CTRON Cs tiJRLD - WorldRadioHistory.Com · Gyrator acts as electronc choke, High -power class -A...

CTRONtiJRLD

+ WIRELESS WORLDSEPTEMBER 1993 £1.95

FREE BOOK OFFER:Exclusive to ELV+WW readers:complete 4 -book microprocessorlibrary for £37.50 price of three!

WS SPECIAL

ICATIO

BarAres.

ugDESIG

T emcontrol!thyri

Technolothe S -P

Front enddistortion

CsDenmark DKr. 70.00Germany DM 15.00

Greece Dra.760Holland DFI. 14

Italy L. 7300IR f3.30

Spain Pts. 780Singapore SS 12.60

USA $6.70

SOR DISTRIBUTION

A REED BUSINESS PUBLICATION

VOICE LINKWITH SPREADSPECTRUMRADIO

PC ENGINEERING a

Electronic Workbenchreviewed

9 770959I I

3 004

THE WORLDS No.1 BEST SELLINGUNIVERSAL PROGRAMMING AND TESTING SYSTEM.

The PC82 Universal Programmer and Tester is aPC -based development tool designed toprogram and test more than 1500 ICs. The latestversion of the PC82 is based on the experiencegained after a 7 year production run of over100,000 units.

The PC82 is the US version of the SunshineExpro 60, and therefore can be offered at a verycompetitive price for a product of such highquality. The PC82 has undergone extensivetesting and inspection by various major ICmanufacturers and has won their professionalapproval and support. Many do in fact use thePC82 for their own use!

The PC82 can program E/EPROM, Serial PROM,BPROM, MPU, DSP, PLD, EPLD, PEEL, GAL, FPL,MACH, MAX, and many more. It comes with a 40pin DIP socket capable of programming deviceswith 8 to 40 pins. Adding special adaptors, thePC82 can program devices up to 84 pins in DIP,PLCC, LCC, QFP, SOP and PGA packages.

The unit can also test digital ICs such as the TTL74/54 series, CMOS 40/45 series, DRAM (evenSIMM/SIP modules) and SRAM. The PC82 caneven check and identify unmarked devices.

Customers can write their own test vectors toprogram non standard devices. Furthermore itcan perform functional vector testing of PLDsusing the JEDEC standard test vectors createdby PLD compilers such as PALASM, OPALjr,ABLE, CUPL etc. or by the user.

The PC82's hardware circuits are composed of 40set pin -driver circuits each with TTL I/O control,D/A voltage output control, ground control,noise filter circuit control, and OSC crystalfrequency control. The PC82 shares all the PC'sresources such as CPU, memory, I/O hard disk,keyboard, display and power supply.

A dedicated plug in card with rugged connectingcable ensures fast transfer of data to theprogrammer without tying up a standard parallelor serial port. Will work in all PC compatiblesfrom PC XT to 4.86.

The pull -down menus of the software makes thePC82 one of the easiest and most user-friendlyprogrammers available. A full library of fileconversion utilities is supplied as standard.

The frequent software updates provided bySunshine enables the customer to immediatelyprogram newly released ICs. It even supportsEPROMs to 16Mbit.

Over 20 engineers are employed by Sunshine todevelop new software and hardware for thePC82. Not many competitors can boast of similarsupport!

Citadel, a 32 year old company are the UK agentsand service centre for the Sunshine range ofprogrammers, testers and in circuit emulatorsand have a team of engineers trained to givelocal support in Europe.

* More sold worldwide than any other ofits type.

* UK users include BT, IBM, MOD, THORNEMI, MOTOROLA, SANYO, RACAL

* High quality Textool or Yamaichi zeroinsertion force sockets.

* Rugged screened cabling.* High speed PC interface card designed

for use with all PC models from XT to486.

* Over 1500 different devices (includingmore than 100 MPU's) supported.Tests and or identifies a wide range oflogic devices.

NOW SUPPLIED WITH SPECIAL VALUEADDED SOFTWARE (worth over £300 ifbought seperately):

* MICROTEC disassemblers for Z8, 8085,8048. 8051, 6809 & 68HC11

* Software supplied to write own testvectors for custom ICs and ASICs etc.

Protection circuitry to protect againstwrong insertion of devices.

Ground control circuitry using relayswitching.

* One model covers the widest range ofdevices, at the lowest cost.

* No need to tie up a slow parallel port.

Two year free software update.

* Speed optimised range ofprogramming algorithms.

