BBC computer Headphone amp satellite TU reception Research and the future electronics technology SATELLITE TV RECEPTION Much has been said and written about satellite TV over the past few years. But what does this entail? Can anyone receive such programmes? What is the cost of a suitable installation? How does the law stand in regard to reception of these signals and the installation of the necessary equipment? In this introductory article we will attempt to give answers to these questions and others, as well as give a theoretical appreciation and practical information on the subject. Satellite TV receiption 10.20 [ Electronic propulsion for satellite 10.30 | Research and the future 10.38 j Software for the BBC computer 10.42 j Computers of tomorrow 10.51 1 projects 723 as a constant current source ... 10.44 Baal Inarl rocic« nrs | information _ ! ■Vl-uu. r 1 guide lines 10.67 Index of advertisers 10.74 1 selex - 16 Measuring techniques 10.52 The heavy weight of electronics 10.54 Unknown transformer data 10.56 Transformer coils in series & parallel 10.58 NOTE: High-power AF Amplifier - 2 will be featured in our Nov’86 issue and not in this issue as mentioned earlier. General Manager: J Dhas Advertising; B M Mehta Television standards 52. C Proctor Road. — what now? [ Standfast House, Bath Place High Street Barnet Two important events took place this summer that will have a pro- Editor: Lon Seymour found effect on TV broadcasting in the future. One, the Peacock Elektuur B.V. Report, is of interest to Britain only and does not really deal with Peter Treckpoelstraat 2 4 technological matters; the other, a CCIR motion, affects the entire western world. In true fashion, the first was given extensive pub- Elektor sari licity, the second hardly any at all. Route Nationals Le Seau; B.P. 53 59270 Bailleul France The Peacock Report will probably affect television broadcasting in j G C P Raedersdort Britain for the remainder of the century. Although it contains many Elektor Verlag GmbH welcome recommendations, only one of these has a direct bear- Susterleld-Stralie 25 ing on the design ot TV receivers. Another, with some importance to the telecommunications industry, suggests that national tele- Elektor EPE communication systems should be allowed to act as common car- Karaiskaki 14 16673 Voula - Athens - Greece riers for a full range of services, including TV programmes. Elektor JCE Designers of TV receivers may have a busy time ahead in view of Via Rosellini 12 the proposal that all new television sets sold or rented in the United Kingdom should be adapted to receive direct subscription [ Ferreira £r Bento Lda. services by 1 January 1988. 1000 Lisboa - Portugal It is interesting to speculate as to how such a direct subscription ingelek S.A. service would work. In all probability, it will entail a form of Av Alfonso XIII*. 141 scrambling. Television sets would then have to be fitted with a special socket into which viewers would have to plug a decoder. International co-ordinating Such a decoder, whatever form it takes, would cost the viewer £r technical manager: money over and above the cost of the television set. Of the various decoding systems in existence, the one used by France's Canal Plus is probably the most cost-effective. In the French system, a subscriber rents a keypad on which a per- -a a. ^ sonal number is keyed in. This number is changed monthly, so that :a e , Elektor oubl, canons and acli.nros the subscriber has to renew it twelve times a year at the appropriate fee. Elektor India are copyright end may not Such a subscription service could not be introduced easily, par- °nVp”bS,r' 0,W ' ,, ""‘ , •' m, “' 0, ' 0, ticularly since the relevant equipment is not yet available. Other than that, the report does not say much about the quality or “ technical requirements of television services. But then, that was not responsibility for foiling to identify such part ot Prot. Peacock's brief, as politicians are not really interested petenl or other protection. in such minor matters. From a technological point of view, a more important event was Printed At: the lamentable decision by the European delegates at the Comite Consultant International de Radiocommunication TruDh 0"set^ (CCIR=lnternational Radio Consultative Committee) to reject a Tfc- Japanese proposal, backed by the USA, for a world standard for high-definition television (HDTV) broadcasting (1125 lines; 5:3 aspect ratio; suitable for projection onto a large screen). Copyright 1986 Elektuur B.V It appears that the Europeans were afraid that Japanese pro- ducers would dominate the world market for video equipment. I The Nutlteilaiids Don't they already? t October 1986 1 0*05 seMIKRON innovation + service IMR - TECH (Consultants to Electron.cs Industry) Tel: (082) - 642 -4191 (022) -642 -4902 Tele*: 11 -71412 AIKA -IN 8. SEA BIRD. 1 14. B.J. Road. Bandra. Bombay - 400 050 SEMIKRON Service brings: Block-package of power diodes SKWD 7000 with integral water cooling. For welding equipment we also supply: ■ Water-cooled a.c. thyristors such as the SKW 1200/12 (1200 Aen). ■ Standard ranges of diodes and thyristors. ■ High-speed diodes, thyristors and trans- formers for frequency control. Make a test installation of SEMIKRON power Compo- nents to prove all the above advantages! For literature giving full techni- cal data and characteri- stics write, phone or telex: SEMIKRON INTERNATIONAL Sigmundstr. 200 D-8500 Nurnberg 82 Tel. 0911/6559-1 Telex 622 155 DC welding offers many advantages e.g. Constant mains load, Smaller transformers, Lower power consumption, Improved cos cp. Faster welding operations, Reduced splashing of metal- and faster welding. Even greater advantages to the above benefits are obtained through using designed for welding applications and ensures: ■ Much greater efficiency due to low forward voltage (typically 1 Vat 10kA). ■ Greatly improved cooling because its low profile gives a large cooling surface. ■ Superior protection against high current overloads Ft = 18 • W A?s and- Ifsm “ 60 kA ■ High current rating of 4000 A at 600V 10-09 CLEANLINESS — The Invisible Dimension of Quality DISCOVER WITH KIRLOSKAR CLEAN AIR SYSTEMS. A must for Dustfree, sterile Atmosphere in Electronics, Aeronautics, Engineering, Medical, Chemical and Pharmaceuticals. Clean Room • Provides up to class 100 Clean t • turnkey protect • Modular T ynnel Clean Room Sys oilers several r*ezr ” ' KIRLOSKAR ELECTRODYNE PVT. LTD. S THE NEEDS OF NEW GENERATION SPECIALISTS IN REPLACEMENT PARTS I the latest ones are very hard to get but this isn't a BIG only, small orders are also WELCOME, ements and we will quote our lowest prices in- SATELLITE TV RECEPTION by J & R v. Terborgh Much has been said and written about satellite TV over the past few years But what does this entail? Can anyone receive such programmes? What is the cost of a suitable installation? How does the taw stand in regard to reception of these signals and the installation of the necessary equipment? tn this article we will attempt to give answers to these questions and others as wet t as give a theoretical appreciation and practical information on the subject. Strictly speaking, the satellite is a repeater station: it receives the pro- gramme(s) from the TV studio via a suitable transmitter and then transmits it back to earth at a different frequency. Most existing and pro- jected TV satellites travel in a geostationary orbit, that is, they remain always above the same point on the earth's surface (but see Electric propulsion tor satellites elsewhere in this issue). To receive the satellite signals, which are trans- mitted in the 10.9 to 12.5 GHz (giga-hertz=one thousand million hertz) band, a parabolic aerial, popularly called a dish aerial or just dish, is needed. The aerial must have an unobstructed view of that part of the sky where the satellite is located (these locations will be discussed later in this article). If the aerial is placed in the garden, even a small tree or washing-line post between satellite and receiving dish can ruin reception. It is possible to put the dish on the roof of a house, but this involves planning per- 10-20 elefclof india oclober 1 986 mission (which is, we understand, not required for installation in the garden). Wherever the aerial is situated, it is important that it is fastened securely (most installation engineers recommend about 200 kg of concrete anchorage in the garden) to prevent its sailing away in high The aerial signal is fed to one or two down con- verters (depending on which programmes are to be received), which are mounted onto the dish. From there the signal is fed to the IDU (indoor unit) tuner, in which it is con- verted into a suitable video input to the conven- tional TV receiver. Although the satellites are about 23000 miles away from the receiving aerial, the colour video signal (and teletext signal) is of excellent quality. In Britain, signals can be received from two different satellites: Eutelsat I - FI, commonly called ECS 1, and Intelsat V - F4. The first carries ten European channels (o* which four in English), while Intelsat transmits four English- language programmes. See also Table 2 in this article and Elektor India, March 1986, p. 3-25. When you want to switch from one satellite to the other, the dish has to be repo- sitioned, and tor this it is best to rent or buy a motor-driven aerial. Full programmes are given in the monthly Satellite TV Europe (£1.50). Costs vary widely Dishes may be rented from DER at an initial outlay of around £750. Complete systems cost from just over £1000 for a DIY outfit to over £3000 for one with a motor-driven dish. So far, the programmes discussed here are