his

New York lab at West Point (a distance of 80.4 km). In 1897, Karl Ferdinand Braun introduced the cathode ray tube as part of an oscilloscope, a crucial enabling technology for electronic television.John Fleming invented the first radio tube, the diode, in 1904. Two years later, Robert von Lieben and Lee De Forest independently developed the amplifier tube, called the triode. In 1920 Albert Hull developed the magnetron which would eventually lead to the development of the microwave oven in 1946 by Percy Spencer. In 1934 the British military began to make strides towards radar (which also uses the magnetron), under the direction of Dr Wimperis culminating in the operation of the first radar station at Bawdsey in August 1936.

In 1941 Konrad Zuse presented the Z3, the worlds first fully functional and programmable computer. In 1946 the ENIAC (Electronic Numerical Integrator and Computer) of John Presper Eckert and John Mauchly followed, beginning the computing era. The arithmetic performance of these machines allowed engineers to develop completely new technologies and achieve new objectives, including the Apollo missions and the NASA moon landing.

The invention of the transistor in 1947 by William B. Shockley, John Bardeen and Walter Brattain opened the door for more compact devices and led to the development of the integrated circuit in 1958 by Jack Kilby and independently in 1959 by Robert Noyce. In 1968 Marcian Hoff invented the first microprocessor at Intel and thus ignited the development of the personal computer. The first realization of the microprocessor was the Intel 4004, a 4-bit processor developed in 1971, but only in 1973 did the Intel 8080, an 8-bit processor, make the building of the first personal computer, the Altair 8800, possible.

Education

Electrical engineers typically possess an academic degree with a major in electrical engineering. The length of study for such a degree is usually four or five years and the completed degree may be designated as a Bachelor of Engineering, Bachelor of Science, Bachelor of Technology or Bachelor of Applied Science depending upon the university. The degree generally includes units covering physics, mathematics, project management and specific topics in electrical engineering. Initially such topics cover most, if not all, of the sub-disciplines of electrical engineering. Students then choose to specialize in one or more sub-disciplines towards the end of the degree.

Some electrical engineers also choose to pursue a postgraduate degree such as a Master of Engineering/Master of Science, a Master of Engineering Management, a Doctor of Philosophy in Engineering or an Engineers degree. The Master and Engineers degree may consist of either research, coursework or a mixture of the two. The Doctor of Philosophy consists of a significant research component and is often viewed as the entry point to academia. In the United Kingdom and various other European countries, the Master of Engineering is often considered an undergraduate degree of slightly longer duration than the Bachelor of Engineering.

Practicing engineers

In most countries, a Bachelors degree in engineering represents the first step towards professional certification and the degree program itself is certified by a professional body. After completing a certified degree program the engineer must satisfy a range of requirements (including work experience requirements) before being certified. Once certified the engineer is designated the title of Professional Engineer (in the United States, Canada and South Africa ), Chartered Engineer (in the United Kingdom, Ireland, India and Zimbabwe), Chartered Professional Engineer (in Australia and New Zealand) or European Engineer (in much of the European Union).

The advantages of certification vary depending upon location. For example, in the United States and Canada only a licensed engineer may seal engineering work for public and private clients. This requirement is enforced by state and provincial legislation such as Quebecs Engineers Act. In other countries, such as Australia, no such legislation exists. Practically all certifying bodies maintain a code of ethics that they expect all members to abide by or risk expulsion. In this way these organizations play an important role in maintaining ethical standards for the profession. Even in jurisdictions where certification has little or no legal bearing on work, engineers are subject to contract law. In cases where an engineers work fails he or she may be subject to the tort of negligence and, in extreme cases, the charge of criminal negligence. An engineers work must also comply with numerous other rules and regulations such as building codes and legislation pertaining to environmental law.

Professional bodies of note for electrical engineers include the Institute of Electrical and Electronics Engineers (IEEE) and the Institution of Electrical Engineers (IEE). The IEEE claims to produce 30 percent of the worlds literature in electrical engineering, has over 360,000 members worldwide and holds over 300 conferences annually. The IEE publishes 14 journals, has a worldwide membership of 120,000, and claims to be the largest professional engineering society in Europe. Obsolescence of technical skills is a serious concern for electrical engineers. Membership and participation in technical societies, regular reviews of periodicals in the field and a habit of continued learning are therefore essential to maintaining proficiency.

