i. 4923261.4921354 8 50 8 54 8 57 Now there’s an easy way to learn Everything About With the Low Cost 8088 Trainer Kit From Dynalog The MICROFRIEND-LC series from Dynalog is growing fast After the unparalleled success of the two low cost kits based on 8085 and Z80, now comes the 16 bit MICROFRIEND-PC/LC. The low cost 8088 trainer kit you have been waiting for. It has the high performance 8088 CPU running at 5 MHz, 8K powerful FIRMWARE in ROM and 8K CMOS RAM for user programs. Other attractive features include- ■ Memory expansion. ■ 48 parallel I/O lines. ■ RS 232C Serial channel. ■ Audio cassette interface. ■ Three 16 bit timers. ■ Prioritised interrput handling. ■ 50 pin expansion Bus. ■ On board EPROM programmer. ■ 24 key multifunction keypad. ■ 8 digit seven segment display. ■ All standard functions and commands available through hex keypad as well as through serial monitor. And the most attractive feature of the PC/LC is the built in software for interfacing the PC/LC with the IBM PC compatible computers, which means you can build up your own 8086/8088 development system after making an economical start with the PC/LC. Dynalog also supplies IBM PC, PC/XT. PC/AT compatible computers and peripherals. For more details, write or call: 14. Hanuman Terrace Tara Temple Lane 1 [) n'i kx Lamington Road, Bombay 400 007 / UyM sv>) Tel: 362421.353029 Tlx: 011-71801 DYNA IN Gram: ELMADEVICE Dynalog Micro-Systems Oscilloscopes so advanced, we even export them. Philips PM 3206. The 15 MHz oscilloscope Fully developed at the totally integrated modern professional electronics factory in India, the PM 3206 is the latest oscilloscope from Philips, the largest manufacturer of oscilloscopes in the country today. Now take a closer look at the PM 3206: □ Full 15 MHz Bandwidth with 5 mV sensitivity o TV triggering for TV and Video applications i ' External Z modulation facility 1 1 Choice of triggering from A or B channels or externally u International approvals like VDE & UL and local CIL To meet specific needs, the PM 3206 is available in four versions: PM 3206/02: Dual trace and general purpose scope PM 3206/LPT : (Long Persistance Trace) In DC and mains versions PM 3206/60: Can operate from 24 V dc battery PM 3206/RM: (Rack Mounting Type) Can also be used as a monitor. However, the Philips PM 3206 is not the only 'scope that comes with the Philips performance/cost assurance The others in the range include the PM 321 7 (50 MHz oscilloscope) and the PM 3262 (100 MHz oscilloscope) Every Philips oscilloscope incorporates state-of-the- art technology and advanced features that conform to international specifications and design standards. Furthermore, they come fully backed by the nationwide service network of Philips. An assurance ol prompt service anytime, anywhere in India. Philips India Industrial & Electro-Acoustics Systems Division Calcutta: 7. Justice Chandra Madhab Road. Calcutta 700 020. New Delhi: 68, Shivaji Marg, New Delhi 1 10 015. Bombay: Band Box House. 254-D. A.B. Road, Worli, Bombay 400 025. Madras: No. 3 Haddows Road. Madras 600006. Bangalore: 73/1. St. Mark's Road, Bangalore 560 001. Philips -the trusted Indian household name for over fifty years 8.15 MOBILE SATCOMS FOR THE FUTURE by Dr John R. Norbury, Rutherford Appleton Laboratory We have come to regard the geostationary satellite as the norm for communication between fixed stations and mobile stations such as ships and aircraft. Recent studies show the advantages of a highly elliptical orbit when planning satellite communications with land-based mobile stations, offering much better coverage at higher latitudes. Nearly all recent proposals for | I satellite communications sys- j terns to provide a service to mobile stations have common features. They include the use | of geostationary satellites, 1 operating at radio frequencies around 900 MHz or l.SGHz; j either low-gain omnidirectional I antennas or higher-gain steer- able directional antennas for j the mobile terminals; and com- I munication on narrow fre- quency bands which permits ] only a single channel to be car- j ried on each allocated fre- i i quency (known as single I channel per carrier, or SCPC, access techniques, which ( | means restriction of data trans- | 1 mission rates to the low figure j of some 16 kilobit s '). I Communication via geostation- ! ary satellites gives global | coverage from a threesatellite constellation, which is ideal for j j most maritime and aeronautical | applications, but it suffers from somewhat severe propagation | ) problems when the line-of-sight j path from the ground station to the satellite is at a moderate i | angle of elevation. This is es- ( pecially so with land mobile | satellite services (LMSS), where the low angle may lead to J multipath propagation effects, attenuation by trees and blockage of the signal by buildings or uneven terrain. These factors place con- siderable constraints- on the type of system that can be planned. For land mobile sta- tions, there has to be a toler- ance of fading of the signal I power by a ratio of about 30:1, which in the communications engineer's parlance is a IS dB (decibel) fading margin, to en- sure a 90 per cent probability of acceptable speech communi- cation over 90 per cent of the terrain covered in suburban and rural areas of North America. Europe, with its more northerly situation and its mountainous terrain both in northern and southern regions, may need an even greater margin if there is to be a good enough service. Cost considerations of land mobile stations call for simple, low-cost antennas; that in turn means the satellite should have a very large effective transmit- ter power to provide a service of commercial standard. This criterion could be met by using high-power transmitters and large satellite antennas, but only at a considerable penalty to the overall system cost. The Molniya Orbit An alternative to the geostation- ary orbit is the 12-hour Molniya orbit, used extensively by the Soviet Union and illustrated in the Erst diagram. It is a highly elliptical orbit which provides a satellite position giving angles near to that at zenith, when viewed from Earth at moderate latitudes, for eight hours of its orbit time. On alternate orbits it provides a further eight hours for a region at the same latitude but 180 degrees different in longitude. For 24-hour coverage over one region means using three satellites in three orbital planes separated by 120 de- grees. Obviously, any such con- stellation of satellites also gives coverage for a region 180 degrees different in longitude from the originally planned region. Elevation angles for Europe and polar regions would be high, as is shown by the 'beam footprints' in the sec- ond diagram. The left-hand part of the diagram shows the view of Earth from a satellite in a Molniya orbit with its apogee at 3.5°W. To the right is the view from the equivalent geostation- j ary position, coverage of the polar region is seen to be ex- | cellent using the Molniya orbit, j in contrast to that provided by : geostationary orbit where the | elevation angle to the satellite is J zero at about 81 degrees North I or South, This means that to pro- vide complete polar coverage, J even for fixed point-to-point I communications services, satel- lites in non-geostationary orbits I are needed. | Several satellite configurations [ are possible for LMSS, selected | to reduce the overall power needed in the satellite and, J thereby, the overall system cost. | Constellations of satellites in low orbits have been proposed | in the USA, and Canadian scien- ] tists have studied 12 and 24- hour elliptical orbits in detail. British studies, published by the UK Institution of Electrical Engineers, have investigated ( the application of Molniya orbits to provide UK coverage I for LMSS. Such systems have j several advantages for Europe. The elevation angles are greater than 60 degrees and j there is the possibility of using j high-gain non-steerable anten- : nas for the mobile stations. I Furthermore, the reduction of j multipath propagation with I such an orbit adds to these fac- I tors to remove many of the con- straints imposed by a geo- stationary orbit system. It means the fading margin that has to be tolerated is reduced to a few decibels, and the gain of the mobile station antenna could be as high as IS dB, so the link can be engineered taking into ac- count a starting advantage of 3 100 times more antenna- to-antenna power being avail- able, from base station to mobile, than in the geostation- ary system. And, although it is necessary to provide a three- satellite constellation for coverage over 24 hours, the launch energy needed to place a satellite into a Molniya orbit is roughly half that for a geo- stationary equivalent. The capital cost of a satellite system tends to be related directly to the amount of radio- I frequency power needed for the link. So any configuration that reduces the power needed per voice channel, as in the case of the elliptical orbit satel- lite, makes the system a great deal more commercially attract- ive. The provider of a satellite mobile service would have the choice of an initial system of satellites working at relatively low radio-frequency power per voice channel, or have many more revenue-earning channels for the same capital cost as in a geostationary system. Studies conducted recently in the UK favour a 12-hour ellip- tical orbit, because it would be the lowest cost option for a demonstration satellite. But the orbit does pass through the Van Allen radiation belts, which could degrade electronics de- and solar panels. A so- called Tundra orbit, taking 24 hours, enables this high radiation environment to be avoided. When deciding on the best orbit for an operational system, it will be necessary to compare the threesatellite Molniya constellation, using a low launch energy and small satellite antenna, with the two- satellite Tundra system where of coverage by (left) a Molniya o it and (right! a geostationary oi launch costs are higher, anten- nas are bigger but the radiation environment is better. Transmission Frequencies Procedures for allocating fre- quencies for radio systems are co-ordinated through the Inter- national Telecommunication Union (ITU). Radio transmis- sions do not respect national boundaries, so agreeing uses of the radio spectrum tends to be rather lengthy. A series of World Administrative Radio Conferences (WARC) are held at suitable intervals to agree in- ternational usage. However, at the last major conference, WARC 79, no part of the spec- trum below 20 GHz was allo- cated to land mobile satellite services in the European re- gion (Region 1), whereas a small allocation at UHF was allocated for use in the Americas (Region 2) and Asia (Region 3). This lack of spectrum is a big stumbling block for any com- mercial satellite land mobile service. A special conference, WARC MOB 87, has been or- ganised to take place during 1987 to tackle the problem. Several solutions seem poss- ible, with frequency slots in the regions of 1.5 GHz, 2.5 GHz and 5 GHz being topics for dis- cussion. Although the con- ference might be mainly de- voted to considering geo- stationary systems, some atten- tion will also be given to elliptical orbit systems. Loughborough University Manchester University Portsmouth Polytechnic Queen Mary College. London University Rutherford Appleton Laborat Coding/decoding Doppler correction Payload Study For several years a university consortium in the UK, whose members are listed in the ac- companying table and whose activities are co-ordinated by Rutherford Appleton Labora- tory, has been studying ad- vanced ideas for satellite communication systems under the banner of Communications Engineering Research Satellite I (CERS). Two ideas that have generated considerable in- | terest are the use of on-board processing of signals in satellite j systems and the application of the Molniya orbit. This group is now in the middle of a two-year project in which an electronic model of a mobile payload with full on-board processing is being built. The design of the proposed payload is outlined in the final diagram. A simple reflector of 1.5 m diameter is planned for the antenna, the necessary steering to point to Earth in a Molniya orbit to be achieved by manoeuvring the satellite. De- pending on the data rate, a transmitter power of between 10 W and 20 W will be needed. Full demodulation and decod- ing of the received signals would be included, using a var- iety of schemes. There are several modulation schemes to be considered, including one in which the carrier is phase- shifted by the data keying pro- cess. Decoding would be poss- ible for a variety of coding schemes. An on-board micro- processor would control an electronic buffer store to allow re-formatting of data and re- transmission using modulation and coding schemes that would be independent of the up-link channel. Access schemes for communi- cation with the satellite are, first, time division multiplexing (TDM) on the down-link to mobile stations with time div- ision multiple access (TDMA) on the return path from mobile station to satellite; second, TDM on the down-link to mobiles with SCPC on the up link. The payload, by using dual chan- nels for each system of access, allows full duplex (simul- taneous two-way) operation. Both up and down channels would operate in the L-band (1.5 to 1.6 GHz), with data rates of 64, 128, 256 or 512 kilobit s '. The motion of the satellite in the Molniya orbit causes a doppler shift in the transmitted and re- ceived signals. It is intended to compensate for this on board the satellite by controlling the 8.22 Demod/ Mobile-to- satellite channels Base station-to-sateilite Demod/ I ) | Variable clock rale \ / TDM/TDMA system (64, 128, 256 Kbits/s) payload to be ig. 3. Proposed scheme for torial regions of the Earth operating with a geostationary satellite. If this mobile satellite solution is commercially viable for Europe, then the cost of the transmitter-receiver, produced in quantity, would have to be comparable with those used in terrestrial mobile systems, namely of the order of £1000. The potential for such tech- nology, in regions where satel- lite systems