trans- mitted by communications satellites with spare capacity. The transmitters on board these satellites are of relatively low power, so that large (1.5 to 1.8 m diameter) receiving dishes need to be used. Many European countries are planning to launch a Direct Broadcasting Satellite (DBS) within the next few months. Such satellites have powerful transmitters, so that rela- tively small (less than 1 m diameter) dishes will suf- fice to receive their signals. These smaller dishes will be much cheaper and much easier to install. The BBC had originally planned to launch their DBS service this autumn, but these plans had to be aban- doned. because of the enormous costs involved. Plans for a British DBS ser- vice are now being developed by the Independent Broadcasting Authority (IBA). As regards direct recep- tion of programmes trans- mitted by communications satellites, the law (at least in the - UK) is not clear. Strictly speaking, no private individual is al- lowed to intercept com- munications transmissions, but the fact that the equipment to do so can now openly be rented or bought from reputable suppliers seems to indi- cate that in this instance the government does not intend to enforce the requirement; for a special receiving licence Compared with terrestial transmitters operating in VHF/UHF TV repeater band, TV satellites 1. have a much larger Fig. 1. Two geostationary satellites (la) providing signals to stations that could not possibly be reached but with an exten- sive network of terrestial transmitters. Figures lb and lc show how a dish aerial is pointed towards the satellite using angle of elevation o and orbital pos- ition with respect to the Greenwich meridian. coverage area (footprint); 2. operate at frequencies of the order of 12 GHz rather than 50. . ,850 MHz; 3. employ FM rather than AM for the vision chan- nels which conse- quently need a bandwidth of 27 to 36 MHz instead of about 7 MHz; 4. may offer more pro- grammes at a time whilst being capable of supporting enhanced multi aural subcarrier systems. The present article is not intended to cover such topical subject aspects as economical viability ot satellites as compared with terrestial transmitter networks, launching schedules and arrangements (ESA, NASA), transponder leasing by international consortia, programme content, legal matters, and the cable vs. satellite debate Neither will it explore details of technical operation, con- struction, and electro- chemical / electromag- netical positioning systems of modern satellites (but see Electric propulsion for satellites elsewhere in this issue), although these would appear highly interesting subjects in view of the fast progress of international space technology and SHF engineering. With the foregoing con- straint as to subject matter in mind, it is instructive to investigate what can be received using a number of given system par- ameters. To this end, a draft may be made of a hypothetical TV satellite receiver system composed of what is currently con- sidered to have represen- tative characteristics; see Table 1. With this system on paper we will assess vision and sound quality by means of guided calcula- tion. As to the target satellite, Intelsat V F-4, which car- ries a mainly British pay- ' load, it should be made quite clear that this is a CSS (communication ser- vice satellite) intended to service cable network headend stations em- ploying large (diameter >3.5 m) dish aerials and very sophisticated con- verter and transposer equipment. It is only the recent development in GaAs (gallium arsenid) technology that has made it possible to receive this satellite with relatively small (diameter 1.5 m) dishes; LNB's (low noise block down converters) incorporating GaAs FETs as ultra low noise active devices are currently being offered at com- petitive prices, enabling private reception of the relatively weak satellite signal. Not all terms used in Table 1 will be clear at a first glance, but they will be explained in due course. First, however, it Is necessary to know the whereabouts of that tiny spot in the sky. Spotting the satellite The hypothetical receiver system just introduced may be considered either station D or E In Fig. la. It should be reiterated how- ever that Table 1 specifies characteristics of a private, not a community (CATV/SMATV) receiver; the requirements for the latter are far more stringent. The revolution time I tor orbiting bodies such as satellites B and C is com- puted from /> = 1 .4081 8333((0'//) + 1) 3 ' 2 [h] (1) where a = altitude ot body above equator [km]; r = mean radius of earth; 6371 [km] For the body to be geo- stationary it must travel at a velocity that results in h = 24 hours. From (1) it is seen that the requisite value for a is calculated as 24=1.40818333((a , //)+1) 3 ' 2 ((0-/637-1 )+ 1) 32 = 17.043236 (a/6371)+1 =17.043236“ 0/6371=6.6227-1 0=35 822 km. The geostationary orbiting plane is already quite crowded with communi- cation satellites, and regulatory action is called for on part of the WARC (World Administrative Radio Conference) to ensure orbital position spacing of not less than 0.2° (about 150 km), while a servicing (parking) orbit is being considered for spare as well as defective satellites at some 100 km further into space. Although gravitational and centrifugal forces are at equilibrium in any orbit, satellites are none the less frequently repositioned by the relevant uplink control centre which obtains its information from monitor- ing telemetric stations. Such positional correc- tions are called for to compensate for satellite movement owing to fluc- tuations in the earth's magnetic field or possible collissions with stray galactic matter such as meteorites; consider that satellite span (solar cells) may be well in excess of 15 metres, while the absol- ute orbital velocity Vo in synchronous orbit amounts to no less than l^o =631. 35 IV (£7+/) [km/s] (2) 1/0=631.35/^ (35822+6371) l/ o =3.07 km/s Given a specific orbital position of B, the receiver dish elevation angle a (see Fig. 1b) will need to be established for the rel- evant latitude of the receiver location within the satellite's service area. Obviously, a decreases as the location is further up north. Therefore, B may be received with, say, a = 22° n the Orkney Islands (=58° N) while * = 29° on the Channel Islands (=>49° N). The requisite angle of elevation also depends in The orbital position of the satellite; if this is pos- itioned at, for instance. 60° E (above the Indian Ocean) like Intelsat V F-1 (see Fig. 1c), a is relatively small (about 10°) at re- ceiver latitude 52° N. This means that the dish aerial should be located in such a manner that clear sight is ensured towards a point just above the horizon. It is evident that the actual distance to the geostationary satellite is more than 35 822 km at, say. 52° N. since allowance should be made for the sphericity of the earth and the fact that the orbital position may not coincide with the longitude of the receiver location. There exists a complex Fig. 2. Rear view of a dish, fitted with the Polar Mount satellite tracking system which enables easy dish positioning for satellites at different orbital positions. (Photograph reproduced here by courtesy of Har- rison Electronics) Fig. 3. Showing the TV satellite uplink station at Lessive, Belgium. (Photo- graph reproduced here by courtesy of Regie Tblefonie en Tblegrafie Belgium) Fig. 4. Basic operation and outlines of dish aerials for use on the 11 to 12 GHz band. The offset parabola shown at the left of Fig. 4c is an attractive alternative for reception of future DB satellites. relationship between orbital position, longitude, azimuth and angle of elevation, and this has been taken as the basis for the design of the so- called Polar Mount track- ing system, the practical version of which is shown in Fig. 2. Once correctly adjusted, the system ensures correct tracking of the polar belt, allowing easy (motorized) pointing of the dish towards satellites at different orbital positions. Many suppliers of satellite receiving equipment can provide customer-specific charts or tables aiding in finding the correct combi- nations for elevation and azimuth. With reference to Fig. la, it is seen that the term EIRP (effective isotropic radiated power) is used to specify equivalent transmitter power, which is the product of aerial gain factor Go and transmitter output power Po, or the sum of these if expressed in dBs; EIRP is expressed in dBs relative to 1 W (dBW) or 1 mW (dBm): EIRP=10logio(A 0 x35°) elev- ation angles such as may be required in, for in- stance, Switzerland and northern Italy. To make clear that dish aerial gain with respect to a dipole (Gasa) rises and half power beamwidth {cpidB) falls with increasing dish diameter, Fig. 5 shows a graph that may be used to estimate the relevant characteristics of our (hypothetical) dish, which is 1.5 m across. The next item to be con- sidered is the LNB (Table 1; Fig. 7). This is basically a high conversion gain, low noise device which trans- poses the 10.95 . . . 