In countries such as Australia, Canada and the United States electrical engineers make up around 0.25% of the labour force (see note). Outside of these countries, it is difficult to gauge the demographics of the profession due to less meticulous reporting on labour statistics. However, in terms of electrical engineering graduates per-capita, electrical engineering graduates would probably be most numerous in countries such as Taiwan, Japan and South Korea.

Tools and work

From the Global Positioning System to electric power generation, electrical engineers are responsible for a wide range of technologies. They design, develop, test and supervise the

deployment of electrical systems and electronic devices. For example, they may work on the design of telecommunication systems, the operation of electric power stations, the lighting and wiring of buildings, the design of household appliances or the electrical control of industrial machinery.

Satellite Communications is one of many projects an electrical engineer might work on

Fundamental to the discipline are the sciences of physics and mathematics as these help to obtain both a qualitative and quantitative description of how such systems will work. Today most engineering work involves the use of computers and it is commonplace to use computer-aided design programs when designing electrical systems. Nevertheless, the ability to sketch ideas is still invaluable for quickly communicating with others.

Although most electrical engineers will understand basic circuit theory (that is the interactions of elements such as resistors, capacitors, diodes, transistors and inductors in a circuit), the theories employed by engineers generally depend upon the work they do. For example, quantum mechanics and solid state physics might be relevant to an engineer working on VLSI (the design of integrated circuits), but are largely irrelevant to engineers working with macroscopic electrical systems. Even circuit theory may not be relevant to a person designing telecommunication systems that use off-the-shelf components. Perhaps the most important technical skills for electrical engineers are reflected in university programs, which emphasize strong numerical skills, computer literacy and the ability to understand the technical language and concepts that relate to electrical engineering.

For most engineers technical work accounts for only a fraction of the work they do. A lot of time is also spent on tasks such as discussing proposals with clients, preparing budgets and determining project schedules. Many senior engineers manage a team of technicians or other engineers and for this reason project management skills are important. Most engineering projects involve some form of documentation and strong written communication skills are therefore very important.

The workplaces of electrical engineers are just as varied as the types of work they do. Electrical engineers may be found in the pristine lab environment of a fabrication plant, the offices of a consulting firm or on site at a mine. During their working life, electrical

engineers may find themselves supervising a wide range of individuals including scientists, electricians, computer programmers and other engineers.

Sub-disciplines

Electrical engineering has many sub-disciplines, the most popular of which are listed below. Although there are electrical engineers who focus exclusively on one of these sub-disciplines, many deal with a combination of them. Sometimes certain fields, such as electronic engineering and computer engineering, are considered separate disciplines in their own right.

Power

Power engineering

Power engineering deals with the generation, transmission and distribution of electricity as well as the design of a range of related devices. These include transformers, electric generators, electric motors and power electronics. In many regions of the world, governments maintain an electrical network called a power grid that connects a variety of generators together with users of their energy. Users purchase electrical energy from the grid, avoiding the costly exercise of having to generate their own. Power engineers may work on the design and maintenance of the power grid as well as the power systems that connect to it. Such systems are called on-grid power systems and may supply the grid with additional power, draw power from the grid or do both. Power engineers may also work on systems that do not connect to the grid, called off-grid power systems, which in some cases are preferable to on-grid systems.

Control

Control engineering

Control engineering focuses on the modelling of a diverse range of dynamic systems and the design of controllers that will cause these systems to behave in the desired manner. To implement such controllers electrical engineers may use electrical circuits, digital signal processors and microcontrollers. Control engineering has a wide range of applications from the flight and propulsion systems of commercial airliners to the cruise control present in many modern automobiles. It also plays an important role in industrial automation.

Control engineers often utilize feedback when designing control systems. For example, in an automobile with cruise control the vehicles speed is continuously monitored and fed back to the system which adjusts the motors speed accordingly. Where there is regular feedback, control theory can be used to determine how the system responds to such feedback.