offer the most practical way of providing mass communication, seems con- siderable. trial-based cellular system, it might be questionable whether a geostationary service will be attractive enough commercially at such a level of coverage. An elliptical orbit system, although resorting to the complexity of operating a constellation of satellites, offers almost com- plete coverage even in urban areas and at greatly reduced signal strength requirement. Further spinoff might be found if these ideas were implemen- ted in a European mobile sys- tem. The technology devel- oped could equally well be ap- plied to both mobile and fixed service systems for the equa- on the vehicle roof with its axis pointing vertically. Dimensions of less than one metre square are possible for this. The power of the mobile transmitter would need to be about 20 W. The only obstructions that may be expected to impair reception are overhead bridges or vegata- tion, or multipath scanering that might occur from very tall buildings. System coverage, in time and space, would be bet- ter than 99 per cent. If the justification for satellite systems to provide communi- cation with mobile stations is that they would fill in all the gaps not covered by a terres- frequencies of its local oscil- lators, using either an on-board control system or ground con-' trol. Different types of traffic such as short, coded messages or voice or facsimile could be accom- modated within the same time frame merely by varying the length of the time slot allocated to each individual service by the multiplexing system. The full capacity of the system, using 4.8 kilobit s - ’ voice coding would be about 50 voice channels. For the mobile station, an anten- na with an angle of ± 15 de- grees could be used, mounted :w PRODUCTS • NEW PRODUCTS • N! memory management and high- speed floating point maths. A real-time clock calendar is stan- dard. Mezzanine options pro- vide VSB(MVMX32) compati- bility, as well as a wide range of I/O possibilities. All system configuration is software- controlled, minimising the need for straps and jumpers. Universal Engineering and Computing Systems Ltd 5-11 Tower Street Newton BIRMINGHAM B19 3UY Telephone: (021) 359 1749 (3630-16F) GMSV07 single board computer General Micro Systems GMSV07 advanced single board computer is now avail- able from UECS. This high-performance CPU for the VMEbus provides ultra-high speed data movement, deliv- ering the performance of a minicomputer at a fraction of the cost. Based on the 32-bit 68020 running at 20 MHz, it features up to 1Mbyte no-wait- state RAM, optional advanced SOLAR POWER GENERATION Research and development into the use ot solar energy as an alternative source ot energy have taken on new importance since the oil crises of the 1970s. Moreover, many of us are afraid of the spread of nuclear power stations, and all of us want to get rid of environmental pollution caused by oil or coal burning power stations. The sun converts 600 ''million tons of hydrogen into helium through nuclear fusion every second, and in the process re- leases enough energy to meet our earthly needs for a million years. Of course, only a tiny part of the solar energy falls onto earth, which is readily seen when it is realized that the sun radiates equally in all direc- tions. Since the average distance from the sun to the earth is near enough 150 million kilometres, the energy (in the form of electromagnetic radi- ation) takes about 8 minutes to reach the earth. In that time, the total energy radiated by the sun has spread over the inside of a sphere of surface area 3xl0’ 7 km 2 . The total surface area of the earth that can be lit by the sun at any one time amounts to 113 x 10 s km 2 . This means that only about 4 ten- thousandmillionth parts of the totally radiated energy falls onto earth. The rest is lost in the universe. The solar energy that reaches the earth can be converted into heat or electricity by various means, mainly solar collectors, magneto-hydro-dynamic (MHD) generators, and photovoltaic cells (normally called solar cells). A major drawback to the wide- spread use of solar power generating systems is their high cost: at present, solar power costs £5-20 per watt as com- pared with a few pence for commercially produced elec- tricity. On the other hand, solar power generation has a number of important advantages: ■ Solar energy is free and in plentiful supply. ■ Electricity can be generated, directly or indirectly, where it is needed, which in many cases would obviate the need for a distribution transmission line system. ■ In the case of most solar power generating systems, there are no moving parts, which simplifies maintenance and enables unattended oper- ation, for instance, solar cells on board satellites. ■ It produces no waste or gases: it is clean. Although the cost of solar power generation is at present such that it precludes the widespread adoption of solar power generating systems, it is expected that prices will fall dramatically over the next 10-15 years. Solar collectors Solar collectors are normally constructed in a way that allows I the incident sunlight to be col- lected and converted into heat. The main types of collector are flat, concave, and heliostat. The flat type has the advantage of being able to operate from dif- fused light: the other two can only work from direct sunlight. All solar collectors operate on the same basic principle: sunlight falls onto a blackened absorbent surface and heats the material immediately under- neath that surface. The material is often water, but it can also be air— see Fig. 1. lb protect the collectors from atmospheric ef- fects and soiling, they are com- monly covered by a sheet of Concave (parabolic) solar col- lectors are able to generate temperatures of up to 4,000 °C. They are usually constructed as a dish similar to satellite TV antennas. Heliostat-type solar collectors make use of plane or concave introduced into the vessel. When an HF voltage is applied, amorphous silicon begins to ac- cumulate on the substrate. Dop- ing of the a-Si is achieved by -adding a phosphorus hydride, such as PH3, for n-type, or a boron hydride, such as B2H6, for p-type. Copper(I)sulphide-Cadmium sulphide, CuzSCdS. The elec- trical characteristics of this type of semiconductor are promis- ing, although research into the material is still going on. From early prototypes, it is clear that both high efficiencies and high power outputs can be obtained. Gallium-Arsenide, GaAs. Although this type of material affords a very high effciency, it is expensive to produce. How- ever, it has a non-linear light- power characteristic, which makes it particularly interesting for use in combination with concave solar collectors. Moreover, compared with crystalline silicon, GaAs does not dissipate so much heat and, therefore, requires less cooling (smaller heat sinks). Cadmium-Selenium, CdSe. This type of solar cell is still in the development stage. Table 1 gives a comparison of these various types of solar cell. A number of other materials are actively being investigated in laboratories all over the world, but at present it does not look likely that these will find com- mercial application in this century. Basic operation of a silicon solar cell The characteristic behaviour of a semiconductor depends on the nature of the constituent atoms and on the way in which these atoms are grouped together. In other words, it is a function of the atomic structure as well as of the crystal struc- ture of the semiconductor. An atom consists of a positively charged nucleus surrounded by negatively charged elec- trons located in discrete orbits (shells) around the nucleus. Electrons can exist in stable or- bits near the nucleus only for certain discrete values of energy, called energy levels of the atom. The allowed energies MHD generators Magneto-hydro-dynamic gener- ators convert thermal energy direct into electricity. A sche- matic representation of such a generator is shown in Fig. 4. The thermal energy is obtained fay heating a gas to some 2500 °C by means of a large concave solar collector. When the temperature of the gas reaches 2500 °C, ionization oc- curs. This causes the gas mol- ecules to accelerate to speeds of well over 300 m/s. The gas is then passed through a mag- netic field, which separates electrons and ions, whereby an electric current is generated. This type of generator is still in its infancy, although large pro- totypes are already in operation in the USA and the USSR. The main problem is the heating of the gas to the high temperature required. None the less, the prototypes work well and show efficiencies of up to 55%. Solar cells Solar cells provide an attractive and promising source of alternative energy. Unlike solar collectors, they provide a means of direct conversion of solar energy into electricity. Types of solar cell Crystalline silicon, Si. By far the largest proportion of solar cells currently manufactured are made from crystalline sili- con. The basic construction of this type of cell is shown in Fig. 6. Its operation will be dis- cussed later in this article. Amorphous silicon, a-Si. Amorphous silicon is, accord- ing to many researchers, the solar cell material of the future, because its production costs are a fraction of the price of crystalline silicon. Amorphous silicon can be formed by a number of methods, such as sputtering, pyrolysis, and high-frequency glow discharge. At present, the glow discharge method is preferred. In this, a substrate is held at a temperature of about 300 °C in a vessel in which the pressure is about 5 ton. Silicon hydrides, such as SiH* or SbHs, or silicon tetrafluoride, SiF*, are Fig. 7. Energy level diagram (simplified) of a semiconductor. J of electrons in an atom are | the electron density, e the elec- 1 represented by horizontal lines j tron charge, and v the average in the energy-level diagram velocity of electrons in the | shown in Fig. 7. Not more than valence band. two electrons can occupy a If the kth electron crosses to the | level: this results in electrons , conduction band, | filling up the lowest possible [ | levels first. Since the atoms in a semicon- | *E"-i.mevi = — evy [2] ductor are closely packed together, there are very many from which it is seen that the J energy levels associated with j vacancy (hole) in the valence each nucleus (because of the [ band can be considered as a interaction between the atoms). | positively charged carrier fully [ This results in the energy-level j analogous to the negatively diagram for the material be- J charged kth electron. The vel- coming an energy-band dia- ocity of the hole is equivalent to | gram (each band contains very that of an electron in the same | many levels). energy level, j The lowest energy band is At absolute zero, all the elec- called the valence band: this is trons occupy the lower energy filled with electrons, since levels, the valence band is filled ] there is an electron for each of to maximum energy, and no the energy levels contained in higher levels are occupied, the band. The upper energy I This level of maximum energy band is virtually devoid of any J is called the Fermi level, Er, | electrons: it is called the con- J which is approximately con- I duction band. There is a small | stant with temperature. When forbidden gap between the | the temperature is at room valence and conduction bands, j level, the electrons in a semi- Because of the thermal energy | conductor are distributed be- of the semiconductor at room I tween the valence band and the temperature, some electrons conduction band, and the Fer- can cross the forbidden gap mi level lies in the forbidden into the conduction band. The gap. consequent empty energy level Since the Fermi level is constant in the valence band is called a throughout the silicon, the h°' e - energy bands at the junctions in The total current resulting from Fig. 8 are distorted, which the electrons in a filled valence causes an electric field accross band is the junction. This field is called the built-in field. j = nev = K = 0 [1] When the silicon p-n ] junction— see Fig. 8a— is in where y is the current density, n equilibrium (no bias), the cur- 8.27 rent 7i resulting from electrons diffusing from the n-side is equal to the current h which arises from electrons leaving the p-side. If a positive voltage is applied to the junction— see ' Fig. 8b— the built-in field is in- creased. The number of elec- trons diffusing from the n- region is then much smaller, since only few electrons have the energy required to over- come the built-in field. How- ever, the number moving from the p-region to the n-region is not affected, because these electrons encounter no field. Therefore, a net current flows, but it is limited by the small number of electrons in the p- region. If the polarity of the ap- plied voltage is reversed— see Fig. 8c— the built-in field is re- duced and I\ is large because the number of electrons in the n-region is so large. As before, Iz from the p-region to the n- region remains unaffected. The net current is then large and corresponds to the forward direction. The net current, 7, under forward-bias conditions is given by the exponential expression /=/«, [exp(eW/i*n— lj [31 where 7o is the reverse satur- ation current, e is the electron charge, V the applied voltage, n a factor between 1 and 2 representing the deviation from ideal diode characteristics, k the Boltzman constant, and T the absolute temperature. In a silicon solar cell, in the absence of incident light (called the dark state), the ex- pression for the dark current, U is identical to that for 7 in for- mula (3). When the cell is illuminated, a photo-generated current, 7 P h, flows as junction reverse cur- rent. This current is directly proportional to the intensity of illumination. From Fig. 11 it will be seen that the net current, 7, is given by 7=— 7ph+7d= 7=-/ P h+/o[exp(eWn*7>-l) [4j The voltage Ui across the load and the current 7 l through it produce an output power Po, which is equal to UlIl or Ii‘Rl, and is the direct result of the in- cident light falling onto the cell. Finally, Fig. 12 shows that the sensitivity of a silicon solar cell is greatest at a wavelength