* NATIONAL SEMICONDUCTOR OPALjrPAL/PLD development software.

* BATCH SOFTWARE for productionprogramming.

Our stocked range of own manufactured andimported Sunshine products include:

* Super fast EPROM Erasers.* 1, 4 & 8 gang EPROM 8Mbit production

programmers.* Battery operated portable EPROM

programmers.* "In circuit" Emulators.* Handy pocket IC testers.

ORDERING INFORMATION

PC82 complete with interface card,cable, software and manual only L..W3

Please add £7 carriage (by overnight courier) forUK orders, £20 for export orders, and VAT whereapplicable.

ACCESS, MASTERCARD, VISA or CWO.Official orders are welcome from Governmentbodies & local authorities.

Free demo disk with device list available.

CPCITADEL PRODUCTS LTDDEPT. WW, 50 HIGH ST.,EDGWARE, MIDDX. HA8 7EP.

Phone now on: 081 951 1848/9

tair11110

VISA

CONTENTS

FEATURES

VOICE LINK OVER SPREADSPECTRUM RADIO 716Spread spectrum techniques inGPS, and now cellular phones, willbe followed by many other civilapplications. James Vincentexamines the technology bydescribing an experimental directsequence voice transmissiolsystem.

AUDIO INDUCTION TECHNOLOGYFOR THE DEAFMagnetic induction loops provide an electroniccommunication channel for deaf people. J P Wilson detailsthe essential design data for induction audio systems.

723

DOWNWARD TREND IS GOODNEWS FOR DESIGNERS 727Designers are being forced to rethink their device supplylevels. Rupert Baines reports on an industry in transition.

DISTORTION IN POWERAMPLIFIERS: THE INPUT STAGE 730Input stage design is not something to be taken lightly forminimal amplifier distortion. Even the fine details ofcurrent distribution at this point can have a surprisinglypowerful effect on distortion. By Douglas Self.

USING MOMENTUM TODETHRONE EINSTEIN 738John Ferguson sends forth an iconoclastic Mechanistictheory into the searing light of the relativistic world.

CAN TEACHERS WAVE BYEBYE TO THE LAB BENCH? 744Electronics Workbench could be ideal for colleges wantingto try electronic simulation and schematic capture - if theycan afford the hardware to run it, says John Anderson.

THE REAL CHOICE FORACTIVE FILTERS 758Op -amp parameters can have a drastic effect on filterresponse. Steve Winder analyses op -amp characteristics.

PROFESSIONAL SERVICES OFFERMicroprocessor Pocket Library: Newnes Microprocessor PocketBook, Newnes Z80 Pocket Book, Newnes 8086 Pocket Bookand Newnes 68000 Pocket Book. Normal price £5.50. Special

ice to EW + WO' readers £37.50 inc P&P. See reader reply

THE SECRET RADIO THATKEPT RESISTANCE LIFELINES OPEN 772Good communication and reliable parachute drops ofsupplies were vital for coordinating Resistance fighters inoccupied Europe. Charles Bovill describes how the S -Phone met both needs - and the enemy never knew.

REGULARS

COMMENT 707Putting pen to paper

UPDATE 708FM stations may close in spectrum shake-up. Mercury tolaunch new mobile phone service, Apple cuts to Newtonlicence, Signal set to "go" for DLR, Woolies pick 'n' mix.

RESEARCH NOTES 713Gate opens for molecular logic, Soft electrons for asmoother etch, Magnetic fields that upset the brain, STMputs electrons in the picture, Computer has designs on itsown patent rights, Optical switch that needs no power.

LETTERS 747Nervous energy, Mortals with golden ears, CFA deathexaggerated, Bring on the de-jaggers, Getting the wind up,Pinching electrons in the ether, Catt's Difficulty, Beware!Danger from falling satellites, Einstein has the last laugh.

CIRCUIT IDEAS 756Gyrator acts as electronc choke, High -power class -Aamplifier, Pulse width detector, Tweaking a D -to -Aconverter, Auto -reverse motor control, Line rampgenerator.

UTILITIES 761About to design a linear regulator circuit or amplifierpower pack? Take a look at A M Wilkes' utility first.

DESIGN BRIEF 763The MCT combines the versatility of a transistor, the lowforward conduction losses of a thyristor and the gatecharacteristics of a mosfet. Ian Hickman puts a new type ofhigh power switching device into a test bed.