Electronics

Electronic engineering

Electronic engineering involves the design and testing of electronic circuits that use the properties of components such as resistors, capacitors, inductors, diodes and transistors to achieve a particular functionality. The tuned circuit, which allows the user of a radio to filter out all but a single station, is just one example of such a circuit. Another example (of a pneumatic signal conditioner) is shown in the adjacent photograph.

Prior to the second world war, the subject was commonly known as radio engineering and basically was restricted to aspects of communications and radar, commercial radio and early television. Later, in post war years, as consumer devices began to be developed, the field grew to include modern television, audio systems, computers and microprocessors. In the mid to late 1950s, the term radio engineering gradually gave way to the name electronic engineering.

Before the invention of the integrated circuit in 1959, electronic circuits were constructed from discrete components that could be manipulated by humans. These discrete circuits consumed much space and power and were limited in speed, although they are still common in some applications. By contrast, integrated circuits packed a large numberoften millionsof tiny electrical components, mainly transistors, into a small chip around the size of a coin. This allowed for the powerful computers and other electronic devices we see today.

Microelectronics

Microelectronics

Microelectronics engineering deals with the design of very small electronic components for use in an integrated circuit or sometimes for use on their own as a general electronic component. The most common microelectronic components are semiconductor transistors, although all main electronic components (resistors, capacitors, inductors) can be created at a microscopic level.

Microelectronic components are created by chemically fabricating wafers of semiconductors such as silicon (at higher frequencies, gallium arsenide and indium phosphide) to obtain the desired transport of electronic charge and control of current. The field of microelectronics involves a significant amount of chemistry and material science and requires the electronic engineer working in the field to have a very good working knowledge of the effects of quantum mechanics.

Signal processing

Signal processing

Signal processing deals with the analysis and manipulation of signals. Signals can be either analog, in which case the signal varies continuously according to the information, or digital, in which case the signal varies according to a series of discrete values representing the information. For analog signals, signal processing may involve the amplification and filtering of audio signals for audio equipment or the modulation and demodulation of signals for telecommunications. For digital signals, signal processing may involve the compression, error detection and error correction of digitally sampled signals.

Telecommunications

Telecommunications engineering

Telecommunications engineering focuses on the transmission of information across a channel such as a coax cable, optical fibre or free space. Transmissions across free space require information to be encoded in a carrier wave in order to shift the information to a carrier frequency suitable for transmission, this is known as modulation. Popular analog modulation techniques include amplitude modulation and frequency modulation. The choice of modulation affects the cost and performance of a system and these two factors must be balanced carefully by the engineer.

Once the transmission characteristics of a system are determined, telecommunication engineers design the transmitters and receivers needed for such systems. These two are sometimes combined to form a two-way communication device known as a transceiver. A key consideration in the design of transmitters is their power consumption as this is closely related to their signal strength. If the signal strength of a transmitter is insufficient the signals information will be corrupted by noise.

Instrumentation engineering

Instrumentation engineering

Instrumentation engineering deals with the design of devices to measure physical quantities such as pressure, flow and temperature. The design of such instrumentation requires a good understanding of physics that often extends beyond electromagnetic theory. For example, radar guns use the Doppler effect to measure the speed of oncoming vehicles. Similarly, thermocouples use the Peltier-Seebeck effect to measure the temperature difference between two points.

Often instrumentation is not used by itself, but instead as the sensors of larger electrical systems. For example, a thermocouple might be used to help ensure a furnaces temperature remains constant. For this reason, instrumentation engineering is often viewed as the counterpart of control engineering.

Computers

Computer engineering

Computer engineering deals with the design of computers and computer systems. This may involve the design of new hardware, the design of PDAs or the use of computers to control an industrial plant. Computer engineers may also work on a systems software. However, the design of complex software systems is often the domain of software engineering, which is usually considered a separate discipline. Desktop computers represent a tiny fraction of the devices a computer engineer might work on, as computer-like architectures are now found in a range of devices including video game consoles and DVD players.

Related disciplines

Mechatronics is an engineering discipline which deals with the convergence of electrical and mechanical systems. Such combined systems are known as electromechanical systems and have widespread adoption. Examples include automated manufacturing systems, heating, ventilation and air-conditioning systems and various subsystems of aircraft and automobiles.