of about 0.8 pm, i.e., at the lower Fig. 9. Photovoltaic effect in unbiased p-n jun Fig. 10. Current-voltage char teristic of silicon solar c 1 1 photon j. 11. Basic circuit of ill Physics of atoms and molecules by B. H. Bransden and C. J. Joachain Quantum physics of atoms, molecules, solids, nuclei, and particles by Robert Eisberg and Robert Resnick 12. Spectral response of a typical si 8.28 elekior tndta august 1987 LASERS: AN OVERVIEW The development of lasers since they first appeared in the early 1960s has been spectacular. In just over 25 years they have become virtually indispensable in such diverse applications as compact disc players, fibre optic communications, surgery, and the Strategic Defence Initiative. The first lasers appeared in | 1960-61 when Javan, Bennett, | and Herriott of Bell Telephone j Laboratories announced the 1 helium-neon laser just after I Theodore Maiman, working at j the Hughes Aircraft Corpor- ation, had made a practical ruby laser. In little over a year later a semiconductor laser had been developed more or less simul- taneously in Britain and the USA. Foundations J An atom may be represented by a Bohr model: Fig. 1 shows that of a hydrogen atom. Bohr con- sidered one electron of charge —e and mass m, moving with speed v, and acceleration v*/r in an orbit around a central nucleus of charge +e. In | classical physics, charges j undergoing acceleration emit I radiation and would, therefore, lose energy. On this basis, the electron would spiral towards the nucleus and the atom would collapse. Bohr therefore I suggested that in those orbits j where the angular momentum J is a multiple, n, of h/2n, the i energy is constant. In the early [ 1920s, de Broglie proposed that an electron may be considered | to behave as a wave of wave- length A =h/p, where h is the Planck constant(4.14 x 10"' s eVs) J and p is the momentum of the moving electron. If the electron can behave as a wave, it must be possible to fit a hydrogen atom. | whole number of wavelengths around the orbit. In that case, a standing wave pattern is set up and the energy in the wave is confined to the atom. If there are n waves in the orbit and A is the wavelength, I nA=2itr j so that, A=2 nr/n=h/p=h/mv from which, I mvr=nh/2n This shows that mvr, the angular momentum of the electron is an I nth multiple of h/2n. In Fig. 1, the electron moving around the nucleus has kinetic energy due to its motion and | potential energy in the elec- trostatic field of the nuclear j charge +e. Bohr calculated the total energy E of the electron in terms of its charge, mass orbital radius, and I the number n which quantizes I the angular momentum. He then assumed that the electron can pass from one energy level to another. If, for instance, the I electron jumps from energy level Ei, corresponding to I n=m, to a lower level £2, corre- sponding to n=m, the differ- ence in energy is released as radiation of energy hi/, where h | is the Planck constant and v is the frequency of the radiation. Therefore, Ei—Ei=hi/=hc/k ' where A is the wavelength of the radiation and c is the speed of j light in a vacuum. I Although Bohr's theory of the hydrogen atom was unable to I predict the energy levels in atoms with many electrons, its : fundamental ideas remain valid. | For instance, the angular mo- I mentum of the electron has quantum values, and the energy levels of an atom have only discrete values: Eo, Ei. Ei. . . E.,; no other or in- termediate energy level is poss- ible. The lowest energy level, £0, is called the ground state energy. All physical systems are in stable equilibrium in the lowest energy state. If an atom absorbs energy, and the energy of the atom reaches one of its discrete levels, Ei, the atom is said to be in an excited state. Once an atom has been excited to a higher energy level, En, it will try to reduce its energy. The energy lost if the atom reverts direct to the J ground state is En— Eo. This j energy is radiated in the form of electromagnetic radiation, ie., quanta of energy hr— see Fig. 2. j These quanta are called photons. The frequency of the photons lies in the range 5 nm to 10 nm. From the foregoing, it | follows that hv=En—E». An atom can interact with a j photon in three ways: absorp- tion, spontaneous emission, and stimulated emission— see Fig. 3. If an atom absorbs a photon of energy hi/, and the difference in energy levels of the atom is equal to hv, the ' with a photon in three ways. photon will raise the atom to a higher energy level. In spon- taneous emission, an atom in level 3 may of its own accord emit a photon hv, leaving the atom in the lower level 2. In stimulated emission, an atom in level 2 may be stimulated to emit a photon hv by interaction with another photon of the same energy. If in a system of atoms with an energy level £V above the ground state there are no photons of energy £V— £o, where £o is the ground state energy, the atoms remain stable. If, however, a few photons of energy En—Eo are introduced, these will immedi- ately stimulate the emission of a number of photons of the same kind. This increases the number of photons in the system, which in turn stimulate the emission of more photons. In this way, an avalanche effect is produced, which results in all atoms in the system rapidly giv- ing up their photons— see Fig. 4. This process is called laser action (light amplification by stimulated emission of radiation). In a system of atoms in thermal equilibrium, the number of atoms in the ground state is much greater than that in a higher energy state. This is called a normal population of atoms. In such a system at tem- perature T, the numbers, n< and ii2, of atoms in two successive states, £. and £2, are related by the Boltzmann formula (in which k is the Boltzmann con- stant - 1.38 x 10'” I K' 1 ): m=iu exp[ — (£2 — Ei)/kT\ from which it is seen that at room temperature (T= 300 K), m is considerably smaller than ni, ie., a normal population ob- tains. If it is possible to make Jt2>ni, a population inversion is produced, which enables laser action to take place. The output from a laser may be continuous (CW operation) as is usually the case with gas lasers, or pulsed as that from solid- state lasers. Table 1 lists a var- iety of lasers and some of their characteristics. Three-level lasers At present, the main solid-state lasers are the ruby (Cr 34 ) and the neodymium/yttrium alu- minium garnet (Nd/YAG) lasers. The ruby laser is a three-level 8.30 ele*..* ind.a aufluST 1987 Various lasers and some of their characteristics. Fig. 5. Artist's impression of the construction of a ruby laser. laser (£2>£<>£o), with a fast decay between levels 2 and 1, and a slow decay between 1 and 0. A typical construction of this type of laser is shown in Fig. 5. Ruby consists of a small con- centration of Cr 34 ions in a lat- tice of crystalline AI2O1 When a high potential is applied to the flash tube, the ions are excited, or pumped, by photons of wavelength 550 nm (green light) and energy £2— £0— see Fig. 6. The excited ions decay spon- taneously to the lower energy state £>, emitting photons of energy £2— £1. The energy state £1 has the special property of having a large stimulated emission prob- ability and a low spontaneous emission probability. It is, therefore, filled with a far greater number of ions than the ground state £0. There is thus a population inversion between these two levels, so that laser action can be initiated, resulting in the emission of red light (A =694.3 nm). Four-level lasers Except in a few cases, such as in the ruby laser, it is difficult to produce a population inversion between a ground state and an excited state, because initially all the atoms are likely to be in the ground state, and more than half the atoms have to be pumped to level 2 before a population inversion can be achieved. An easier method is possible in a four-level laser in which a population inversion is created between two excited levels— see Fig. 7. Initially, all the atoms are in the ground state, £0, and none in the ex- cited states 1, 2, and 3 (£,<£ 2 <£ 3 ). Level 3 is chosen so that it has a fast decay to level 2, and pumping between levels 0 and 3 immediately produces a population inversion between levels 2 and 1. As level 2 begins to fill up by the stimulated emis- sion at frequency (Ei—E>)/h, the population inversion will decrease. To minimize this ef- fect, level 1 is chosen so that it has a fast decay to the ground state. Gas lasers are examples of a multi-level system, which can be pumped by an electrical dis- charge rather than by incident radiation. An important model is the He-Ne laser, in which the I Fig. 7. Operation (simplified) of a four-level laser. | active material is a mixture of [ helium and neon gases con- I tained at low pressure inside a I long quartz tube with optically I plane mirrors at each end— see | Fig. 8. Two terminals near the I ends of the tube enable a high | potential to be applied to pro- duce a discharge in the gas mixture. A typical construction | of a He-Ne laser tube is shown in Fig. 9. In an electrical discharge, the helium atoms are raised to the I 2'S and 2 3 S levels which are [ metastable— see Fig. 10. By col- lision with these atoms, the neon atoms are excited to level 3, so that a population inversion | is produced and laser action occurs as explained above. The wavelength of the emitted light J depends on the reflectivity of the mirrors between which the I gas is placed. Oscillation will take place at the wavelength for j which this reflectivity is a maxi- j mum. In Fig. 10 it is-typically- 633 nm (red light). It is seen that I two other beams are also gener- ! ated: one at 3.39 pm and one at | 1.15 pm, but these are effec- I Polarization of laser light [ Although laser light is coher- ent, because all the photons (or waves) are in phase, polariz- ation is random -see Fig. 11. To linearize the polarization, a Brewster window as shown in ! Fig. 9 is used. Such a window is a disk of plane glass (see Fig. 8) which is set at the Brewster angle to the incident light to en- sure that only light of a given wavelength is passed. Brewster's law states that when light strikes a glass surface at an angle of incidence given by tan '(/?), where n is the refrac- tive index, the reflected light is plane polarized. At this angle of incidence, the refracted ray makes an angle of 90° with the reflected ray. Resonance cavity The laser emitter is placed be- tween parallel mirrors so that photons can be reflected back and forth many times, resulting in the build-up of a large photon density by the avalanche effect. j | It is, of course, necessary that 1 one of the mirrors be partly transparent, so that some of the light can get outside the tube used. I The mirrors may be plane or curved as shown in Fig. 12. 1 When plane minors are used, part of the emission may be | reflected spuriously outside the i system. Such losses must be kept small: reflections must be I higher than 99%. When con- I focal minors are used, the beam is kept exactly parallel within the cavity. The slight divergence at the exit is con- trolled by a collimating lens. I Beam spread Many laser tubes are marked with their internal beam radius, n>], from which the beam diameter, Dx, at a distance m can be calculated: where 20, the angle of spread, is equal to Unib\ / is the wave- length of the laser light. If, for instance, a He-Ne laser, operating at a wavelength of 633 nm, has an internal beam radius of 0.375 mm, the beam | diameter at a distance of 100 m j Dm=20m = 2nri/nrbi =2x100 x 633x10-*/ | 3.142 x 375 x 10'® = 107.5 mm. Lasers and their applications Since the development of lasers continues at a spectacular speed, only an outline of the state of the art will be given. He-Ne lasers, because of their small output (0.1— 10 mW) are best suited to use in labora- tories and measurement tech- niques, but are also used for medical purposes. Their wave- lengths are 632.8 nm, 1.15 nm, and 3.39 pm. Argon-ion lasers, with outputs of up to 15 W, are frequently used in medicine for photo coagulation. Their bluish green light (488 nm and 514.5 nm) is selectively absorbed by hae- moglobin and melanin. Their main application, however, lies in the field of eye surgery. Carbon-dioxide (COz) lasers, operating in the infra-red tively suppressed by filter ac- tion of the minors. Fig. 9. Cross-sectional view of a He-Ne laser (courtesy of Siemens). region (9.6-10.6 nm), are pri- marily used in industrial appli- cations: hardening; drilling; welding; refining; and ageing are but a few of these. The use of a CO; laser for industrial welding is illustrated in Fig. 13. The dye laser is pulse operated and pumped by a xenon Dash tube— see Fig. 14— or by a pulsed beam from another laser. Continuous tuning of this type of laser is possible by mak- ing the grating that forms one end of the resonant cavity rotat- able. With its very narrow line width and large frequency range, the dye laser is emi- nently suitable for use in spec- troscopy and in the chemical industry. Solid-state lasers find almost universal application in measurement techniques, be it the exact distance from the earth to the moon or the speed of motor vehicles. Many of these techniques are by- products of military research. The only solid-state laser to be used in the medical world is the Nd/YfiG laser. Because of its high power output (>100 W continuous) and operation in the infra-red region (0.9— 1.35 j;m), this type of laser is par- ticularly suitable for operations in soft tissues, such as the removal of tumors in the oesophagus. Solid-state lasers can produce pulses of extremely high power: a power of 100 TW (=10'* W!) at the peak of a 2 ns pulse has been reported. Such enormous powers are needed in the strategic defence in- Fig. 14. Artist's impression of a dye laser. itiative and in research into nuclear fusion. The exit beam diameter of such lasers is ar- tificially increased to about 1 m to prevent vaporization of the lenses. By far the most common lasers nowadays are semiconductor or injection lasers. These lasers are based on the fact that a population inversion of elec- trons can be achieved by apply- ing a voltage across the p-n junction of doped gallium- arsenide (GaAs) material. Semi- conductor lasers are available for operation from the near ultraviolet to well into the infra- red regions. An artist's impres- sion of the construction of a GaAs laser is shown in Fig. 15. Semiconductor lasers are of prime importance in modem communications, optical memories, and in compact disc players. In fibre optic communi- cations, for instance, they enable transmission rates of 1400 Mbit/s to be achieved. Without the small dimensions of the diode laser, it would not have been possible to develop the compact disc player. A very recent development based on the diode laser is compact disc video. In the fore-front of laser devel- opment is the excimer laser which uses diatomic rare-gas hallides as the active material. This type of laser was de- scribed in the March 1987 issue of Elekior India. Commercial considerations In 1986, the world market for lasers amounted to more than £425 million. The largest sectors were research and develop- ment, and materials processing —see Fig. 16. When studying this figure, it should be borne in mind that the diode laser for a ] CD player costs only about £3, ] whereas an industrial model j may cost as much as £30,000. It is expected that the laser market will have grown to j around £1,000 million by the [ early 1990s. References: Advanced level physics' by M. Nelkon and P. Parker Optics and its uses by G F. Lothian Physics of atoms and molecules by B. H. Bransden and G j. joachain Quantum physics of atoms, molecules, solids, nuclei, and particles by Robert Eisberg and Robert Resnick Physics by David Halliday and Robert Resnick Fundamental University Physics by Marcelo Alonso and Edward J. Finn 8.33 THE COMPACT DISC In the mid-1970s, engineers at the Dutch electronics company Philips felt they had developed just what the world had been waiting for. They called it the Laser-Vision videodisc. This is an optically scanned disc which gives an hour of colour video and sound. Unfortunately for Philips, the video disc arrived too late: too many people already had a video cassette recorder. Undismayed, the engineers continued their development, and in early 1979 Philips unveiled a trimmed down version of the videodisc, much smaller and containing sound only. It was called the compact disc. Because of the very favourable reception of the compact disc system, Philips felt it had a new I world standard to replace the | conventional (and vulnerable) gramophone record. Wisely, it ' came to a joint agreement with 1 Sony to perfect the system. The first compact disc player went on sale in Japan in late ; 1982, and in Britain six months j later. At that time, a player cost I around £500 and the discs about £10. Now, just over four and a half years later, a reasonably good player can be bought for under £200, and it is expected that prices will be under £100 by Christmas. The discs have, however, risen slightly in price to about £12-14. Production technique | The master (or blank) is made of | a glass disc that is ground and polished to optical flatness— J see Fig. 1. This is coated with a layer of photoresist, the thick- ness of which is controlled very accurately. The coating is oven- cured, after which the disc is ready for cutting. Strictly speak- ing, the term "cutting" is incor- rect, because the recording is created photographically, but because of some parallels with the production of a vinyl gramo- phone record, it has been re- Cutting is carried out by a con- tinuously operating helium- neon (HeNe) laser, which is intensity-modulated by the audio signal via an acoustic modulator. In the absence of an audio signal, light can pass through the modulator, but with an audio input light is scattered. The laser travels from the centre of the disc to the outer as the master revolves. The rota- Fig. 1. Some of the stages in the production of a compact disc. 8.34 i id etching tional speed of the disc is re- duced gradually in a way to ensure that the speed of the laser beam over the surface of the blank remains constant. The photoresist is then devel- oped during which the unex- posed areas are hardened. Subsequent etching removes the exposed areas, which has the effect of creating pits in the surface of the resist as shown in Fig, 3. These pits represent the digital information of the audio input. The disc is then given a thin silver coating to make it elec- trically conductive. At this stage, it would be possible to produce a commercial com- pact disc from the master. How- ever, to preserve the master, only a few (negative) copies, called "father" are made. From these, a number of (positive) intermediate copies, called "mother" are made, and these in turn produce a number of "sons” (negative). The sons are the dies used to stamp compact discs. Since there are an even number of processes, the com- pact disc is identical to the master. The compact disc is made of 1.2 mm thick polymethylmetha- crylite, better known as Perspex, or of Makrolon, a polycarbonate plastic. The sur- face of the side of the CD that contains the audio information is then given a thin layer of alu- minium, followed by a protec- tive coating of laquer. The thickness of the aluminium layer is of the order of only 10 nm, while that of the laquer is about 5—10 jim. This side of the disc is called the label side, because the identifying label is printed or affixed here. The information is read from the disc by a laser at the underside. i.e., through the Perspex or Makrolon. The laser, therefore, sees the pits as bumps. A typical construction of a laser pick-up unit is shown in Fig. 4. Structure of the compact disc Figure 5 gives a cross-sectional view of a compact disc. The lead-in track contains all the necessary information regard- ing the recorded music or speech. A total of some 20,000 tracks are contained within the 33 mm wide recording surface. The digital data are defined by the length of the pits and the distance between them. The length of the pits varies from 833 nm to 3.56 jim, their width is 500 nm, and their depth is 110 nm. The distance between two adjacent tracks is 1.6 jrm. The disc contains some 7x10’ bits. At a constant linear vel- ocity— CLV— of 1.2 ms', the maximum playback time is 74 minutes. The Perspex from which the made has a refrac- disc base is five index, diameter of the laser beam when it enters the Perspex is 0.8 mm, but because of refrac- tion this is reduced to 1.7 jim at the recording surface— see 8 I Fig. 6. This small diameter is j one of the reasons that, say, a | dust particle of 0.5 mm does not | affect the reproduction of the disc. Pits and bits The pits and the reflective (alu- I I minium) surface represent | logic Os and Is respectively. I When the laser beam is fo- | cused on a pit, ideally no light should be reflected. To achieve that, the depth of the pit, a, is approximately equal to ,1/4 n, where J is the wavelength of the J laser light and n is the refractive 1 index of the disc base. Since the diameter of the laser j j beam at the recording surface | j is 1.7 Mm, and the width of a pit | is 0.5 Mm, some light is reflected | | from the pit. Because of the relationship between the depth | of the pit and the wavelength of j the laser light, there will be a phase difference between light reflected from a pit and that | reflected from the aluminium ; layer of 2/1/4=180° (in an ideal I case). This means that due to the interference effect the two reflected light beams will j cancel one another. In practice, j this cancellation will not be complete, however, but the re- j duction in the total reflected light is none the less sufficient to actuate the focusing detector unit. The reflected light is i j consequently modulated in a 1 I manner that depends on the J length of the pit. The optical system The laser, optical system, and j detector are contained in one unit as shown in Fig. 7. The col- j lection and telescope lens assemblies focus the light emanating from the laser diode. The correction prisms shape | this to an annular beam. This I beam is deflected by a routeing mirror assembly to a polarizing beamsplitter and //4 plate as- sembly, where the plane of polarization is shifted by 90°. From there, the beam passes through the objective lens to the recording surface of the CD. The reflected light is taken from the objective lens, aligned parallel, and then falls onto the ,1/4 plate. The plane of polariz- ation is again shifted 90°, after which the beamsplitter directs the beam to the focus error prism, from where it travels to the detector (photo-sensor). Fig. 8. The SONY laser pick-up In the Sony laser head used in commercial CD players, shown in Fig. 8, the photo-sensor is a four-quadrant type. This head contains two extra sensors (E and F) for the side spots. Con- trol signals from these sensors drive the two-axis device. Input signals for the audio amplifiers, servo systems, and associated circuits in the com- pact disc player are also taken from the photo-sensor unit. Sampling frequency | The sampling frequency should ; 1 be greater than twice the fre- quency of the highest audio fre- quency the system is required to process. Taking also the anti- j aliasing requirement into ac- I count, a world-wide standard of 44.1 kHz was chosen. With a sampling frequency of 44.1 kHz, the upper audio fre- quency range must be limited to just above 20 kHz. Although this is considered satisfactory by many, there are also many who feel that this limitation is unacceptable. Since manufac- turers of CD players can not change the agreed sampling frequency, they have devel- oped a technique called digital filtering or oversampling. In oversampling, the original sampling frequency is seem- ingly doubled or even quad- rupled by electronic means. In twice oversampling, there are 44,100 real samples coming off the disc, and a special elec- tronic circuit adds a sample be- tween each pair of real ones to give a total of 88,200 samples. These added samples are an I electronic prediction as to what | they would have been had they been recorded on the disc. In four times oversampling, the number of predicted samples | increases to three between each pair of real ones. In some I CD players, the previous thirty samples are used to predict every set of three guessed samples. With all oversampling, extra bits are generated: one in twice oversampling, and two in four times oversampling. These bits are in addition to the 16 bits already coming from the disc. | Unfortunately, the signal pro- cessing circuits of CD players i can cope with 16 bits only, so that, ironically, some of the in- formation has to be discarded. Disc production At present, there are only a dozen or so CD producers in the western world and two in the USSR. Most of these made their name through gramo- phone record production and have been in existence for a l long time. j The largest CD producer is cur- rently FolyGram, a subsidiary of Philips, with plants in Federal Germany and Britain. The first British company to produce CDs was Nimbus of Monmouth, which started in 1984. There is I now also Thorn-EMI in Swin- don. Since worldwide pro- duction at present amounts to only about 100 million per year, it is clear that with nearly 20 million CD players in use in the western world demand out- strips supply, which will keep the price of the disc high. It will take a year or so yet before supply will start catching up with demand: only then is there a likelihood of CD prices com- ing down from their present level. The biggest bottleneck in pro- duction is the metallization of the disc with aluminium which ensures that the disc can be I read by the laser in the player. ! Until recently, this was done in large chambers that hold hun- dreds of discs at a time. It takes about 15 minutes to create a vacuum in the chamber and another 10 minutes' to deposit the aluminium. New machines from Balzer in Switzerland bring the cycling time down by more than a half. These evaporation chambers are held at a perma- nent vacuum. The discs are loaded at one end on a con- veyor and passed through a series of bulkheads that create a pressure gradient from at- mospheric to high vacuum and up to atmospheric again. j In spite of the strict clean-room [ procedures at CD production j plants (in most the disc does not | come into contact with humans until it has been given the pro- j tective lacquer coating: all | previous operations are per- formed by robots), the rejection J rate remains high at over 10% ; over the entire production pro- cess. It should, of course, be | realized that this involves no fewer than 60 stages from tape j mastering, through disc mas- J tering, electroplating, pressing, I metallization, and so on, to | packaging. j An interesting aspect of the [ siting of a CD production plant j is that the foundations must be I very stable: deep rock is pre- j ferred. because its natural j movement is not more than a j few micrometres at very low fre- [ quency. This stability require- ment becomes clear when it is j realized that the track dimen- ' sions of the high density master discs are less than 1 micro- | most of the world's large record companies have confirmed | their backing. The video picture signal is recorded towards the outer edge of the disc, where it is easier to get a high tracking speed. Normally, a digital audio disc spins between 196 and 486 rev/min to give a constant linear velocity (laser tracking speed) | of 1.2 m/s. This is too slow even for analogue video. The snag with the new system is that it is linked to TV standards, at least as far as the video sec- tion is concerned. For PAL CDV discs, with 25 pictures/s, the rotational speed varies from 1512 to 2250 rev/min, giving it a CLV of between 9.2 and 10.2 m/s. For NTSC video (30 pictures/s), the spinning speed will be 1815 to 2700 rev/min, resulting in a laser tracking speed of between 11 and 12 m/s. The CD video During the preparation of this article. Philips. Sony, and a number of other Japanese manufacturers announced the CDV player. This type of player, 1 whose commercial launch is planned for the coming J autumn, can handle normal j audio compact discs as well as I the new CDV discs which hold j 5 minutes of colour video as well as 20 minutes of sound only. It appears these manufacturers' 1 intention to use CDV as a means 1 of marketing pop music video clips. Polygram, Philips' sub- sidiary record, CD, and tape 1 manufacturing plant in Federal Germany, is in full support of the new system, and claims that Commercial aspects j During the 1980s, the audio equipment market in genera! 