11.75 CSS band into an inter- mediate frequency (IF) of 950. . ,1750 MHz, using a 10.0 GHz local oscillator. The 11 GHz amplifier stages as well as mixer and local oscillator are usually all-GaAs technology ensuring a low noise figure (3 dB), good stability over a con- siderable temperature range, and high IF gain. The indoor unit, lastly, is a wideband FM TV tuner which accepts the IFi 10-24 6lektor India October 1986 Fig. 5. This combination of curves enables a quick estimate to be made of primary focus dish characteristics, since it shows theoretical power gain with respect to a 1/21 dipole, effective surface and 3 dB directivity as functions of dish diameter. Fig. 6. Showing how equivalent receiver noise temperature is calculated and plotted as a function of either FoC/[dB] (7) The figure 114 is an empirically established, sufficiently conservative (actor that does, however, not include any additional attenuating influences such as heavy rainfall, snow, hail, tog, passing aircraft or sudden disturbances in the rel- evant section of the atmos- phere. The additional attenuation caused by adverse weather con- ditions may rise to as much as 0.6dB/km, while meteorite showers and satellite positioning errors cause an even more dramatic increase of /. It it is known that (7) gives us /M14+20log.o38800 = 205 dB for reception of Intelsat V F-4, it should be remembered that some 210 dB may be a more practical value in view of the prevailing weather conditions in most ot western Europe In many instances of adverse weather con- ditions, either an auto- matic signal strength monitoring system, the rel- evant CATV /SMATV auth- orities, or even the transponder leaseholder may arrange for a logo to be shown notifying viewers of the possibly impaired vision and/or sound qual- ity. With Intelsat V F-4's EIRP of +44 dBW and / = 205 dB, a so-called isotropic aerial, which is a hypothetical reference device offering unity gain (<9>=l— 0 dB), would receive a power level of EIRP— /=(44— 205)= -161 dBW (8) if located at the centre of atolttor india October 1986 10-25 the satellite’s downlink beam on the earth's sur- face. Since an isotropic aerial has an effective sur- face, S/so, of s/so=A*l4n [m 2 ] (9) which on 11.5 GHz (A = 0.025 m = 2.5 cm) s«o=4.97x10' 5 m 2 . Since this isotropic surface offers unity gain, a real aerial with an effective surface of 1 m 2 iwst have a power gain Gi of (?/=10logio(l/S/K>) [dBi] (10) which in the present case gives 20107 — +43 dBi Note the difference between 3.5 m) to 10-26 eleklor India October 1 986 cope with less than favourable weather con- ditions. Downlink budget The foregoing paragraphs have led to two important figures which counteract and thus form weights on the carrier-to-noise [On) balance: Cin =PFD— fPn/srsA [dB] (12) which in our case goes down in favour of the car- rier weight: 07=— 118— (—127.66)= +9.66 dB This figure is not bad at all, considering that CATV /SMATV authorities demand approximately +15 dB for their set-up which necessarily includes a 3. . .5 m dish. In practice, 0/7=10 dB has proved to be a satisfactory value for private reception. Figure of merit It ought to be recalled here that both Limn and On have a bearing upon the RF input of the system; they provide neither a direct measure for, nor any conclusive information about, what happens at the indoor tuning unit's output, i.e. on the colour TV set. Manufacturers of TV satellite receiving equip- ment generally use the figure of merit, expressed as the gain-temperature ratio GIT to specify the relative quality of their GIT= 10l °9"jaTa +(1-0)290 +7.| [dB/K] (13) Where G = aerial gain (power factor, not in dB); a = sum of losses between preamplifier (LNB) input and point of maxi- mum PFD in aerial system (power factor, not in dB). For our hypothetical system with parameters as per Table 1 we can assess GIT at GIT= lOlog.o [ 20,000x0.8 1 [0.8x45+(1— 0.8)290+300 10logio(16, 000394) dB/K = 16.1 dB/K, provided of course that the overall conversion gain is high enough (which condition seems tc be satisfied with Gc at 80 dB) and that the input noise figure of the indoor unit is not more than about three times that of the LNB (see literature references [1] and [2] ). The calculation of GIT shows quite conclusively that pre-LNB losses fa) can degrade the overall system performance to a very high extent; all attenuation in the form of filters, polarizers or lengths of waveguide fitted to the LNB input may have a detrimental effect on the system sensitivity, just like the moth sheltering against the rain in the feed horn of our LNB (Fig. 7); reception was only restored to normal after the insect had been removed. The signal-to-noise ratio, SIN. is, finally, a measure of vision quality at the receiver system output: SIN=PFD+GIT+x [dB] (14) In which x is generally given as 147.3 dB for systems operating at a bandwidth of 36 MHz (Literature reference [7] ). Our equipment therefore offers 5//V=(— 118J+16.1+147.3 =45.4 dB which is more than ad- equate for excellent vision and sound quality, as proven by Fig. 14. Tuning across the transponders on Intelsat V F-4 reveals a channel assignment as shown in Table 2a, while Table 2b shows the programmes carried by ECS-1. High-power transponders: DBS It has already been noted that the foregoing calculations apply to private reception of a satellite intended to ser- vice CATV /SMATV systems, and it should be clear by now that receiver dish size is highly dependent on satellite EIRP. Planned as early as in 1972 and assigned their orbital positions during WARC 1977, DBSs (direct broadcast satellites) have, regrettably, become the subject of heated debates in which technical arguments are rapidly superseded by the wildest speculations about pro- gramme content, overspill capacity ot downlink beams, and exotic modulating systems intended to make recep- tion as costly as poss- ible. Scrambling is often wrongly identified with the D2-MAC system, which has, in tact, been developed with entirely different aims In mind and which Is basically but an en- hanced version ot existing PAL/SECAM standards. All of these speculations are quite premature, since the first pair of European DB satellites are not due for launching until later this year (both Russia and Japan already have DBSs in operation), and it will take controi/uplink centres at least half a year to complete extensive test procedures As to the mandatory minimum PFD level DB satellites must be capable of producing within the centre footprint, RARC 83 (region 2, the Americas) relaxed the original —103 dB(W/m 2 ) require- ment stipulated by WARC 1977 to —107 dB(W/m 2 ), thereby formally recog- nizing the rapid progress in SHF semiconductor technology over roughly 6 years It was also agreed that a Cln ratio of 14 dB and a figure of merit GIT' = 10 dB/K should be • Table 2a. Intelsat V F-4 orbital position: 27.5° W EIRP: +44 dBW channel bandwidth: 70 MHz Table 2b. Eutelsat 1 F-t IECS II orbital position: 13° E. All data subject to change. target design values for individual receivers. (Literature reference [6]). Figures 9 and 10 sum- marize a part of what was agreed upon during WARC 1977, although it must be pointed out quite clearly that these are but recommendations to indi- vidual countries of which, at present, only France (TDF-1) and Federal Germany (TV-Sat) are on the verge of putting a national DBS in orbit; the EBU (European Broad- casting Union) intends to have ESA launch the Olympus (L-Sat) DBS which is to have a very large pan-European beam (see Fig. 11). The Scandinavian Tele-X is probably the next in line, while Italy, Ireland, Luxemburg, and Switzerland have their projects still in the plan- ning stages. To understand why DB satellites can be received by relatively small dishes (60. . .90 cm across) and low-cost LNBs for the 11.7. . .12.5 GHz band, you need merely interpret the foregoing calculations in the knowledge that the EIRP levels of these transponders will be of the order of 60 ... 65 dBW (1 to 7 MW, Po" a 27S W), ensuring good signal strength within the relevant coverage area. With these PFD and EIRP levels, and using a 1.5 m dish, the SIN and On of our hypothetical receiver (slightly modified to suit the somewhat higher DBS frequencies) would be improved by no less than about 15 dB The dish could, therefore, be reduced to around 75 cm diameter and still give a more than adequate signal for high-quality sound and vision. The D2-MAC / Packet system (D = data transmission; 2 = bitrate division factor with respect to C-MAC; MAC = multiplexed analogue components, i.e. luminance and chrominance signals are separately stored and pro- cessed; Packet = digital coding of aural subcar- riers) is claimed to offer a •lector india October 1 986 10-27 HIBIH hIRIRImBHiRIH mibibiribibibibibiq HloIH EilDIIEiioioiDiEiinininin CT M n « ri b g'~a ■ n m pi i m rri b g^a b b ipth BIMIHIS 53 IHIB yisiyiailiSMS mm liiaiairii Slel HIMIHISIH MraSBBBSBeH HIM BiBfleieieiaiBisieia Fig. 9. WARC 77 assign- ment of orbital positions for a number of DB satellites. Not shown are CS satellites already orbiting the earth at sometimes very low spacings, crowding the available room above the equator. Fig. 10. WARC 77 assign- ment of channels within the DBS band and associated polarization standard. Note that a single frequency may be occupied by two transponders, provided these employ different polarization directions. Fig. 11. Projected footprint of the EBU DBS called L- Sat. PFD= - USdBW/m 1 contour represents the limit of community reception, while PFD= - 103dBW/m 3 should enable excellent reception by privately owned receiver systems. TDF-1 and TV-Sat arq expected to produce even higher PFD levels within their necessarily smaller elliptical coverage areas. (Courtesy of EBU Brussels) Fig. 12. Example of a DBS baseband signal, i.e. un filtered frequency spec- trum at the output of the system's FM demodulator. Fig. 13. This is what may be required to receive future DBS transmissions, provided the site is located within the relevant service area (footprint) of the satellite. Fig. 14. The system re- ferred to in this article is in fact nor hypothetical at all; these photographs were taken to prove that recep- tion of ECS- 1 is quite ad- equate given the limited means as compared with cable distribution headend