NEW PRODUCTS 767Round up of all that's new in electronics.

APPLICATIONS 778Fast 10 -bit converter with power -down mode, Pressuregauge with bar indicator. Slotted ferrites for non -contactcurrent measurement, Planar power inductors - lowerprofile and higher efficiency, GaAs tuner for VHF and UHF.

A GUIDE TO INTEGRATION 782Integration is fundamental to electronics. In this extractfrom his book Understand Electrical and ElectronicsMaths, Owen Bishop shows how to go about it.

In next month's issue: The top ten audio IC poweramplifier chips are put to the Ben Duncan test. Wepresent a detailed, independent assessment ofdistortion performance using the test equipmentnormally reserved for the best in hi-fi equipment.Also in this issue: Cold Fusion warms up.OCTOBER ISSUE IS ON SALE SEPTEMBER 30

September 1993 ELECTRONICS WORLD + WIRELESS WORLD 705

PORTABLE %APPLIANCE TESTING`,N

MEET THE MAN WHO DIDN'T

TALK TO SEAWARD ELECTRONICHealth & Safety requirementsencourage all organisations whichuse electrical appliances to

implement regular testing procedures.Keeping and maintaining regular recordsof every appliance in an organisationcan he a major headache.Now, for the first time, an instrument isavailable which not only simplifies thetesting process, but which also automaticallyrecords results in its memory - eliminatingall unnecessary paperwork.Seaward's award winning andcompetitively priced PAT1000 isthe world's first microprocessor controlledPortable Appliance Tester.

Simple to use, the PAT1000Portable Appliance Tester guides the userthrough the test sequences.

All test results are recorded in amicroprocessor memory enabling recallor printing out, giving the user evidencethat the correct tests have been performedat the right time on every appliance.

Provides protection to the user and theequipment with electronic interlocks.

Tests insulation, earth bonding, flashtests, load tests and earth leakage.Offers comprehensive test facilities forClass 1 and 2 appliances.Available for testing all appropriateAustralian and other Internationalstandards.

For further information contactEMONA ENTERPRISES PTY LTD, 86 Parramatta Road, P.O.Box 15, Camperdown, NSW 2050

For all other countries contactSEAWARD ELECTRONIC, Bracken I lill, South West Industrial Estate, Peterlee, Co. Durham, SRS 23J, England

CIRCLE NO. 105 ON REPLY CARD

SETTING

HIGHER

STANDARDS

COMMENT

EDITORFrank Ogden

081-652 3128

DEPUTY EDITORJonathan Campbell

081-652 8638

CONSULTANTDerek Rowe

DESIGN & PRODUCTIONAlan Kerr

EDITORIAL ADMINISTRATIONLorraine Spindler

081-652 3614

ADVERTISEMENT MANAGERCarol Nobbs

081-652 8327

SALES EXECUTIVESPat Bunce

081-652 8339

ADVERTISING PRODUCTIONShirley Lawrence

081-652 8659

PUBLISHERSusan Downey

EDITORIAL FACSIMILE081-652 8956

CLASSIFIED FACSIMILE081-652 8931

SUBSCRIPTION HOTLINE0622721666Quote ref INJ

SUBSCRIPTION QUERIES0444 445566

NEWSTRADE ENQUIRIESMartin Parr

071 261 5108

BACK ISSUESAvailable at £2.50

081-652 3614

41 RE Etkiii BUSIDNESSPUBLISHING

Putting pen to paperAs a freelance technical journalist Iam constantly invited to put thelatest technology to the test. It

strikes me that companies often pursuetechnology as much for its own sake thanfor fulfilling a genuine need. The computersector seems especially prone to this.

Take PDAs, for example. The PersonalCommunicators, or Personal DigitalAssistants, either available or coming fromApple, EO, Amstrad and Casio. Withoutdoubt Apple's Newton is a remarkablepiece of technology. It recognises cursivescript, which in plain English meansjoined -up writing. It plays a clever trick tomake users teach it the peculiarities of theirwriting, by challenging them to a game.

But there were glum faces at Apple inCupertino when I was asked for honestcomments and asked in reply for goodreasons why I would want to use a $700+PDA, rather than a 50p notepad and pen.Writing on the LCD screen is nowherenear as easy as on paper.

Because of the communications feature,came the reply. You can write notes on themove and send them electronically. Foranother $150 extra for the modem feature,Newton can send faxes or electronic mail.(Provided of course that you can find afriendly phone socket).