1 grew moderately in size, but 1 hardly at all in value. The excep- j J tion was the CD player sector, | j which saw a boom towards the 1 end of last year that continued J into this year. An estimated 1 192,000 players were sold in I j November and December alone: a three-fold increase I compared with the same ! months in 1985. If these new j buyers follow their prede- cessors' purchasing patterns, | the sale of CDs should rise l quite sharply. Gramophone Magazine's CD survey showed i that 69% of CD player owners ( own more than 20 discs. How- ever, although compact discs 1 offer hitherto unattainable qual- I ity, at nearly twice the price of j LPs and cassettes they still ap- peal mainly to the serious I music enthusiast. J Figures just released by the I British Radio & Electronic j Equipment Manufacturers' A s- I sociation (see Table) show that j during last year CD player deliveries were at more than four times the level achieved in 1985. The major development in 1986 was the increasing avail- ability of combination products, primarily CD music centres, I which contributed to a high level of consumer interest. These products accounted for over one tenth of total music centre deliveries. The CD separates sector was very ac- tive and registered a more than three-fold increase over the 1985 results. These represent faster growth than that achieved by any other consumer elec- tronics product. Together with the 'ordinary' three pin regulator IC's a few different types for special use are becoming available. The LM 2930 from National was primarily intended for use in the car, but it also has other applications. The 1C has a few quite useful characteristics, such as the fact that the difference between input and output voltage need only be 0.6 V. Changing the input voltage's polarity is no longer a disaster and short voltage peaks of up to 40 volts naiics no damage. Other character- include voltage limitation and car stabiliser ritpri although thermal protection and useful, are less spectacular. Since the output voltage is 5 volts (there is also an 8 volt version) and the maximum current is 200 mA, this regulator will be ideal for use with instruments (speedometers computers) rather than in audio. The circuit is extremely simple. Both capacitors have to be mounted close to the 1C in order to prevent oscil- lation. In most applications the 1C must be mounted on a heatsink; this can be connected to ground. The maximum input voltage is 26 V. M FILTERS: THEORY AND PRACTICE — 1 by A.B. Bradshaw The design of filters remains a topic of considerable interest to practitioners in many branches of electronics, in spite of the fact that many of such networks can nowadays be purchased at rela- tively low prices. None the less, there are still many occasions when a filter has to be designed from scratch. This series of articles will look at the theory underlying such design, and in the last part two practical designs will be discussed in detail. As long as there has been elecronic engineering there { has been a need for filters: low- l pass, high-pass, band-pass, and ' j band-stop. Basically, a filter is an 1 I electrical network that will pass | signals with frequencies within certain ranges and suppress signals with other frequencies. I A network is essentially a j number of impedances con- nected together to form a l system the behaviour of which | depends on the values of the | resistances, capacitances, and j inductances from which it is I made up, and on the way in which they are interconnected. In the 1920s, Zobel developed I the so-called image parameter theory, which formed the foun- dation for filter and network | design until comparatively re- cent times. This theory met the needs of designers working on filters for multi-channel tele- phone links and VF teleprinter links quite adequately. Television, radar, data trans- mission, and other techniques developed during the 1940s and 1950s showed up the limitations of image-parameter theory. The higher precision and more ex- act characteristics required of filters from then on caused the image-parameter theory to give way to the modern network theory that uses synthesis tech- niques and digital computers. One of the latest products of modern filter technology is the surface acoustic wave filter, which has an exciting perform- I ance, and can already be ob- tained at relatively low prices. Murata, for instance, produce a | 10.7 MHz SAW that retails for ' less than £5 (available from Cirkit), and Plessey make units I for the IF stages in TV I receivers. Surface acoustic wave filters have a superb rectangular fre- quency response with constant group delay: >t is possible to make these filters and shape the two features separately— this is unique to SAWs. Their only disadvantages are a high insertion loss (20—30 dB), which results from the necessi- ty of suppressing certain trans- mission reflective modes, and the necessity of temperature stabilization of the unit for cer- tain applications: there is usually a price for everything! General network concepts Networks can be shaped like a T, a n, or an L, as shown in fig. 1. There is also a ladder network and a lattice network. The boxes in the diagram (and all others in this article) represent j an impedance. This impedance may be a pure resistance, a reactance, or a combination of the two. It is customary to show series impedances in half, i.e. Z,/2, and parallel (or shunt) im- pedances double, i.e. 2Z\. It will be seen that this eases the calculations. Most networks and filters are unbalanced and one side is usually grounded. A notable exception is found where high levels of electromagnetic hum or RF interference prevail. This I situation can arise in sound | studios, particularly when these 1 are co-sited with their parent j transmitters. In these cir- cumstances, the sound line dis- l tribution system is usually ! balanced. The balanced ar- rangement is made to cancel | out induced currents in each I leg of the lines. -T0 □ b Characteristic impedance Zo The characteristic impedance, j Zo, is defined as the value of the input impedance of an infinite I number of cascaded identical I networks— see Fig. 2. From this definition, it follows that a net- work with a terminating im- | pedance of Zo behaves as if it were infinitely long. Such a net- j work is said to be matched- see Fig. 3. Since the network in fig. 3 is in- finitely long, no signals can return from the far end. This reasoning also applies to a net- work matched in its own Zo. It | should be noted that even if a | network is not matched in its I own Zo, and contains lossy im- pedances, it can be shown | mathematically that it still tends to behave as though it were in- I finitely long. Symmetrical or asymmetrical A network is symmetrical if its input and output terminals can be interchanged without caus- ing any change in its electrical performance. Zk 2 at one pair of terminals of network N produces a like im- pedance at the other pair of ter- minals. Similarly, an impedance Zki at the other pair of terminals produces a like impedance at the first pair of terminals. Im- pedances Zki and Zk 2 are called iterative impedances. If the two iterative impedances are equal, their common value is the characteristic im- pedance of the network. In Fig. 5, impedance Zn con- nected across one pair of ter- minals of network N causes an impedance Z12 at the second pair of terminals, and an im- pedance Z12 connected across the second pair of terminals causes an impedance Z11 at the first pair. These two im- pedances are called image im- pedances. In symmetrical networks, the iterative and image im- pedances are equal. Maximum power transfer theorem For a generator to supply power ! to a load at maximum efficiency, A network is asymmetrical if its In Fig. 6, ZGcjsi +Zicjs2= Characteristic im- pedance 1 =(Zccosi +ZLCOS2)+ Symmetrical T network. In Fig. +XZcsin®i +ZLsin2) [1] 1 so that 7 Zt 2 =Zg 2 +Zi, 2 + KU-pO - + 2ZcZlcos(Oi— <> 2) ■ — • N [ 3 Now, I=U/Z r and Z. p =Zi/2+Zz(Zi/2+Zot)/ P=I*Zi cos4>2 /(Z2+Zi/2+Zot) 14) from which after c oss- where Zl cos2 is the resistive multiplication and making (real) part of Zl. From this P= ZOT = vW2+2l 2 /4) [5] , ,121 When the terminating output IZc 2 +Z1 2 + 2ZcZicos(«>i— <>2)1 Differentiating |2] with respect impedance is removed a Fig. 8, “ to Zl with 4>2 constant gives 8 dP/dZ\.= c HZhr{I] 1 -O -U 2 cos«i2(l— Z g 2 /Zl 2 ) IZc 2 /ZL+2Zccos(4>i-2 with Zl constant gives dP/d9z= 9 . . iPZUfZc 2 + ZL 2 )sin2 + 2ZcZLsin 1 ) (Zg 2 + Zl 2 + 2ZgZlcos(<1> 1 — 2)) 2 sc _ Maximum power transfer will take place when dP/d< t>2=0, which requires that Ztsc =Zi /2 + (Z2Z1 /2)/(Za +Z1/2) sinit>2= — (2ZGZLsini)/ /(Zc 2 +ZL 2 )=-sin, also called propa- gation coefficient or image transfer constant, is of special significance in network theory. It is a complex number, of which the real part, o, is called the alteration coefficient or im- age attentuation coefficient, and the imaginary part, p, is called the phase shift coeffi- cient or the image phase constant. /i//2exp(a) gives the amplitude variation, whence o=log«(/i//2) [nepers] [17] Similarly, h /AexpO?) gives the phase be- tween the currents, whence p=loge[I\/h) [radians] [18] One neper (Np) equals 8.686 decibels (dB), 8.40 Summing the voltages around the loop A-B-C-D in Fig. 14 gives [exp(y)+exp(— y)]/2= =coshy=l+Zi/2Z2 [19] 14 m n - R /exp(2y)Zi + [/exp(2y>- /exp(yjZ2—[/exp(3y>— — /exp( 2 y)JZ 2=0 Note that when y=0, coshy=l; the hyperbolic cosine' is sym- metrical about the Y axis. W*”* ' ~j 1 — j ' ^ r which, when divided by in our September issue. /exp(2y) gives Complex numbers For some readers, ii may be useful to reconsider briefly the properties of complex numbers, sometimes called complex quantities. A good way to understand complex numbers and algebraic op- erations with them is to consider that they represent a point in a plane. In the diagram, the complex number a+jb=5+3j represents a point P, which has the abscissa a =5 and the or- dinate b = 3. The distance of P from the coordinate centre is the hypothenuse of the right angle ONP, which is OP = /(a 2 + b 2 ) = v r (25 + 9) = 5.83 The angle 9 that OP makes with the x-axis is given by tan 9 = NP/ON = b/a = 3/5 = 0.6 Instead of representing a complex quantity by its rectangular coordinates a and b in the form a + jb, it can also be represented by its polar coordinates, ie„ the distance, r, of P from the coor- dinate centre, 0, where which is often abbreviated to rcisB or rcjs9, and sometimes writ- ten as r /9. which is read as "r at the angle 9". Note that mathematicians use ’T to denote the concept of /—l, while electrical engineers use the to avoid confusion with the use of "i" to represent an electric current. The distance r in the diagram is called a vector by methematicians but a phasor by electrical engineers. Two complex numbers are equal only if their real and imaginary parts are equal. If, therefore, a+jb=c+jd then a=c and b=d Consequently, (a+jb)*(c+jd)=(a+c)+j(b+d) The same rule applies to the subtraction of two complex numbers. The multiplication of complex numbers is carried out in the conventional manner, but it should be bome in mind that/ 2 = — 1 and, therefore, all higher powers of j can be eliminated: j'=j 1 /*=-) >*=+1 /*= j and so on. (a + jbXc +jd)=ac+jad *jbc *fbd = = (ac—bd) +/ad + be) The division of complex numbers is carried out by multiplying both numerator and denominator of a fraction by the conjugate of the denominator. For instance, in the fraction ( a+jb)l(c+jd ), multiply both parts by (c—jd). Thus, (a +jbXc-jd)/(c+jdic-jd ) = =[(ac+bd)+j(fic-ad)]/(c 2 +d i )= =(ac+bd)/(c 2 +d 2 ) +J[(bc—ad)/(c 2 + d 2 )) r=V(a+b) and the phase angle 9 between the radius r and the x-axis, where 9=arctan (b/a) which is read as "the angle whose tangent is b/a". With reference to the diagram a=reos9 and Z>=rsin9 so that the complex quantity, P=a+jb, can also be written as P=r(cos9+/sin9) 8.41 WHERE STUDENTS MAKE THEIR OWN CHIPS by Brian Lawrenson, Department of Physics and Electronic Engineering, University of Dundee Electronics engineering graduates entering industry often meet difficulties in translating theory into practice. The advanced technology of making chips is probably one of the biggest hurdles they have to face. A Scottish university is tackling the problem by getting its students to turn out chips in the laboratory. When the Irish dramatist George Bernard Shaw wrote his condemnation of the teaching profession, “He who can, does. He who cannot, teaches", he came close to identifying one of the main difficulties in educat- ing engineering students. It lies in ensuring that the teaching of theoretical principles is firmly identified with the real world of engineering design and manu- facture. Most degree courses in elec- tronics engineering include lectures on the principles of semiconductor devices, ex- plaining how such components as transistors and diodes operate. At the University of Dundee we have taken this study one stage further: under- graduates are regularly design- ing and making silicon chips as part of their normal project and laboratory work. Our involvement with this branch of engineering began several years ago because the University is close to makers of semiconductors in central Scotland, in an area known as 'Silicon Glen 1 . When we took part in the usual type of organ- ised student visit to these I companies it was clear that many of the undergraduates were very attracted to their high technology and ultra-clean working conditions, so we invited a number of semicon- ductor engineers to make regular visits to the University to contribute to some of the lec- ture courses. From these early beginnings we have been able to set up a microelectronics laboratory which has all the facilities needed to design, manufacture and test silicon integrated cir- cuits. The very high capital costs usually associated with