stations. The indoor unit as part of the system will be described in next month's issue of this magazine. further improvement of about 2 dB SIN as com- pared with conventional PAL/SECAM coding, while being recognized as capable of supporting multiple aural subcarrier arrangements such as the one shown in Fig. 12. Multi- language transmissions (EBU), as well as, for instance, VCR timekeeping and high-quality stereo programmes (compressed bandwidth systems like Panda-Wegener), public data services and Teletext over satellite. . no wonder DB satellites are expected to become a revolutionary force in the TV era just ahead of us. Next month we will publish the first part ot a design for an Indoor Tuning Unit for satellite TV reception. Literature references [1] Lenz R, DL3WR: Noise in receiver systems. VHF Communications 4-75. [2] Aerial amplifiers. Eiektor Electronics (UK), February 1980. [3] Kernot, R J: The use of the European communi- cation satellites for tele- vision transmission. Source: see [4]. [4] Scott, J M C and Neusten, M: Experience of accessing Eutetsat transponders from transportable earth- stations. Colloqium of the Electronics division of the IEEE, digest No. 1986/32. [5] Evans, D S and Jessop G R: VHFUHF manual. Third edition; The Radio Society of Great Britain (RSGB). [6] EBU review (technical j. several articles in issue Nos. 200, 202 and 215. [7] Kathrein Ha us & Antenne, English edition 1985. [8] Satellite TV receiving equipment. Eiektor India, March 1986. Bu Since T is the international symbol for both period and thermodynamic tem- perature, it is used with these different meanings in formulas 1 and 5 re- spectively. Similarly, a is used for altitude in for- mula 1 and for sum of losses in formula 13. (Ed) 10-29 ELECTRIC PROPULSION FOR SATELLITES by Dr Anthony Martin, Culham Laboratory, Abingdon, Oxfordshire. Fuel is a significant fraction of a communications satellite’s mass. A large part of it is needed for the rockets which keep the spacecraft stationary in orbit relative to tracking stations on the ground. Electric propulsion systems to replace chemical rockets promise large savings in fuel mass, with correspondingly greater communications payloads. Magnets Baffle Anode cathode corrected by using an on- Most commercial satellites are destined tor geo- stationary Earth orbit. That is, an orbit with a period of 24 hours, which means the satellite rotates about Earth at the same rate as Earth revolves about its axis. The satellite will then appear to be fixed in the sky, so the antennas receiving its signals do not have to be steered or moved to track it. The greater part of Earth's long-range communi- cations are now routed this way, Including inter- continental telephone calls, and television from the other side ot the world is familiar on our screens. Plans are already well advanced for direct broadcasting satellites which will relay signals with such a high power that they can be picked up by a relatively small dish antenna in the home, bypassing the need for large receiving stations. But an uncontrolled satellite would not remain fixed in its 24-hour orbit for very long. Because the gravitational pull of the Sun and the Moon on a satellite distorts its orbit, it would wander about the sky and make tracking dif- ficult. This effect has to be 10-30 eleklor india October 1 986 board propulsion system to correct the velocity by about 50 metres per sec- ond in one year. So, orbit control to keep a satellite 'fixed' in the sky or 'on station’ is essential and the satellite must be able to provide this. Indeed, ability to correct the orbit may often decide the useful lifetime of the satellite: once it is unable to keep station, the com- munications payload is switched off and the satellite is abandoned. Propulsion systems Any orbit control system must be reliable and have a long life, which means Schematic layout of an electron bombardment ion engine. about 10 years for modern spacecraft. It should also weigh very little, for every kilogram that is not used for payload reduces the revenue that the satellite earns. Satellites now in service use chemical rockets for orbital control. The pro- pellants used are allowed to react in a rocket chamber and the prod- ucts from the reaction are expanded through a nozzle to produce a jet of | fast-moving gas. The vel- | ocity of the jet, or exhaust, | which the propulsion | system can achieve i$V j important. The amount of ] propellant that has to I be used to provide the 50 metres per second ! change in velocity is related exponentially to the ratio that the velocity change bears to the exhaust velocity. The j higher the exhaust vel- ocity, the lower the mass of propellant that must be carried to keep the j satellite on station. Monopropellant rockets | have exhaust velocities of | about 2.2 kilometres per | second and, for a 10-year mission, must carry 200 grams of propellant for every kilogram of satellite mass at the start ot oper- ations. Bipropellant sys- | terns have an exhaust | velocity of about three kilometres per second, which means they must carry about 150 grams of propellant for every kilogram of satellite, but at the expense of a heavier, ' more complex rocket system. Electric propulsion However, the energy j available from a chemical reaction is limited. To reach higher exhaust velocities, the source of energy must be decoupled from the pro- pellant. It is here that elec- tric propulsion systems offer an alternative to chemical rockets. Electric power is used to accelerate propellant to much higher velocities, in the range of 30 to 40 kilometres per second. That means only 12 to 17 grams of propellant are needed per kilogram of satellite mass. Of course, the mass of the electric rocket and its power sup- plies must be reckoned as part of the propulsion system, but even so it can be seen that the mass gains possible with this type of system are very large. The case of a 2-tonne satellite, typical of those that will be used tor the most important communi- cations links until the end of the century, can be used to illustrate the point. The main propulsion requirement that has to be satisfied is the ability to provide a velocity change of 50 metres per second every year tor station keeping and a further 60 metres per second at the beginning ot the mission to place the satellite in the correct initial orbit. If an electric propulsion system were used to carry out these manoeuvres, gains in the payload frac- tion of about 25 per cent could be realised, com- pared with the mass that would be needed by a chemical propulsion system with its much greater need tor fuel. To emphasise this figure, the Comparison of electric propulsion with chemical rockets, for a 10-year North-South station-keeping duty. Data are for a two-tonne geostationary satellite, with initial station acquisition and 50 metres per second vel- ocity change per year. 10-31 saving in mass of a 2-tonne satellite could be as much as 280 to 300 kilograms if electric pro- pulsion were used instead of chemical rockets. This contrasts with the total payload mass of a modern telecommuni- cations spacecraft: The Olympus-1 satellite being built for the European Space Agency by British Aerospace, and sched- uled for its first launch in 1987, has a communi- cations payload of 307 kilograms. Electric thruster In an ion thruster, pro- pellant is accelerated by electric forces to high velocities to produce thrust. For this to happen, the propellant must have an electron removed from the atom, leaving a positive ion. By far the most flexible means of carrying out this ionisation process is for electrons to bombard the propellant atoms and knock off an electron. So in an electron bombardment ion thruster electrons are emitted from a cathode and acceler- ated to a cylindrical anode, colliding on the way with propellant fed into the discharge chamber where the pro- cess happens. At the front of the chamber is an ion extraction system, usually consisting of fwo grids with a large number of small holes drilled in them. An electric potential, usually in the range of 1000 to 1500 volts, is applied across the grids, thereby causing the ions to be pulled from the discharge and accelerated through the second grid to form the beam. If only the ions were extracted from the dis- charge, the satellite would build up a large negative charge very quickly. So a neutraliser is included, to eject electrons and balance the charge on the spacecraft. All the foregoing has to do with the thruster part of the system. But a complete 10-32 eloktor india October 1986 system needs an electrical power source That might be the main solar-cell array powering the satellite The array is usually oversized relative to the needs of the payload, to allow for solar-cell degradation over the lifetime of the satellite Alternatively, the source might be the bat- teries carried by the satellite to support it through periods when the solar cell array is in eclipse from the Sun. Typical stationkeeping thrusters need a few hun- dred watts of power to operate them, which is a small fraction of the several kilowatts which are available on board large communications satellites. The need to draw power from the spacecraft, rather than from the chemical reaction of conventional rockets, governs the design of the propulsion system as a whole. High exhaust velocities, achieved by high accelerating voltages, reduce the amount of propellant needed. But they also mean that the power unit, which converts the output from the solar array or battery to the voltages required by the electric I propulsion system, becomes heavier and I heavier with a corre- | sponding need for higher power. So there is