The term mechatronics is typically used to refer to macroscopic systems but futurists have predicted the emergence of very small electromechanical devices. Already such small devices, known as micro electromechanical systems (MEMS), are used in automobiles to tell airbags when to deploy, in digital projectors to create sharper images and in inkjet printers to create nozzles for high-definition printing. In the future it is hoped the devices will help build tiny implantable medical devices and improve optical communication.

Biomedical engineering is another related discipline, concerned with the design of medical equipment. This includes fixed equipment such as ventilators, MRI scanners and electrocardiograph monitors as well as mobile equipment such as cochlear implants, artificial pacemakers and artificial hearts.

Power engineering

Rural 3 phase distribution transformer

Power engineering is the subfield of electrical engineering that deals with power systems, specifically electric power transmission and distribution, power conversion, and electromechanical devices. Out of necessity, power engineers also rely heavily on the theory of control systems. A power engineer supervises, operates, and maintains machinery and boilers that provide heat, power, refrigeration, and other utility services to heavy industry and large building complexes.

History

Power engineering was one of the earliest fields to be exploited in electrical engineering. Early problems solved by engineers include efficient and safe distribution of electric power. Nikola Tesla was a notable pioneer in this field.

Power

Transmission lines transmit power across the grid.

Power engineering deals with the generation, transmission and distribution of electricity as well as the design of a range of related devices. These include transformers, electric generators, electric motors and power electronics.

In many regions of the world, governments maintain an electrical network that connects a variety electric generators together with users of their power. This network is called a power grid. Users purchase electricity from the grid avoiding the costly exercise of having to generate their own. Power engineers may work on the design and maintenance of the power grid as well as the power systems that connect to it. Such systems are called on-grid power systems and may supply the grid with additional power, draw power from the grid or do both.

Power engineers may also work on systems that do not connect to the grid. These systems are called off-grid power systems and may be used in preference to on-grid systems for a variety of reasons. For example, in remote locations it may be cheaper for a mine to generate its own power rather than pay for connection to the grid and in most mobile applications connection to the grid is simply not practical.

Today, most grids adopt three-phase electric power with an alternating current. This choice can be partly attributed to the ease with which this type of power can be generated, transformed and used. Often (especially in the USA), the power is split before it reaches residential customers whose low-power appliances rely upon single-phase electric power. However, many larger industries and organizations still prefer to receive the three-phase power directly because it can be used to drive highly efficient electric motors such as three-phase induction motors.

Transformers play an important role in power transmission because they allow power to be converted to and from higher voltages. This is important because higher voltages suffer less power loss during transmission. This is because higher voltages allow for lower current to deliver the same amount of power as power is the product of the two. Thus, as the voltage steps up, the current steps down. It is the current flowing through the components that result in both the losses and the subsequent heating. These losses, appearing in the form of heat, are equal to the current squared times the electrical resistance through which the current flows.

For these reasons, electrical substations exist throughout power grids to convert power to higher voltages before transmission and to lower voltages suitable for appliances after transmission.

Components

Power engineering is usually broken into three parts:

Generation

Generation is converting other forms of power into electrical power. The sources of power include fossil fuels such as coal and natural gas, hydropower, nuclear power, solar power, wind power and other forms.

Transmission

Transmission includes moving power over somewhat long distances, from a power station to near where it is used. Transmission involves high voltages, almost always higher than voltage at which the power is either generated or used. Transmission also includes connecting together power systems owned by various companies and perhaps in different states or countries. Transimission includes long meduim and short lines.

Distribution

Distribution involves taking power from the transmission system to end users, converting it to voltages at which it is ultimately required.

Optoelectronics

Optoelectronics is the study and application of electronic devices that interact with light, and thus is usually considered a sub-field of photonics. In this context, light often includes invisible forms of radiation such as gamma rays, X-rays, ultraviolet and infrared. Optoelectronic devices are electrical-to-optical or optical-to-electrical transducers, or instruments that use such devices in their operation.

Electro-optics is often erroneously used as a synonym, but is in fact a wider branch of physics that deals with all interactions between light and electric fields, whether or not they form part of an electronic device.

Optoelectronics is based on the quantum mechanical effects of light on semiconducting materials, sometimes in the presence of electric fields.