this type of work have been largely avoided by obtaining used professional equipment I from industry, either by do- I nation or by paying a modest amount. It has often meant that we have had to wait patiently for suitable items to become avail- able and a lot of time has been spent on repairs and modifica- tions. It has taken some 11 years for the laboratory to reach its present level of operation. In spite of the fact that the term silicon chip has been much paraded by journalists and broadcasters, relatively few people know how the devices operate or how it is possible to make something which is so I small and yet so complex. Most chips are made from sili- con (Si), an abundant chemical element which has two for- j tuitous properties. First, it is easy to oxidise its surface in a furnace to produce a stable coating of silicon dioxide glass | (silica), which is an excellent | electrical insulator. Second, it I is easy to change the value of its electrical conductivity by ; adding relatively small amounts j of either phosphorus or boron. The electric currents which ; flow in silicon are due to the movement of negatively- charged electrons (n- type Si) or, on the other hand, due to what appear to be positively-charged particles known as positive holes (p- type Si). The latter behaviour is somewhat surpris- ing: it arises from the way in which electrons interact with atoms of silicon, especially in the presence of certain other types of atom such as boron. The simplest active component i of an integrated circuit is the | MOSFET (Metal Oxide Semi- I conductor Field Effect Transis- tor). Its structure is shown in the first diagram, where phosphorus and boron have been used selectively to form regions of differing conduc- tivity and silica of varying thickness has been used to pro- vide electrical insulation where required. This transistor will switch on if an electric field is created below the gate elec- trode by applying a positive voltage to it. Electrons are at- tracted into the central region of the device and a current can then be made to flow through it from the source to the drain contacts: the MOSFET is then acting as a switch which has no moving parts and which is actuated by electrical means. This switching property means that groups of transistors may be used to transmit and process information presented in the form of a binary code. A com- plete digital integrated circuit may have more than 100 000 MOSFETs formed just below the surface of the silicon and in- terconnected by a top layer of fine aluminium tracks. A most important characteristic of such a circuit is that it is fairly insen- sitive to differences in the per- [i 8.42 1.8 mm square made by a I formance of individual tran- sistors; as long as each switches on and off, all is well. In making a chip, the details of every feature in one layer of its I structure are first recorded on a high definition photographic I plate. For instance, all of the j sources and drains in the entire I circuit would appear in the photograph as transparent rec- tangles on a black background | each measuring some five micrometres square. Because the whole chip will be only about five millimetres square or | even smaller, there is room on 1 the plate for the detail con- j tained in at least 200 identical I chips, arranged on the photo- I graph like a sheet of stamps. This detail is then transferred to the surface of the silicon by a process called photolitho- | graphy. It means coating the | oxidised surface of a thin disc J of silicon (called a silicon wafer) with a layer of a substance ! known as photoresist, which is | sensitive to ultraviolet light. Using the photographic plate and UV light produces an image of the circuit features in the photoresist which, when treated with acid, reveals the source and drain regions as tiny rectangular holes in the silica and exposes the surface of the silicon. Manufacturing processes that follow include the diffusion of J boron or phosphorus through ! the holes in the silica into the surface of the wafer, in furnaces at temperatures of about 1100°C. Finally, the whole wafer is coated with a thin layer of alu- minium in a vacuum chamber, and photolithography etches it into the pattern of metal tracks which connect the transistors. The details of each layer derive from its particular photomask. Fig. 2. Photomicrograph of a ch Our microelectronics labora- tory is equipped for all these processes, in a suite of clean- rooms with a filtered air supply to exclude dust. It has it own photographic unit with cameras for making the photomasks. One cleanroom is reserved for photolithography and another for the 10 electrically heated furnace tubes with their associ- ated gas supplies. The largest area has probing equipment for making electrical test measure- ments on the finished wafers. There is also equipment for sawing the wafers into separate chips and for mounting them in their familiar plastic boxes with metal legs. Project Time A student at Dundee who opts for this work will spend some 300 hours of project time designing and making a chip to his or her own specifications. The circuit has to be fairly simple, of course, with fewer than 150 transistors. We find that MOSFETs with p- type channels are simplest to make and we are usually content to turn out devices with a separation of 10 »im between source and drain. The layout details for each chip are designed with the aid of a mainframe com- puter and are stored on mag- netic tapes. Initial artwork is produced from the tapes using equipment for reproducing weather satellite photographs (we have not been fortunate enough to obtain the appro- priate pattern generator for this stage). The main fabrication processes are then undertaken. To make working conditions as realistic as possible, everyone wears a full set of cleanroom clothing resembling a surgical hood and gown. Students are also asked to assess the cost of bringing each design to fru- | This activity provides a wealth I 1 of practical experience and puts the importance of the lec- | ture courses into perspective, j Students are encouraged to consult key research papers as j well as standard textbooks, to i help them identify causes of j unexpected results or failures that arise from time to time in the work. The aspiring inte- grated circuit engineer soon j begins to appreciate the im- j portance of mastering the re- quired blend of electronics j engineering, solid state phys- ics, chemistry, crystallography and metallurgy, backed-up by computer-aided design tech- niques. Almost 40 Honours students have now been introduced to integrated circuit engineering through the work of the micro- | electronics laboratory. Post- 1 graduate research and indus- trial contract work are sup- i ported, too. Dundee graduates | | in this field are now working I for major manufacturing | companies in the UK and over- seas, including GGC, Plessey, Ferranti, Hughes Microelec- tronics, National Semiconduc- tor, Motorola, British Telecom, 1NMOS, Mullard and Siemens. The chips our former students have been working on include ! the Transputer and many other ! devices. Through this we gain useful feedback about the con- tent of lecture courses and the areas that are interesting for 1 research. Employers have been enthusi- astic about this practical ap- proach to microelectronics. When our graduates go to their first job interviews carrying sili- con chips that they have made themselves, their starting sal- aries have been good. E1W PRODUCTS • NEW PRODUCTS • NEW Extensive range of phone sockets | As part of its rapidly expanding service supplying high-quality [ Japanese components to the | UK, Watts International an- | nounces an extensive range of phone sockets. ' The Emuden manufactured range features a remarkable number of socket types, includ- | ing single, double, and multiple j connectors. The multiple units | are available as complete items, or they can be ganged from a variety of single or double in- terlocking connectors. Com- } plete multiple assemblies are j available with three, four, five, six, and ten connectors. The outer earth cap may be stan- ' dard or chamfered. Watts International Components Limited Suite 6 Wyvera House 46-48 High Street BOGNOR REGIS P021 ISP Telephone: (0243) 868322 (3630-28F) ATN FILMNET DECODER by J & R v Terborgh Believe it or not, but a few days after our May issue was sent to the printers, the engineers at ATN Filmnet altered the station's scrambling system. The next thing that happened was Sky Channel abandoning encoding altogether for roughly a fortnight. An article written for naught and circuits doomed to end up in the junkbox? Here is the updata! The speed at which dramatic changes take place in the satel- lite TV world is very hard to keep up with. After the publi- cation of " , we saw Europa TV dissappear and their 3WH transponder temporarily as- signed to 3-SAT, Music Box change into Super Channel, and ATN Filmnet adopt the all on ECS-1. The launch of the DBS services for Federal Germany and France was post- poned for the umpteenth time, the flat dish was developed to aid in individual reception, Europe witnessed an invasion of relatively inexpensive LNBs of Far Eastern origin, and many of our readers embarked on setting up their own reception system. Roundabout March 26 of this year, pay-TV channel ATN Filmnet selected scrambling mode 2 as a follow-up of mode 1, which was analysed in l2 ', and had been in operation since September 1, 1986. Until that memorable day, it was not gen- erally known that their Matsushita scrambling system can be programmed to provide several scrambling modes. It has now evolved that sub- scribers’ decoders incorporate a microprocessor-based system that selects and combines a number of essentially simple decoder blocks. This selection goes by wholly unnoticed to the registered subscriber, and is ef- fected with the aid of a special code packet transmitted via the digital subscriber’s data chan- nel at 7.02 MHz in the baseband —see Fig. 1. It must reiterated here that the scrambling system itself is not digital, and it is readily seen that the power of 8.44 OtektOT ,n<„a august .987 this multi-mode scrambling system resides in the possibly large number of available com- binations of methods, rather than in the complexity of each individual method. Therefore, now that mode 2 is operative, the use of the mode 1 decoder proposed in ' 2l is the same as using no decoder at all, since in both cases the picture is completely unintelligible. And yet, the essence of the newly adopted encoding method re- mains very simple, and is but an extension of mode 1, so that the design idea brought forward in 2 remains the basis for any further designs. New circuits for new modes: 2 all The circuit diagram in Fig. 2 shows how the functional blocks of Fig. 3 in 2 have been worked out into a practical decoder. The 7.6 MHz FM re- ceiver for obtaining the com- j posite blanking signal is pur- posely shown as a separate unit here to make clear that it is always required for decoding ATN Filmnet, whatever scram- bling mode is, or will be, adopted (it may well be that mode 3 or even 4 is operative when this article is being pub- lished. . .). The PLL and pulse timing sections are largely identical to those used in the Sky Channel decoder 2 , so that a detailed description of these is not required here. As already stated, mode 2 is an extended version of mode 1. In addition to shifting the DC com- ponent of the blanking, and inversion of the entire signal, the polarity of the video signal is now toggled for each raster in | the interlaced picture. This is very simple to put right by dividing the 50 Hz blanking I component by two in bistable ' FFv and alternately selecting the DC -corrected VIDEO or ] VIDEO signal from the NE592 differential amplifier on the signment in the baseband transmitted by vision/sound/PSU board in the Elektor Electronics Indoor Unit for Satellite TV Reception Provision has been made to ensure that a viewable signal is always available at the AC coupled and CVBS-1 output. This is effected by rectifying the raster pulses in Di-Cjii-Rjs and using the logic level so ob- tained to select between the decoder output and the re- ceiver's CVBS-1 signal. LED Dr lights when the signal from ATN is encoded. When it is not encoded, which is sometimes done on purpose between films and during announcements of forthcoming programmes, the carrier at 7.6 MHz is simply left unmodulated. Since it was intended to keep the decoder as simple as poss- ible, no provision has been made for automatically selec- ting the correct frame polarity of the video signal. This means that the sync button may have to be pressed a few times to obtain a properly decoded pic- ture. The decoder remains syn- chronized when ATN switches between encoded and non- encoded transmissions, but loses synchronization if the signal strength at the receiver input is too low, since spikes then upset the operation of the filters and the PLL Type 4046. In practice It must be pointed out here that the decoder experiments dis- cussed require a certain amount of feeling for dealing with RF and video signals. Also, the material presented here is essentially but a design idea, and the construction and align- 8.45 infra-red light gate The use of an infra-red light source is an obvious choice for this type of application. In the first place, for intruder alarm applications the light beam must be invisible, which limits the choice to infra-red or ultra-violet light. Ultra-violet light can cause visible fluorescence of certain materials, which makes it less suitable than infra-red. In the second place, relatively powerful solid-state infra-red sources, and infra- red sensors, are available at modest cost, whereas there are no solid-state UV sources commercially available. The circuit described here uses the Siemens LD241 infra-red emitter and Bl’W 34 I R photodiode. Although these devices are not exorbi- tantly priced, neither are they inex- pensive, so in order to minimise the number of IR emitters necessary to achieve a given range the transmission system should be as efficient as possible. Since the light level received at several metres distance from