an opti- mum point between a reduction in propellant mass and an increase in powersupply mass. Future j propulsion j Even with the prospects of j all the benefits to be J gained, communications I satellites still do not use j electric propulsion, but rely on chemical rockets. Why? There are several reasons. Although electric propul- sion systems are capable of increasing the payload by 20 to 25 per cent on a wide variety of satellites, it is only recently that com- munications spacecraft with masses of more than one tonne have become j relatively commonplace. | Previous generations of vehicles had masses of about 750 kilograms and the extra payload that might have been added was not considered enough to warrant the cost of developing the propulsion system. The UK 10-centimetre diameter ion thruster, designed for station- keeping of multi-tonne communications satellites, now being tested with xenon pro- pellant. It is only quite recently that communications satellites with powers of several kilowatts have become operational. Elec- tric propulsion systems would absorb a small fraction of the total power available, in contrast to earlier available powers of 500 to 1000 watts; the pro- pulsion appeared to require too large a frac- tion of that lower power to gain easy acceptance. Also influencing accept- ance are natural resist- ance to change and reluctance to adopt what is often seen as a com- plex system of strange thrusters, power supplies and controls, compared with the chemical rockets which might be thought relatively simple because they are so familiar. It is only recently that the benefits from such a change have become so potentially great as to compel this attitude to be rethought. One major objection to the use of electric pro- pulsion has been the choice of propellant for most of the work carried out on thrusters, namely mercury. It is almost ideal as a propellant because it is heavy, dense and easily | stored. But it is not an I ideal material as far as [ spacecraft designers are concerned, for it amalga- I mates rapidly with many metals such as copper, gold and aluminium, which means that the | spacecraft structure, elec- trical wiring, power- producing solar cells and so forth could all be j vulnerable to attack, j Another problem with mer- I cury is to do with the fact I that it is liquid at normal [ temperatures. Care must be taken to heat the pro- pellant to a vapour before introducing it into a . 1 thruster, and to keep it as a vapour. If it condenses, it could lead to breakdown of high voltage insulation, shorting out of power supplies, damage to solar cell arrays and other serious problems. All these disadvantages and problems with pro- pellant are eliminated, or I at least greatly reduced, if a gas is used instead of a | liquid metal. The favoured candidate is the rare gas xenon, which is inert. It does not contaminate or | react with the elements of space systems so it removes most worries about the structural integrity of long-life spacecraft. It does not condense upon compo- nents, so it does not cause electrical trouble. The power supply systems are simpler, too for no supplies are needed to heat and vaporise the propellant. That means better system reliability. The problems of economic justification and power requirements hitherto associated with electric propulsion have dimin- ished in importance, while those to do with choice of propellant and complexity of power supply have been reduced by thruster developments, so the time is ripe for this novel pro- pulsion technique. A very successful pro- gramme of work on the development of electric propulsion systems, led by the UK Royal Aircraft Establishment at Farn- borough in collaboration with Culham Laboratory and several industrial firms, ended in 1978. The reasons for not continuing further had nothing to do with any failing in the systems that had been developed, which were at least as advanced and efficient as any others, but with the economics and other arguments to do with small, low powered spacecraft which I have already outlined. The pro- gramme has now been re- activated with a view to providing ion thrusters for station-keeping of multi- tonne satellites. The work is based around thrusters of 10 centimetres diameter operating with xenon pro- pellant instead of mercury. How far development reached in the previous programme is shown by the fact that the same thrusters are being used; the only modification needed was to remove components that were used to vaporise the mer- cury and keep it in vapour form. Present plans call for a test flight on board a satellite in 1989. and commercial implementation soon after. Britain is not alone in such work, of course: all the leading space nations are planning to test electric propulsion systems in the next tew years. The USA is due to fly a satellite with two mercury devices. Japan has already flown a small mercury system and operated it for 200 hours in space, and is developing a 12-centi- metre xenon system. Germany has plans to fly a 10-centimetre xenon system for a six-month test on the European Retrievable Carrier (EURECA). With so many contenders it is obvious that electric propulsion is about to come of age and find more and more appli- cation in the 1990s. The focus ot work will then shift away from the proot-of- principle of a new propul- sion system and concen- trate more upon providing the most efficient, flexible and commercially attract- ive product foi commer- cial users. Though electric propulsion will have been a long time coming, it will soon be here to stay. IKs -lmJ"! r'smito', SYaT rr. ‘T-*- r:r munimenlr). S 1.440 IPIessey), TDA20O2 ISGS/ATI.S). UAAI70 (Siemens) and 4ISI For the construction protects m Elektor. India we try to use standard components wherever 1 Raytheon). Resistor* t 1 possible This may not always be obvious at first sight, so some further explanation is given here £* EiHirrir The values arc specified urine ‘1’ lor 1.(1001) and M (or 1.000.00011; Ihe decimal polnl 470000011. Capacitors ft Tv* ,N«I)0I IN40O4 red IN400S are reel* *1 Icier oil V.’unlen other. ,«• speeilied. • j.rit in* oa«s aaiis Sd'SSssrifisirs oT.Et'mjoio lH = 100 Q R33:R3S = 33 k Pi = 100 k linear P2 = 100 k logarithmic potentiometer' P3;Ps= 10 k logarithmic stereo potentiometer' The user may set the equalizer am- plification between —18 dB to +2 dB by means of P4. Buffer As outputs the frequency-corrected monitor signal, while R31 prevents the IC from being damaged in the case of a short cir- cuit at this output of the mixer module. Output module 2 is fitted on a ready- made PCB as shown in Fig. 3, and this job should present very few problems if the guidelines given in previous articles in this series are properly observed. Finally, Fig. 4 shows the layout for the front panel foil (FPL) which goes with the pres- ent output module. Mixed matters In order that the proposed mixer can be carried easily from one site to another, the completed mod- ules are fitted into an aluminium 46x34x14 cm (width x depth x height) photographer’s case. Before detailing the preparation of the case, a number of brief hints will be given regarding the separate modules that constitute the mixer. Obviously, the slide potentiometers should be mounted in the correct position, that is, the resistance should change from slow to fast when the slide is moved upwards (higher signal level). Ensure easy movement of the slider knobs by inserting 2 mm spacers be- tween the slider potentiometers and the module front panel. Cut the spindles of the PCB-mounted potentiometers to a length that enables the knobs to be seated just above the front panel. All input and output sockets must be isolated with respect to the module panels. As already explained in the first part of this series (see Elektor Electronics India, May 1986), the mains earth may be disconnected from the mixer power supply to pre- vent trouble with hum in complex equipment setups. As to the 78L15-79L15 replacement for the Type XR4195 dual regulator chip, it should be pointed out quite clearly that the three-pin ICs must only be used as a very last resort, since they may produce audible clicks at power-on. The Type U267B VU meter ICs may be damaged when the DC level at their input pins exceeds 5 V. In order that short-duration signal peaks can- not reach this level, zener diodes rated at 4V7 may be connected in parallel with the 1 (iF capacitors at the IC input terminals. In addition, a 470 ohm resistor is connected in series with the Type 1N4148 rectifier in each VU meter circuit; these modifications are easily brought about on the PCB for output module 1. Most LEDs can be mounted direct onto the PCBs to allow their pro- truding through the front panels. Where this is not possible, they should be fitted with two-component glue and suitable lengths of wire to connect to the relevant PCB points. The construction of the power supply is likely to present most diffi- culties, but the photograph in this article should prove helpful in offer- ing sufficient information to bring the matter to a successful end. Figure 5 shows the outlines of a support screen which serves to improve the mechanical stability of the supply module, as well as to offer an effec- tive shield, useful for the suppres- sion of undersirable stray magnetic fields (transformer hum and mains- induced noise). The screen may also be used to secure PCB and front panel, but due care should be taken I to mount the mains switch in an elec- trically safe position, well isolated from the screen. In any case, the mains switch should fit snugly into the relevant front panel hole. The voltage regulators on the mains supply PCB get hot after prolonged