Photoelectric or photovoltaic effect, used in: o photodiodes (including solar cells) o phototransistors o photomultipliers o integrated optical circuit (IOC) elements

Photoconductivity, used in: o light-dependent resistors o photoconductive camera tubes o charge-coupled imaging devices

Stimulated emission, used in: o lasers o injection laser diodes

Lossev effect, or radiative recombination, used in: o light-emitting diodes or LED

Photoemissivity, used in o photoemissive camera tube

Important applications of optoelectronics include:

Optocoupler optical fiber communications

Photodiode

A photodiode

Photodiode closeup

A photodiode is a semiconductor diode that functions as a photodetector. Photodiodes are packaged with either a window or optical fibre connection, in order to let in the light

to the sensitive part of the device. They may also be used without a window to detect vacuum UV or X-rays.

A phototransistor is in essence nothing more than a bipolar transistor that is encased in a transparent case so that light can reach the base-collector junction. The phototransistor works like a photodiode, but with a much higher sensitivity for light, because the electrons that are generated by photons in the base-collector junction are injected into the base, and this current is then amplified by the transistor operation. However, a phototransistor has a slower response time than a photodiode.

Principle of operation

A photodiode is a p-n junction or p-i-n structure. When light of sufficient photon energy strikes the diode, it excites an electron thereby creating a mobile electron and a positively charged electron hole. If the absorption occurs in the junctions depletion region, these carriers are swept from the junction by the built-in field of the depletion region, producing a photocurrent.

Photodiodes can be used under either zero bias (photovoltaic mode) or reverse bias (photoconductive mode). In zero bias, light falling on the diode causes a voltage to develop across the device, leading to a current in the forward bias direction. This is called the photovoltaic effect, and is the basis for solar cells in fact, a solar cell is just a large number of big, cheap photodiodes.

Diodes usually have extremely high resistance when reverse-biased. This resistance is reduced when light of an appropriate frequency shines on the junction. Hence, a reverse-biased diode can be used as a detector by monitoring the current running through it. Circuits based on this effect are more sensitive to light than ones based on the photovoltaic effect.

Avalanche photodiodes have a similar structure, but they are operated with much higher reverse bias. This allows each photo-generated carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device.

Materials

The material used to make a photodiode is critical to defining its properties, because only photons with sufficient energy to excite an electron across the materials bandgap will produce significant photocurrents.

Materials commonly used to produce photodiodes:

Material Wavelength range (nm)

Silicon 1901100

Germanium 8001700

Indium gallium arsenide 8002600

Lead sulfide

Applications

Photodiode schematic symbol

P-N photodiodes are used in similar applications to other photodetectors, such as photoconductors, charge-coupled devices, and photomultiplier tubes.

Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, and the receivers for remote controls in VCRs and televisions.

In other consumer items such as camera light meters, clock radios (the ones that dim the display when its dark) and street lights, photoconductors are often used rather than photodiodes, although in principle either could be used.

Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a better, more linear response than photoconductors.

They are also widely used in various medical applications, such as detectors for Computed tomography (coupled with scintillators) or instruments to analyze samples (immunoassay). They are also used in Blood gas monitors.

PIN diodes are much faster and more sensitive than ordinary p-n junction diodes, and hence are often used for optical communications and in lighting regulation.

P-N photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, avalanche photodiodes, intensified charge-coupled devices or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night-vision equipment and laser range finding.

Comparison with photomultipliers

Advantages compared to photomultipliers:

1. Excellent linearity of output current as a function of incident light 2. Spectral response from 190 nm to 1100 nm (silicon), longer wavelengths with

other semiconductor materials 3. Low noise 4. Ruggedized to mechanical stress 5. Low cost 6. Compact and light weight

7. Long lifetime 8. High quantum efficiency, typically 80% 9. No high voltage required

Disadvantages compared to photomultipliers:

1. Small area 2. No internal gain (except avalanche photodiodes, but their gain is typically 102103

compared to up to 108 for the photomultiplier) 3. Much lower overall sensitivity 4. Photon counting only possible with specially designed, usually cooled

photodiodes, with special electronic circuits 5. Response time for many designs is slower

Phototransistor

The fact that traditional transistors (these days called Bipolar Junction Transistors or BJTs) are photosensitive has been known ever since they were invented. This is one of the reasons that they are usually sealed in light tight cans - because unwanted light is a source of noise. Phototransistors are very similar to ordinary BJTs except they are designed for use as detectors. Transistors are amplifiers and in a phototransistor the amplifier gain is controlled by the amount of light striking the device.