the transmitter will be very low, the receiver must have a high gain. This immediately excludes the simpler types of photoelectric switch that use a continuous light beam and a DC-coupled receiver, since a high- gain DC coupled receiver amplifier would be prone to offsets, temperature drift and other effects that could lead to poor sensitivity on the one hand, or false triggering on the other. The choice therefore falls on an AC modulated light beam and AC-coupled receiver, since a high AC gain can be achieved without offset problems. Such a system can be either narrowband or wideband. The advantages of a narrow- band system are a higher signal-to-noise ratio and less susceptibility to ex- traneous interference, either in the form of ambient light or transients on the supply lines. The disadvantage of a narrowband system is that the trans- mitter and receiver frequencies have to be accurately aligned. In a wideband system, the light source is simply pulsed on and off, and the amplification stages of the receiver have a fairly large bandwidth. The advantages of this system are simplicity and ease of alignment, but the disadvantages are poor signal-to-noise ratio and suscepti- bility to interference. However, advan- This article describes an infra- red light source and detector, which can be used in a wide variety of applications ranging from intruder alarms to automatic garage door openers. When the light beam from the infra-red source is interrupted, the receiver circuit detects this and energises a relay. tage may be taken of the fact that the infra-red emitting diode will withstand a peak current that is much larger than the average current ( 1 A peak as against 1 00 mA continuous). Small duty-cycle, high-power pulses may thus be trans- mitted, which will give an improved signal-to-noise ratio over a larger duty- cycle transmission of the same average The effects of external sources of inter- ference may be reduced by careful attention to constructional layout, mounting the unit in a screened box, and suppression of the supply lines. With these precautions a wideband system can give quite acceptable per- formance and was thus chosen because of its other advantages. Transmitter circuit The simple transmitter circuit is shown in figure 1. It consists of a SS5 timer connected as an astable multivibrator, driving an output transistor which switches the IR emitter on and off. The duration of the transmitted light pulses is about lOps and the repetition rate is just less than 1 kHz. The average current drawn by the circuit is about 1 2 mA and the peak current through the IR diode is around 700 mA. The LD241 is available in three versions, LD 241/1, LD 241 /II and LD241/III, which have different radiant intensities. For the same forward current, the light output of the LD241/II is typi- cally l’/j times, and the light output of the LD 241/IIi typically V/i times, that of the LD 241 /I. The power supply for the transmitter is not critical provided the output voltage is no greater than 6 V, as this could result in the maximum current rating of the LD 241 being exceeded. A suitable circuit is given in figure 2 and can be built up on the board for the ‘Local Radio’ power supply (Elektor 22, February 1977). Note that the component values for this circuit differ from those of the original circuit (see parts list) and that the following components are omitted: R5, C2 (replaced by R6), Dl, D2, T1 (base and emitter connections linked on the p.c.b.). Receiver circuit The receiver circuit is shown in figure 3. A BPW 34 infra-red photodiode is operated in the reverse-bias mode. The leakage current of this diode varies with the light received from the transmitter, which causes a varying voltage to appear across resistor R2, the gate resistor of the FET source-follower T 1 . The signal appearing at the source of T 1 is fed to IC1 , which is used as an ampli- fier and limiter. P 1 varies the sensitivity by altering the reverse bias voltage of the diode. When light pulses are being received from the transmitter, a negative-going pulse train with an amplitude in excess of 1 V peak-to-peak appears at the output of IC1 (pin 8). This turns T2 on and off continually, charging up Cll. T3 is thus always turned on, T4 is turned off and relay Re is not ener- gised. When then light beam between the transmitter and receiver is interrupted, the amplitude of the pulse train from the output of IC1 will fall. T2 will be cut off, Cl 1 will discharge, T3 will turn off and T4 will turn on, pulling in the relay. Once the light beam is re- stored the relay will, of course, drop out again, but can be made to hold in for several seconds after the light beam has been restored by adding the com- ponents shown dotted. R12 should be 8.47 4k 7 and Cl 2 can be from 10^1° 100 M, depending on the desired hold-in time. Alternatively a latching arrangement may be used that will hold the relay in until a reset button is pressed. Power supply A power supply for the receiver circuit is shown in figure 4. This is virtually identical to the power supply for the ‘Local Radio’ and may be built on the Construction Printed circuit board and component layouts for the transmitter and receiver are given in figures 5 and 6. Construc- tion of the transmitter should present no problems. When constructing the receiver, great care must be taken with the layout due to the high sensitivity and large band- width. The leads to the BPW 34 photo- diode must be as short as possible, as otherwise they may pickup interference. The relay should preferably not be housed in the same box as the receiver, as the magnetic field set up when it is energised may completely saturate the sensitive receiver input stage, causing the relay to drop out immediately. The receiver will then begin to function again, the relay will pull in and the whole process will repeat. If the relay must be mounted in the same box as the receiver, then it should be mounted as far as possible from the receiver input stage, and must be magnetically and electrically screened. The receiver itself should be mounted in a metal box for screening, the only holes in the box being for relay and supply leads, an adjustment hole for access to PI and a hole for the photo- diode. Since the photodiode is sensi- tive to visible as well as infra-red light, it must be fitted with an infra-red filter (obtainable from photographic suppliers) if the unit is to be used in daylight. Even with the infra-red filter, direct sunlight should not be allowed to fall on the photodiode, since its large infra- red content could affect the diode biasing and hence the receiver sensi- tivity. Some kind of hood or tube to screen the diode may be necessary in such circumstances. Adjustment The transmitter diode and receiver diode should be aligned with one another, although the radiation pattern of the one and the acceptance angle of the other are so wide that a slight misalignment will have little effect (but remember that a screening hood or tube on the photodiode will reduce the acceptance angle). The circuit is then checked for reliable operation at close range by breaking the infra-red light beam, after which the transmitter and receiver are moved progressively further and further apart, whilst PI is adjusted to obtain the maximum range. If the photodiode is well screened from ambient light, this adjustment will have little effect and the wiper of PI can simply be turned fully clockwise. As it stands, the circuit will function at distances of up to 6 meters between the transmitter and receiver. If lenses are used to concentrate the transmitted light into a much narrower beam and to focus the received light on the photodiode then much greater ranges can be achieved. However, the physical 8.48 * TUN = BC 547A.BC1 07 A = BPW34 IR photodiode ,03, 04 - 1N4148 Notes on the TBA 120 The TBA 1 20 is produced by several manufacturers, and several different versions are available. All of these should function satisfactorily in the receiver circuit. However, in some cases it may be necessary to omit R6 (see figure 3) or connect it to ground instead of +Ub, to obtain the best signal-to-noise ratio. To check this the output of the 1C should be monitored, either on an oscilloscope, or by con- necting a pair of high impedance (> 500 Cl) headphones between pin 8 of the IC and +Ub- When receiving a signal from the transmitter a 1 kHz signal should be heard (or seen). The effect of omitting R6, or connecting it to ground, can thus be investigated. The optimum result is indicated by the loudest (highest amplitude) signal. Care should be taken when altering R6 not to disturb the relative positions of the receiver and transmitter, as this could give false results. K selex 26 HIGH FREQUENCY COMPONENTS These types of components are also generally known as RF components. These are mostly passive elements used in a radio receiver, (or even a transmitter.) Due to continuous technological developments, new types of components keep on appearing on the market and slowly the old types find their way to the hobbyists junk boxes. For a quick reference, we are giving here a short components. Rotary Condensors Rotary condensors are mainly used for tuning applications. A gang condensor such as the one shown in photograph 1 is used in the tuning stage of of two stacks of metalic fin#, one called the stator The rotor stack is mounted on the spindle and by change the overlapping area between the stator and Minimum requirement of condensors ganged together — one for the tuning circuit and the other for the condensors combined together, in case of AM/FM radios. Fins of the AM condensor are larger than the one for FM. Photograph 1 shows a double — AM/triple FM gang condensor. In transistor radios we find mostly miniature rotary condensors in a plastic casing. The dielectric material used here Even the plates of the stator and rotor are thin metal foils. This helps in increasing the packing density and reducing the Trimming capacitors are also used on these gang condensors for accurate tuning of the oscillator frequency. These are small plastic film dielectric. In case of ceramic disc trimmers, a ceramic disc selex coated with silver forms the Trimmers mostly have very low capacities ranging from 5 to 15 pF for FM and upto about 100 pF for AM. The gange condensors have values from 200 pF to 600 pF- Coils and Filters These are inductors, either core. The number of turns of copper wire, coil diameter and the material of the core decide the inductance value. iThe more the number of turns, higher is the inductance. A ferrite core gives more inductance value compared to an air core for the same coil winding. If the moving it in or out of the coil winding can give a variable inductance value. A variable inductor can also be used to tune the radio receiver. This is called Variometer tuning. The lower the frequency to be tuned, the higher must be the inductance value. Thus, an air core coil is useful for Ultra Short Wave tuning. Medium Wave tuning requires ferrite core coils. Coils are also used in filter circuits, which allow only a particular range of frequencies to pass through to the next stage. Internal circuit and frequency response of a bandpass filter is shown in figure 1. It consists of two coils coupled with a ferrite core and connected with two capacitors in parallel. Older types of these filters, and I.F. Transformers used to be very large in size, but in the modern transistor radios, their sizes have shrunk to just about one cubic centimeter. Even the modern fixed value inductors have sizes and appearance similar to small resistors. Ceramic Filters Similar to coil filters, we also have ceramic filters to work as band pass filters. One such filter is shown in 8.51 selex figure 2. with its circuit symbol, pin configuration and the frequency response. These look similar to plastic film capacitors, but may have 3 to 5 terminals instead of just two. They contain disc ceramic resonators in place of the resonant circuits made of coils and capacitors. The resonators are excited into mechanical vibrations only at the resonant frequency and can couple the incoming signal to the next stage. At all other frequencies, they do not get excited into vibrations and act as open circuits. Such type of ceramic filters are found in miniature transistor radios in the I F. Stages in both AM and FM just two terminals coming out. Inside the can, these two terminals are connected to a quartz crystal disc. It behaves similar to the ceramic resonator. It can be excited to vibrations through AC voltage. This happens only at the resonant frequency of the crystal, which is printed on the can. The quartz crystal can be used in an oscillator circuit and gives a very precise and stable oscillator frequency. The quartz crystals are also very popular in watches and clocks in addition to their application in radio Quartz Crystals These are packed in small 1 Figure 4: Germanium point contact Figure 5: Circuit symbol and capacitance curve of a tuning Photograph 2 : Quadruple gang condensor and variable inductor. Photograph 3 : Different trimming condensors: ceramic trimmers, plastic trimmers ant air trimmers. 