use of the mixer and should, there- fore, be fitted with suitably cut, angled metal brackets which are secured to the existing heatsinks and the front panel. This modification necessitates the use of insulating washers and bushes with the regu- lator ICs. The circuit diagram of the power supply (see Elektor Electronics India, May 1986) erroneously shows R4 and Rs to-have values of 3k6 and 220 Q respectively, while the parts list indicates the correct values for these resistors. Transformer Tri should be a Type 13014, not a Type 11014 as stated, while D6 is shown with reversed polarity in the circuit diagram; however, it appears correct on the PCB layout. Finally, R9 and Rio have been shown as 47 k types in the circuit diagram of the stereo module (page 53), whereas the correct value of 22 k is stated in the parts list. Constructors of the above modules are advised to correct the wrong values of the indi- cated parts in the circuit diagrams. A home-made flight case The proposed mixer is contained in an aluminium case, and consists of six M1C-LINE modules, two stereo modules, one output module type 1. one output module type 2 and the power supply module. Assuming that all modules have 10-48 I been completed as per the guide- lines given in the articles, and that the mixer is equipped with the above module configuration, the number of 13-way sockets to receive the corresponding module plugs amounts to eleven. The 13-way sockets are wired to form a bus structure; the earth connec- tions at pins 3, S, 7, 9, 11 and 13 are wired from socket to socket with heavy duty, stranded wire of 1.5 mm 2 cross-sectional area. If the intercon- necting wire lengths are about 17 cm, the mobility of each of the modules is ensured; this is of para- mount importance for measurements in individual units while the mixer is still functional, since the additional wire length enables the user to readily remove modules from the cabinet for that purpose. However, 3 Capacitors: Ci;C3;C«;C6;Cii; C32 = 10 p;40 V bipolar electrolytic C2;Cs;C3t = 10 p C7;Cs=1n5 C9;Cio;C25= 100 n Ci2;Ci3 = 220 n Ci4;Ci7;Ci8; C23=100 p;16 V Ct5;Ct6=22 p;6V3 Ci9;C2O = 160 n C2t;C22 = 470 p; 16 V electrolytic C24 = 10 p;28 V electrolytic C26, C27= 100 p C28;C29 = 10p;16 V electrolytic C3O=470 n Semiconductors: Di;D2 = 1N4148 ICi;IC2;ICs;IC4= TL072 ICs = TEA2025 IThomson) IC6 = 7812 IC7=XR4195 ICa=TL071 Miscellaneous: St = 2-pole, 3-way rotary switch socket, insulated 2 off 3-way Cannon IXLR) sockets Ki= 13-pole PCB-type connector to DIN41617 Knobs for poten- (4 mm spindle) potentiometer Front panel foil Type 86012-5F' PCB Type 86012-5' 13-way sockets to DIN41617 as required * available through our Readers' Services Fig. 3. Compo- nent mounting plan for output module number two. The true- scale track layout for this PCB may be found on the centre pages in this issue. 1986 1 0-49 switch terminals. Point 9 on the PCB is connected to the case ground with a suitable length of wire and a solder tag. Finally, the photographer’s case needs to be tailored in order that it can receive the completed modules; to this end, holes should be drilled in the front panels and the rim inside the case to enable the modules to be secured with self-tapping screws. Depending- on the type of case, its lid may have to be modified on the inside to allow for the projecting parts (knobs, switches) on the front panels; it should, of course, be poss- ible to close the lid! The photographs included with this article should be studied carefully to enable individual adaptations to a particular case to be made. A$-GS it should be pointed out that ex- cessive bus wire lengths are to be avoided in view of the increased susceptibility of the modules to ex- ternally generated noise. Socket pins 4, 6, 8, 10 and 12 are con- nected with light duty wire (cross sectional area 0.1S mm 2 ), and the power supply pins 1 and 2 are "bussed'' using 0.75 mm z type wire The mains transformer is mounted in the upper right hand comer of the aluminium case. The wires from the mains socket should be run direct to points 1 and 2 on the supply PCB, while the transformer primary winding is connected to points 3 and 4; the secondary windings are wired to points 5-6 and 7-8. The mains earth isolating switch S2 is preferably fit- ted at the rear of the case; neon lamp Lai is connected direct across the Fig. 4. Front panel foil layout and drilling template. Fig. 5. Showing the additional screening bracket for the power supply unit. 10-50 COMPUTERS OF TOMORROW by David Fishlock Launched in 1983, the United Kingdom's five year Alvey Programme in ad- vanced technology takes its name from John Alvey, the British Telecom execu- tive who led a government inquiry to determine the country’s response to the challenge of Japan's national quest for the fifth I generation computer. | The name caught on, and people soon began to talk of the £350 million funds made available for research and develop- ment as 'Alvey money". Funds are provided jointly | by the British Government (£200 million) and industry j (£150 million), and are channelled to the re- search teams through a specially established executive agency called the Alvey Directorate. In 1981, when Japan disclosed its idea of a national research and development programme, I it soon became clear that | no one knew what form the fifth generation com- \ puter might take. The first j four generations were i defined as those based on valves (vacuum tubes), transistors, integrated cir- cuits (chips), and very large scale integration i (VLSI) or big chips. Would | the fifth generation be an optical computer perhaps, J or a machine that spoke | a human language? Working through consortia The Alvey Directorate has no laboratories of its own. From an office in London it co-ordinates a nation- wide research and de- velopment programme involving British teams drawn from around 60 companies, including foreign owned ones such as Philips and IBM, 46 universities and poly- technics, and five national The Cad fly robot from CEC Research Laboratories is part of Britain's Alvey Programme. The robot, seen here fitted with two six-axis force sensors, can be programmed to perform intricate tasks on irregular and delicate sur- faces — even, for example, writing on an egg. laboratories. It works through consortia or groupings of academic and industrial researchers who have come together to put up a joint proposal for Alvey money. The direc- torate has backed more than 100 out of 550 such proposals, adding up to a programme with seven broad areas of attack: 1. VLSI: One micrometre geometries or less. 2. Software'engineering. 3. Intelligent knowledge based systems (IKBS). 4. Man-machine interfaces (MMI). 5. Systems architecture. 6. Large demonstrators (prototype fifth gener- ation computers). 7. Infrastructure and com- munications. The section of the pro- gramme attracting the greatest interest is the one involving large demon- strators. These are seen as prototype computers, novel concepts that will give the participating companies a worthwhile commercial lead in five to seven years time Of seven detailed proposals submit- ted, the directorate finally chose the four that best matched its overall pro- gramme These are: * A £6.5 million project to develop a new com- puter system for the British Government's health and social security operations. It is led by ICL. * A design to product demonstrator that is, in effect, a demonstration ot a fully automatic factory. It is led by GEC and Lucas- CAV. * A speech input word processor and work- station project, expected to form a key part of any fifth generation computing system. It is led by Plessey. * A £7.5 million five year project to develop a mobile information system with such capabilities as route guidance and fault location in the electricity supply system. Racal Elec- tronics is the lead firm here Community rival In parallel with the Alvey Programme the European Community launched the European Strategic Pro- gramme of Research in In- formation Technology (Esprit), of which three British companies — GEC Plessey and ICL — were among the 12 founding fathers from the European electronics industry. Esprit was conceived as a way of enhancing the strength of European in- dustry. Collaborators have free access to all the technology arising, but are free to choose what they use to design and make their own products. Two new and related European research pro- grammes in advanced en- abling technologies are now taking shape, spurred by the success of both Esprit and Alvey. These are Eureka and the recently launched Basic Research in Industrial Technologies for Europe (Brite), which seeks to extend the prin- ciples and lessons of Esprit into the more traditional areas of European in- dustry. Britain is playing an important part in help- ing to shape both projects, elekio. India octobm 1986 1 0-51 selex-i6 Measuring Techniques Chapter 2 In the previous chapter of this series, we discussed the different types of meters and multimeters, and the basic types of measurements that can be carried out with them. In this chapter, we shall see how different types of components can be tested with a multimeter. However it should be noted here, that a multimeter cannot be used for determining how well a component functions, but it can check for defects. A multimeter can be used for testing components like Electrolytic Capacitors, Transistors. Diodes and quite obviously the Resistors. The multimeter must be kept in the resistance range. In this case, the internal battery of the multimeter serves as the current source. The terminals of the multimeter generally give reversed polarity in the resistance range, that is, the COMMON