Light arriving in the device is absorbed and creates charge carrier pairs in many different places. The action is different depending on where the charge carrier pairs are created. However, in an intentional phototransistor, absorbtion is intended to take place in the depletion zone between the emitter and the base. Creation of charge carriers here causes a current to flow in the E-B circuit which is then amplified by the transistor action of the device. These are much lower in noise and have a higher output than APDs, but are significantly less responsive than either APDs or p-i-n diodes. The major problem with phototransistors is materials. We would like to use silicon or gallium arsenide but these have too great a bandgap energy and are limited to detecting wavelengths shorter than about 1 micron. Germanium is usable in the 1300 nm band but while it is easily possible to build transistors with smaller bandgap materials (such as InP) there is no established technology to do so. This takes away much of the potential cost advantage in the 1550 region. The major use of phototransistors is in non-communications applications using visible (or near visible) light. Alarm systems (light beam detection) and remote controls for TV sets and automobiles are among the most common uses. Phototransistors are occasionally built as part of an integrated circuit. In this configuration they are referred to as Integrated Preamplifier Detectors (IPDs).

Photomultiplier

Photomultiplier tubes (photomultipliers or PMTs for short) are extremely sensitive detectors of light in the ultraviolet, visible and near infrared. These detectors multiply the signal produced by incident light by as much as 108, from which single photons can be resolved. The combination of high gain, low noise, high frequency response and large area of collection have meant that these devices still find applications in nuclear and particle physics, astronomy, medical imaging and motion picture film scanning (telecine). Semiconductor devices like avalanche photodiodes have replaced photomultipliers in some applications, but photomultipliers are still used in most cases.

Structure and operating principles

Schematic of a photomultiplier tube coupled to a scintillator.

Photomultipliers are constructed from a glass vacuum tube which houses a photocathode, several dynodes, and an anode. Incident photons strike the photocathode material which is present as a thin deposit on the entry window of the device, with electrons being produced as a consequence of the photoelectric effect. These electrons are directed by the focusing electrode towards the electron multiplier, where electrons are multiplied by the process of secondary emission.

The electron multiplier consists of a number of electrodes, called dynodes. Each dynode is held at a more positive voltage than the previous one. The electrons leave the photocathode, having the energy of the incoming photon. As they move towards the first dynode they are accelerated by the electric field and arrive with much greater energy. On striking the first dynode, more low energy electrons are emitted and these, in turn, are accelerated toward the second dynode. The geometry of the dynode chain is such that a cascade occurs with an ever-increasing number of electrons being produced at each stage. Finally the anode is reached where the accumulation of charge results in a sharp current pulse indicating the arrival of a photon at the photocathode.

Usage considerations

Photomultiplier

Photomultiplier tubes typically require 1000 to 2000 volts for proper operation. The most negative voltage is connected to the cathode, and the most positive voltage is connected to the anode. (Negative high voltage supplies are usually preferred.) Voltages are distributed to the dynodes by a resistive voltage divider, though variations such as active designs (with transistors or diodes) are possible. The divider design influences aspects such as the frequency response and rise time, and therefore may be critical to an application.

While powered, photomultipliers must be shielded from ambient light to prevent their destruction through overexcitation. If used in a location with high magnetic fields (which will curve electron paths), they are usually shielded by a layer of mu-metal.

Integrated optical circuit

An integrated optical circuit is one or more circuits composed of solid-state optical components on a semiconductor or dielectric substrate. Components include light

sources, optical filters, photodetectors, and thin-film optical waveguides. An example of an integrated optical circuit is an opto-isolator (or opto-coupler) which allows one circuit to interact with another while remaining electrically separated from the other. A 2005 developmentsolved a quantum noise problem that prevented silicon from being used to generate laser light, permitting new integrated optical circuits to use high-bandwidth laser light generated within the circuit itself as a signal medium

Digital circuit

A digital circuit that acts as a binary clock, hand-wired on a series of prototyping sockets.

A digital circuit is