1 1 3 V ; *" jr selex receivers and transmitters for giving high accuracy and stability. They are aslo used in computer circuit to generate precise clock frequencies. Diodes There are two types of diodes for the HF applications which differ considerably from the normal types of diodes. special diodes which operate in the high resistance direction. As in case of other diodes, this diode also does not allow any current in the reverse direction. However, it presents a small capacitance between anode and cathode. This capacitance changes itself with change in the blocking voltage applied across the diode. Because of this used in car radios and small battery operated sets. Second type of diode specially used in HF circuit is the point contact Germanium diode. Its internal construction is shown in figure 4. Its tiny bent spring in the glass casing is even visible from outside. The tip of this spring presses on a Germanium crystal disc at the other end of the diode. This type of diode is able to rectify small AC voltages. Silicon diodes are not useful in this respect because of their higher threshold voltages The demodulator stages of radio receivers always use Germanium diodes. However, new types of integrated circuits are being developed which will replace the needs for many discrete HF components. property these diodes are selex more exciting, we intended to just give the problem in in the next issue. However, to avoid the excessive excitement, the solution is Let us see how the trick circuit functions. The principle of operation is described in figure 3. Diodes are connected in parallel to the switches and lamps. The circuit is supplied with an AC voltage. With DC supply the circuit would never function The battery shown in figure 1 is not a real DC battery but a pseudo battery which has a ' convert e DC ir 3 AC. The circuit of this DC to AC converter is hidden inside the casing of the battery as shown in figure 4 Once we know that the supplied with AC and not DC. and that the switches and lamps are connected with parallel diodes, it is easy to understand the functioning of the circuit. If we close switch SI, then current flows through SI, DS2, DLa2 and Lai, and th< lamp Lai glows. If S2 is closed, then current flows ! through S2. DS1, DLal and i La2. and the lamp La2 glows. If both switches are closed, both the lamps glow simultaneously. In reality they do not glow simultaneously but appear to glow simultaneously, because one lamp glows 8.55 during the positive half cycle and other glows during the negative half demonstrate the trick without using coloured lamps and switches can connect the diodes behind the board on which the lamps and switches are mounted. But for those who also want to do the trick with coloured lamps and switches, it is essential that the diodes must be fitted inside the bulbs. This is a difficult job but not an impossible taks. How it is done is shown in figure 5. In order to install the diodes inside the bulbs, the soldering tin at the soldering position of the bulbs is removed carefully. This will make the glass bulb loose from the screw The diodes are soldered below the switches, so a coloured lamps are used, it will be necessary to take them out from the sockets repeated a few times will be enough to take out the glass bulb. The diode can now be soldered at the two terminals and the glass bulb re-inserted into the screw cap. This is the most difficult part and must be handled very carefully. To prepare the two functional trick lamps, we may require lamps depending on your skill! The first 1C produces pulsating DC voltage using good washers. The componemts required to construct this trick circuit are very few and the construction is simple. But The circuit of the pseudo battery which gives an AC output is constructed on a small piece of SELEX PCB as shown in figure 7. The circuit diagram is shown in figure 6. The first 555 1C is used as a multi-Vibrator and produces a pulsating DC Voltage. IC2 inverts the output signal of IC1 in such a manner, that between the two outputs of the ICs a rectangular AC voltage is produced. After constructing this circuit, it is pasted on a 9V miniature battery and then enclosed in an empty case of a larger size battery. The trick circuit is constructed on a piece of cardboard as seen in figure 1 in such a manner that it very obviously looks like a 8.56 8.57 selex extent. However, even this circuit will not be able to receive a far away station. Because we still do not have the required sensitivity to get the weak signals to drive our headphones. This can be achieved by using an amplifier stage between the parallel resonant circuit and the detector diode, as shown in figure 4. The amplifier A amplifies the high frequency carrier waves before they reach the demodulating circuit. But an amplifier needs a battery to function longer work by drawing power from the radio waves. As we have an amplifier in the circuit, it is possible to receive even far away stations. However, this once again poses a problem of selectivity because even other signals which have frequency values nearer to the tuned frequency can also now be I amplified sufficiently to Figure 4 : An RF amplifier stage improves the sensitivity of the selectivity. This uses a double rotary disk condensor. Figure 6 : The simplified circuit diagram of a 'Superhet' receiver. The mixer stage to produce a fixed frequency output of 455 KHz. The oscillator is so designed that it always produces a frequency which is ahead of the frequency of the antenna signal just by 455 KHz. This frequency is known as the Intermediate Frequency (I.F.). The subsequent stages are tuned to this frequency. drive the headphones. This calls for one more tuning circuit after the amplifier, as shown in figure 5. We can continue in this manner and go on increasing sensitivity and selectivity. But a problem with this approach is that for best results, all of these tuned circuits must be tuned equally for every different radio station we overcome this problem, an ingenious method is used The tuning is done only in the first stage for the desired station and then that frequency is converted to a fixed intermediate frequency in the next stage, (called the Mixer stage) The remaining stages are all tuned only once during the construction of the circuit to the intermediate frequency and need not be retuned for every different radio station. This type of radio receivers are called Superheterodyne Receivers. 4 Y The most important part of this 'Superhet' receiver is the mixer stage which produces the intermediate frequency signal (I.F. signal) for the remaining part of the circuit, independent of the frequency of the radio station tuned in by the tuning circuit of the receiver. What the mixer does is that, using a variable capacitor which is physically ganged with the tuning capacitor, it drives an oscillator to produce a frequency which is exactly 455 KHz more than the resonant frequency of the tuning circuit. This oscillator frequency is then mixed with the signal received from the radio station. As the oscillator frequency always remains ahead of the radio station frequency by 455 KHz, the output of the mixer stage has always the same frequency of 455 KHz. The audio signal is still super-imposed on this new carrier wave of 455 KHz. 5 Y Figure 6 shows the simplified circuit of a Superhet receiver. The tuning circuit has a range of 500 to 1 600 KHz (MW range). The oscillator is so designed as to have a range of 955 KHz corresponding to the tuning circuit range. If the tuning circuit is tuned to 500 KHz, the oscillator has frequency of 955 KHz. When these two signals are fed to the mixer, it gives an output with a frequency of 455 KHz. If we now move the tuning knob further till the receiver is tuned to 1000 KHz, the oscillator automatically changes its oscillation frequency to 1455 KHz. The difference is once again 455 KHz and the mixer output still remains at 455 KHz. The remaining part of the receiver never knows the actual frequency of the radio station to which we have tuned our receiver. From the output of the mixer stage onwards, the receiver works as if we have tuned to only one radio station having a carrier frequency of 455 KHz. This is why the remaining tuned stages are called I.F. stages. To improve upon the performance, we can even Superhet stages. The output of the I.F. stages can once again be passed through a pair of oscillator and mixer to produce a second Intermediate Frequency. This is called a double Superhet. One can even have a triple Superhetl 8.58 pw PRODUCTS • NEW PRODUCTS • NEV P C B COATING CRC Acryform is now manufactured in India under a technical license agreement with world renowned CRC Chemicals, Europe. CRC Acryform is a fast drying, single component conformal coating for application on printed circuit assemblies. Having very good solderability and excellent flexibility, CRC Acryform protects assembled PCB's from humidity, fungus and corrosion. The coating provides insulation against high voltage arcing and corona shorts. It is excellent from production standpoint as it is easy to apply, has a long shelf life (2 years), and is easy to remove if rework becomes necessary. CRC Acryform is easy to handle with a dry to touch time (air drying) of 30 minutes. As CRC Acryform is transparent, coated components are easily identified. Single components may be replaced by soldering or desoldering directly through the coating. The cured film can be removed by using chlorinated solvents such as methylene chloride or trichloroethane. contact Acryform Division: BHARAT BIJLEE LTD. Electric Mansion, 6th Floor Appasaheb Marathe Marg Prabhadevi Bombay 400 025 ICE FOR 8 BIT pPs Advanced Electronic Systems introduce their model ICE-8 which features * Fully buffered data, address and control signals to the target * No restrictions on the target processor's memory and I/O space ‘ 8K emulation memory can be mapped into any 8K block in target memory range * Break points with loop * I/O ports can be polled upto 256 times with sampling duration adjustable from 1 to 256 * Resident assembler and two-pass dis-assembler " Upload/down load facility for executable files to/ from a host computer system (like IBM PC) ** '' ' #-JT ^ For further details contact: ADVANCED ELECTRONIC SYSTEMS 106, 8th Main, Malleswaram West, Bangalore 560 055 MOTORS MAGNA MOTORS introduce permanent magnet motors of voltage ranges 12 to 110 volts speeds 1500 to 8000 RPM, and wattages upto 250 watts. These motors find applications in Switchgear units as charging motor for Oil/Air/Vacuum Circuit breakers, isolators, welding machines, lab equipments, telecommunications and domestic appliances with intermittent and semi continuous duty cycle. For further details contact: MAGNA - MOTORS No.88A/3, (204), 3rd Cross llnd Main Prakashnagar Bangalore 560 021 PROM PROGRAMMER Professional Electronic Products introduce their Universal PROM Programmer PP-85. PP-85 eliminates the Connection Adapter' approach; the selection of the PROM type is through a command. PP-85 has 32 K bytes of internal data RAM to program 32 K 8 byte 27256 EPROM. The PP-85 basic unit contains all the hardware and software to program the range of EPROMs from 2716 (2 Kx8) to 27512 (64 K x8) EPROM. Besides it can also program a number of EEPROMs. Wherever applicable intelligent programming algorithms have been used to reduce the programming time. PP-85 has LED display and a key board for entering the command and data. In addition, an Rs 232C serial I/O port is provided for down loading the data from a computer or an MDS. Software is available at present to interface PP-85 with IBM PC compatible computer. Separate Programming Modules are also available for the following; 1. Gang programming Module: Programs 8 NMOS EPROMs at a time. 2. Microcontroller programming Module: Programs 8741, 8742, 8748, 8749, 8751 & 8755. 3. Bipolar PROM Programming Module: Programs nearly all Signetics, T1 (Texas Instrument), NS (National Semiconductor) and AMD (Advance Micro Device) bipolar PROMs. 4. PAL Programming Module: for popular PALS (Programmable Logic Arrays). For further details contact: PROFESSIONAL ELECTRONIC PRODUCTS Opp. Old Octroi Post, Delhi Road, Meerut 250 007, U.P. AUTO RANGE PANEL METER PRESTIGE ELECTRONICS introduce their Autoranging digital Panel Meter Display is 3 ’/j Digit 12.5mm Red, Green or Yellow. Range selection is automatic depending on input voltage. Ranges are 1.999V, 19.99V, 199.9V & 750V DC overall accuracy is 0.25% ± 1 Digit for DC & 0.7% ± 1 Digit for AC models. Dimensions are 48 x 96 x 190mm (% Din size) Cutout 45 x 96mm. Input supply is 230V ± 10%. For further details contact: PRESTIGE ELECTRONICS 62/A, Push pa Park, Mai ad (E) Bombay 400 097 Tel: 693805 8.62 nN aecraoNic €XTRnvnGRNzn TO G€N€RRTe GR€AT€R PROFITS Visit TAIWAN €L€CTRONIC FAIR 6-12 OCT. 87 With BANGKOK-PATTAVA-HONGKONG-TAIUJAN-SINGAPOfl€ 14 DRVS *Rs. 13990/- + US S 100 & •Capture Neuj Market ‘Improve efficiency ’Reduce Production Cost ‘Increase Productivity. Also Visit RSIR €L€CTRONIC FAIR At SINGAPORC 19-22 Oct ’87 Ullth Bangkok & Pottaya 10 DRVS ‘Rs. 6890/- + US S 50 CLAttiC Unit G-1. Khiro Indl. Cstote. S.T. Rood. Behind Khiro Nogor. Sontocruz (LU). Bombay - 400 054. Tel: 6120487 - 6128545. Hnccifterl nds nrinsrlisfirs inriftx 8085 MICROPROCESSOR TRAINER built Wanted Diploma Electronics Engineer in EPROM programmer, power supply. with minimum 1 year experience for 2K CMOS/RAM with dry cell back up testing and servicing Analog and CMOS expandable to 8 K. 12 K user EPROM Digital modular circuits of European installed Rs. 2975/- All inclusive machines Apply immediately to M M EPROM Eraser Rs 500/ Contact: CORPORATION 3/319 Sun Industrial NEW AGE ELECTRONICS. Third Floor. Estate. Lower Parel. Bombay 400 013 Laxmi Mahal. Near Vandana Cinema. Agra Road, Thane - 400 602 Attention ZX-SPECTRUM users: Superb programmes available for tape back-up Bhatnagar. 45-A. Pologround. Udaipur- ABC ELECTRONICS 8 04 ADVANCED VIDEO LAB 8 18 APEX ELECTRONICS 8 14 BMP MARKETING 8 64 CHAITANYA ELECTRONICS 8 67 CHAMPION ELECTRONICS 8 61 CYCLO COMPUTERS 8 14 DEVICE ELECTRONICS 8 63 DEWAN RADIOS 8 04 DYNALOG MICRO SYSTEM 8 13 DYANTRON ELECTRONICS 8.16, 8.18 8.67 ECONOMY ENGINEERING 8.71 ELECTRONICA SALES 8.16 GALA ELECTRONICS 8 02 For sale : HM 312. Dual Trace. 20 MHz Scientific Oscilloscopes with Component Tester and 3'/i digit ZE 1501 Zenith Benchtop DMM in perfect working condition. Contact: M M. Corporation. HCL 8 59 IGE 8 07 JUNIOR COMPUTER 8 66 LEADER ELECTRONICS 8 08 LOGIC PROBE 8 10 MECO INSTRUMENTS 8 67 NCS ELECTRONICS 8 08 PECTRON 8 04 PHILIPS INDIA 8 15 PIONEER ELECTRONICS 8 64 PLASTART ELECTRONICS 8 08 PRECIOUS ELECTRONICS 8 69 ROCHER ELECTRONICS 8 18 SAINI ELECTRONICS 8 16 SEMICONDUCTORS LTD 8 09 SIEMENS LTD 8 17 CORRECTIONS D-A converter for Universal control for I/O bus stepper motors January 1987 p. 1-21 Feburary 1987 p. 2-31 In Fig. 3, the order of the databits Do-Dr Table 5a should be amended as foHows: should be reversed both at the bus con- M21 = 50h. In Table 5b. the databyte at nector and the inputs of ICi . M3E should read OOn. TEXONIC INSTRUMENTS 8 71 TRIMURTY ELECTRONICS 8 16 UNLIMITED ELECTRONICS 8 71 VIKAS HYBRID 8.14 VISHA ELECTRONICS 8.75 8.72 R.N No 39881/83 WEST -228