terminal becomes the plus pole and the other terminal becomes the minus pole. Resistance measurement is the main function of the resistance range, and details about resistance measurement with the multimeter are always given in the operating manual of the multimeter. Use of this range for testing other components is described below: Electrolytic Capacitors A multimeter cannot be used for measuring the capacity of an electrolytic capacitor, but it can be used to check whether the capacitor is defective or not. Electrolytic capacitors are indispensible parts of most electronic circuits. They contain a fluid called electrolyte which can evaporate in course of time and the capacitor is said to have run 'dry'. The capacity to store the charge is thus lost and the two terminals of the capacitor are no longer fully insulated from each other. The capacitor thus starts conducting a leakage current. For testing the electrolytic capacitors, a high resistance range is to be used (50 K or 100 K). The polarity of the terminals should be carefully maintained because electrolytic capacitors are polarised components. When an electrolytic capacitor is connected to the multimeter for testing (with correct polarityl) the needle deflects quickly and then starts falling back slowly. The needle should ideally go back to show infinite resistance. If it doesn't, then it means that the capacitor is leaky. The quick deflection of the needle is caused by the charging current, as the multimeter's battery charges the capacitor. As the capacitor reaches the fully charged condition, the current reduces and the needle starts falling back. When the capacitor is fully charged, the needle shows infinite resistance. Only in case of capacitors above 500 fxF, a small leakage current is permissible and the needle mayjiot reach the infinite resistance mark. The same measurement can be repeated after about 30 seconds, with same polarity to check if the capacitor can retain the charge. This time the deflection of the needle is very small because the capacitor is already charged and does not draw a substantial charging current from the battery. If the voltage rating of the capacitor is quite high, it can be touched with the reversed terminals of the multimeter. This time also the needle will deflect quickly and start falling back. Capacitors below 10V rating should not be subjected to this test. Also, in case the internal battery of the multimeter is a high voltage battery, this reverse test should never be carried For testing small value capacitors, the multimeter must be kept in the highest resistance range. In case of capacitors below 100 nF, no visible deflection can be observed. Diodes To test the diodes, the multimeter can be kept in 2 K or 20 K range. The diode is connected to the multimeter probes, first in the forward direction and then in the reverse direction. In the forward direction the meter should read a resistance value around 1 K and in the reverse direction it should read a very high or infinite resistance. Please note that when connected in the reverse direction, the probes of the multimeter should not be touched, otherwise the body resistance will come in parallel and a deceptive reading will be shown. The resistance reading given by selex the diode in forward direction is the result of the 0.6 V drop across the (Jiode. Diodes in an electronic circuit can also be tested directly on board, provided the circuit is disconnected from its power supply. If the resistance shown in both the directions is same, remove one end of the diode from the PCB and check again. If same readings are given, the diode is defective. Sometimes it may so happen that the diode works properly at low voltage, but behaves as a short circuit if a high voltage is applied in an actual circuit. Transistors Transistors are built with two PN junctions, which act as diodes. The Base-Emitter diode or the Base — Collector diode can be checked as discussed previously for individual The direction of the diode junctions is different in NPN and PNP transistors as can be seen from figure 3. When resistance is measured between collector and emitter, it should be an infinitely high value. If a high resistance (about 100 K) is connected between collector and base, the multimeter should show a slight deflection when connected as shown in figure 4. The resistance of 100 K need not be an actual resistor physically soldered between collector and base. It can be replaced by the body resistance by touching the base and collector simultaneously by a wet finger, which causes a small current to flow through the base thus giving rise to a noticeaole amount of collector to emitter current, depending upon the gain of the transistor. In case the pin details of a transistor are unknown, this type of measurement can be used to find out the Collector, Base and Emitter pins. Connecting a resistance between base and emitter collector current. 10-53 selex The Heavy Weights Of Electronics If a mains operated equipment is lifted, it can be immediately noticed, where the transformer is located. This is because it is the heaviest part of an electronic equipment operated directly from the mains power supply. Transformers are the heavy- weights of electronics, and every designer tries to keep them to the minimum. This heavy structure of metal plates is unavoidabe because the magnetic field produced by the primary winding of a transformer must have a suitable medium so that the strength Figure 2 shows the shape of a transformer core, through which the field lines pass. All line pass through the where the energy is transformed from electricity to magnetism and magnetism to electricity again. The direction of field lines reverses every half Transformer cores are generally as shown in figure 2 and 3. but toroidal cores as shown in figure 4 are found to be more efficient. In this case the windings are distributed throughout the circular shaped block of laminations. Figure 4 shows a comparison between two transformers of same capacit y The small toroidal transformer can manage the same capacity as that of the conventional transformer shown on its side. Toroidal core transformers are more expensive due to two reasons, due to the material of laminations and because of the complex procedure of winding the coils. As the toroidal core cannot be I split up in two pieces as in j case of the conventional transformer, the coils have j to be wound as shown in figure 5. At first, the wire is wound on the circular toothed ring which runs | through the torodial core | and then wound on the core by rotating that ring as well | as the core. Insulated copper wire is used for winding the coils of a transformer. The thickness of the wire used depends upon the current which must flow through the winding safely. The length of wire required depends on the number of turns - which in turn depends on the voltages involved. The magnetic field generated by the primary winding depends on the current flowing through the winding as well as the number of turns in the winding. The secondary voltage induced depends on secondary winding. All these relations can be briefly stated by the following formula : W prim U prim Wsec ~ Usee, where W is the number of turns in the winding and U is the voltage. A concrete example will make this clearer: A transformer has 2300 turns in its 230V primary If a secondary voltage of 12V is required, how many turns will be required in the secondary winding ? 2300 _ 230 W sec ' W sec 1 2 W sec = 120 the secondary winding must have 120 turns. A transformer data sheet generally gives the ratio of the number of turns to voltage as “Turns per Volt". In the above example, this ratio is 10 turns/volt. Toroidal transformer windings are wound directly on the core body. However the conventional type of transformer has windings on a plastic former which sometimes may have separators to sepa rate the primary and secondary windings. Small transformers are sometimes cast with plastic compounds to make them safe and reduce hum. 10-55 selex Electronic hobbyists frequently come across a typical problem - a transformer which has no markings on it. Many transformer manufacturers find it unnecessary to give the data on the transformer, and this leads to serious difficulties for a hobbyist. What can be done in such a situation ? First of all, let us find out if this is a mains transformer or some other type of transformer. Audio transformers from old valve type radios look very similar to mains transformers, but they are not. There is no reliabe way to identify a mains transformer just from it appearance. So a little research must be done, to find out the source from where this transformer The use of an unidentified transformer will finally depend on its ratings, and this can be roughly estimated from its size. As a general rule, a high capacity transformer will always be larger in size. This size increases on one hand due to the thicker and larger lamination block and on the other hand due to the thicker wires used for higher current capacities. Transformer rating are specified at their maximum values and need not be fully loaded at all the times. The load will depend on which equipment we connect at the output. Table 1 provides a rough reference data for frequently used transformers. This table can give you a rough idea about the ratings of the transformer from its size. The next step is to find out which terminals belong to which winding. A multimeter can be used for this purpose. A multimeter in the lower ranges of Ohms can measure the winding resistance. The higher the resistance, the thinner and longer is the wire and higher is the relevant voltage for the winding. With this measurement, mostly it will be possible to locate the primary 230V Unknown Transformer Data If we notice that more than two terminals are connected to the same winding, it must be a winding with several taps on it. For example, it may be a 0 — 6 — 9—1 2V winding with a total of 4 terminals. Once you identify the primary and secondary windings, almost half the job is over. Do not apply the mains voltage directly to the primary at this stage. It is better to test for the turns ratio using a low AC voltage of the order of a few volts. About 3 volts from a small i bell transformer would be I ideal. Connect this voltage to a pair of terminals on the secondary side and then measure the voltage appearing on the other terminals on the secondary as well as primary side. Remember to switch the multimeter into the AC Volts range. When measuring the induced voltage on the primary side, be careful not to touch the terminals. 10-56 elaklor in selex because it may have directiy induced 230V or even more! If the transformer under test is a 3V transformer itself, then the measured voltage on the primary will be 230V. This may be a rare case, and mostly the ratios of voltages measured will require further calculation to get the exact winding details. Example : An unidentified transformer has two windings. When 3V AC is connected to the secondary side, the measured voltage on the primary is about 92V. This gives us the ratio as 92:3, which when modified can be stated as 230:7.5. From this we know that the transformer under test is a 7.5V mains transformer. A practical situation will not be so simple, and mostly there will be many taps on the secondary, and primary side. If you suspect more than one winding to be the 230V primary, the one with the thickest wire will be the actual primary winding. If the measured values do not lead to any conclusion, either the transformer is a damaged one or it is not a mains transformer at all. Once we establish the voltage ratings, we must also estimate the current ratings. This can be done by using the VA (Volt Ampere) ratings given in table 1. Dividing the VA rating by the voltage rating will give the current rating. This is based on the fact that the VA rating of the transformer holds good both at the primary and secondary P = U.l (VA) VA is used in AC instead of Watts because it is not always necessary that the voltage and current will always maintain the same phase relation. For two transformers with same VA ratings, the lower the secondary voltage, higher is the current rating. For example, a transformer with 10 VA rating and 10V secondary will have 1 Amp current rating on secondary, where as a 10VA transformer with 3V secondary will have 3.3 Amp current rating on the secondary. Practically, the above rule is not quite accurate because the transformer consumes energy in the core and in the windings and becomes hot. These losses can be as high as 15%. The primary VA rating must be greater than the secondary VA rating. by this amount. The current rating can be found by using the data given in table 1 . The | thickness of wire can give an accurate indication of the current rating. The diameters of wires given in table 2 are inclusive of the insulating varnish coating. Generally it is enough to know the secondary voltage and current rating of a mains transformer for all practical applications. The methods of identifying a transformer described above are not 100% accurate. However with a little bit of intuition one can decide the suitability of a transformer for a specific application. Digilex-PCB is now available! Price: Rs. 85.00 + Maharashtra Sales Tax. Delivery charges extra: Rs. 6.00 Send full amount by DD/MO/PO. Available from: precious ° ELECTRONICS CORPORATION Journal Division 1 1 , Shamroo Vrthal Marg (Kiln Lane) Off Lamington Road. Bombay-400 007. 10-57 selex 'Phase' to describe the direction of the winding. selex Parallel Connection We have seen the effect of connecting two secondary windings in series. It is not always necessary that the windings be connected in series. Transformer windings can be connected in parallel also. In case of a series connection, the voltages are added. In case of a parallel connection the voltage remains same, but the current capacity is doubled. Even in case of the parallel connection, the phases must be correct. Figure 4 shows a parallel connection of two 1 5V secondary windings. The phase of the windings must again be checked by trial multimeter in AC V range and connecting the windings as shown in figure 5a and 5b. The windings should be also checked by connecting the terminals as shown in figure 6a and 6b. If the connection as in 6a gives a zero voltage reading, then the connections in figure 5a are the correct parallel connections. For reasons of safety, measurements should also be carried out as in figure 6b; if this gives zero voltage, then the connections in 5b are correct One important point should be kept in mind before connecting the two secondary windings in parallel. The windings may not be exactly identical and there may be a slight difference between the voltages at these windings. will flow in these windings even there is no external load connected to the secondary side. This current may heat up the transformer. If the heating is excessive, the transformer must be replaced. When two parallel windings are to be connected to a rectifier bridge it is advisable to rectify the outputs of both the windings separately and then connect the two rectified outputs in parallel. This avoids the heating of the transformer under no load conditions. 10-59 10-61 classified ads. Advertisers Index Contact; TEKNO ASSOCIATES K-40 Gandhi Nagar. Gwalior - 474002 Ph 24806 "DON’T READ THIS ADVERTISEMENT! I than 66% on hundreds ol Electronic Books. Write today for FREE Book-list | SRI KRISHNA ASSOCIATES, B 25. Mahesh Co-operative Housing Society. I Dattapada Road, BOMBAY 400 066. EQUIPMENT SERVICING Excellent and ments, measuring instruments, medical , gadgets, computer keyboards, enter | Contact: Anil V Joshi 2A/22 ' Geetan- jali", New Siddarth Nagar. Goregaon (W) Bombay - 400 062 Wanted computer instructor for com teaching materials. Reply Post Box 84 I AKOLA 444 001 (Maharashtral ' MICROPROCESSOR ENGINEER" undertaken or completed. Freshers not wanted Apply Managing Director. P O Box 91 22. Bombay 400 025. CORRECTIONS Battery guard (Aug/Sept 1986 p. 57) Diode Dr has been shown with the wtong polarity in the circuit diagram. The positive terminal of C2 should be connected to the Infra-red light switch (February 1986 p. 2.33) The parts list to Ihe project should be modified to read: P. = 10 M preset for horizontal mounting on PCB Cj;Cx;C« = 10 p;10 V; tantalum C10 = 10 n;100 V ACE COMPONENTS 10.06 APEX 10.66 ATRON 10.08 BELL SYSTEM 10.69 BINATONE 10.04 COMTECH 10.08 CYCLO 10.12 DEWAN RADIOS 10.72 DEVICE 10.61 DISCO 10.69 DYNALOG MICRO SYSTEM 10.76 ELECTRONICA 10.62 ELECTRONIC GMBH 10.12 ELCIAR 10 62 ELECTROKITS 10.72 GRAFICA DISPLAY 10.06 GREAVES SEMICONDUCTORS 10.16 IEAP 10.66 INDIAN ENGINEERING .... 10.70 J M ENTERPRISES 10.71 KIRLOSKAR 10.11 LEADER ELECTRONICS . 10.12 LUXCO ELECTRONICS . 10.63 MECO INSTRUMENTS 10.73 MOTWANE 10.15 NCS ELECTRONICS 10 64 PIONEER ELECTRONICS . . . 10.10 PRECIOUS ELECTRONICS 10.65 SAINI ELECTRONICS . . 10 66 SCIENTIFIC 10.06 SEMICRON 10.09 SMJ ELECTRONICS 10 13 SOLDRON 10.69 SONODYNE 10.02 S.S INDUSTRIES 10 14 SUPERB PRODUCTS 10.14 10.64 SWASTICA 10.07 TESTICA 10.62 TEXONIC 10.10 THERMAX 10.08 TRIMURTI 10.72 UNLIMITED 1064 VASAVI 10.10 VISHA ELECTRONICS 10.75 YABASU 10.70 PI A for Electron (Aug/Sept 1986 p. 45) The designations of pins 10 and 19 of IC2 have been interchanged: pin 10 should be G ND. and pin 19 should be P = Q. Electronic rotary switch (Aug/Sept 1986 p. 70) Pins 1. 8. 9, 10. and 15 of 1C. and pins 12 and 23 of IC2 should be connected to ground. R.N. No. 3988 1/83 mh/byw 228 WHICH Microprocessor? Microprocessors are now being used In a wide range ol products, from Mixers and Washing Machines to highly sophisticated Industrial Robots. Have you decided which microprocessor you will be using for your new products? Will it be 8085. Z-80. 8086. 8088. 6802. 6502 or will it be the powerful 32-Bit 68000? Whichever Microprocessor you select . Dynalog has a suitable Training and Development System for Tel: 362421, 353029 Telex: 011-71801 DYNA IN Gram: ELMADEVICE Branches and representatives at: Pune. Bangalore. New Delhi. Hyderabad and Chandigarh 14. Hanuman Terrace, Tara Temple Lane. Lamington Road, Bombay 400 007 Training And Development Systems Based On : 8085, Z-80, 8086, 8088, 6802, 6502 And 68000. Dynalog Micro Systems otter you the most comprehensive range ot Microprocessor Training and Development Systems. Starting with the Low Cost Systems based on 8085. Z-80 and 8088rthe Top of the range covers Single Board Computers based on 6502 and Z-80, and a Full Fledged Single Board Development System based on 68000 The Single Board Computer-Super 80 based on Z-80 has on board Interlaces tor Centronics Printer , Video Monitor, ASCI I Keyboard, RS-232- C Serial Communications and Floppy Disk Drives of both 5V, and 6 inch types It is fully compatible with CP/M Operating System The standard features ol the MICROFRIEND Series Training and Development Systems include Hex Keypad, Seven Segment LED Displays, On board EPROM Programmer. Timer/Counters, Parallel and Serial I/O Porta STD Busori edge connector, Powerful Monitor Firmware In EPROM. Detailed Documentation/Operating Manuals etc Systems like the MICROFRIEND-III also offer facilities for programming in BASIC, FORTH or 8085 ASSEMBLY LANGUAGE with built in interfaces for Video Monitor. ASCII Keyboard and Printer