OK, I said, let's use Newton to log intoTelecom Gold, via Tymnet. No go.Newton has no terminal emulation. Thisshuts off the market for on -the -moverswho have spent the last ten years learningemail and Hayes AT commands, and havegrown one arm longer than the other byhumping round a keyboard PC with bigenough batteries to keep it running for

longer than a short -haul hop.Software houses appear just as indifferent

to real needs; new does not necessarilymean better. The thick manuals that comewith new software equate to a lostweekend. The latest version of IBM's OS/2comes on 25 disks, and sentences anyonewilling to try it to a whole new world ofunexpected compatibility problems whilelearning a new command vocabulary.

I would try Hayes' new version ofSmartcom if it came with a utility thatautomatically converts all the telephonedirectories and logon scripts from myexisting comms software.

I would upgrade to the warring Word orWordperfect if either had a simple utility toconvert a decade's worth of text filesstored in an old format, without the hassleof going through ascii export routines. Butinstead of writing in facilities that peopleactually want, the purveyors of this modernsoftware have spent their time on piling infancy features that most users will neverdiscover, let alone need, want or use. In sodoing they have missed the simple trick ofwinning new customers who are currentlytrapped into continuing with old software.

They miss the trick because, in their madrush to innovate, the innovators do not stopto think what people might actually want.They have lost sight of the purpose ofinnovation, and the object of the wholeexercise.

All of which prompts a thought. Perhapsthe best thing to come out of the recessionis the buying public's growing resistanceto things that are new for the sake of beingnew.

Barry Fox.

Electronics World + Wireless World is publshed monthly By post, current issue £2.25, back issues (if available) £2.50.Orders, payments and general correspondence to L333, Electronics World + Wireless World, Quadrant House, TheQuadrant, Sutton, Surrey SM2 5AS. Telex:892984 REED BP G Cheques should be made payable to Reed BusinessPublishing GroupNewstrade: IPC Marketforce, 071 261-5108Subscriptions: Quadrant Subscription Services, Oakfield House, Perrymount Road, Haywards Heath, Sussex RH16 3DH.Telephone 0444 441212. Please notify a change of address. Subscription rates 1 year (normal rate) £30 UK and £35 outside UK.USA: $116.00 airmail. Reed Business Publishing (USA), Subscriptions office, 205 E. 42nd Street, NY 10117.Overseas advertising agents: France and Belgium: Pierre Mussard, 18-20 Place de la Madeleine,Paris 75008. United Statesof America: Ray Barnes, Reed Business Publishing Ltd, 205 E. 42nd Street, NY 10117.Telephone (212) 867-2080. Telex23827.USA mailing agents: Mercury Airfreight International Ltd Inc, 10(b) Englehard Ave, Avenel NJ 07001. 2nd class postage paidat Rahway NJ Postmaster. Send address changes to above.Printed by BPCC Magazines (Carlisle) Ltd. Newtown Trading Estate, Carlisle, Cumbria, CA2 7NRTypeset by Marlin Graphics 2-4 Powerscroft Road, Sidcup, Kent DA14 SDT©Reed Business Publishing Ltd 1992 ISSN 0959 8332


UPDATE

FM stations may close inspectrum shake-up

!MmallIMIFf

Band

I

Band

11

VHFHigh

i Band

NATO BandIV

28 47 68 87.5 108 136

30MHz

FM broadcasting by BBC andcommercial stations could comevirtually to a stop in little more than 15

years, if plans to supersede it with digitaltechnology are put into effect. The proposalis expected to be among therecommendations of Sir Colin Fielding'sindependent spectrum review committee,which is preparing an assessment of presentand future use of the 28-470MHz spectrum.

If the Government follows its practicewith previous spectrum reviews, it willaccept and implement most of thecommittee's conclusions.

Terrestrial digital audio broadcasting. T -DAB, could open in a temporary parkingband in about two years. But the committeeproposes a strict timetable for rehousing it inBand II: from 2007 onwards, national and

173 225

300MHz

regional FM transmitters would close tomake room, leaving only a few communitystations on FM. The remainder of Band IIwould be occupied by ancillary servicessuch as programme links and talkback.

Broadcasting was just one of the areas ofspectrum activity discussed by committeemembers when they presented theiremerging conclusions at a seminar inLondon. A major problem facing them islack of time: the CEPT is beginning its ownEuro-study of the VHF and UHF bands, andthe DTI wants to have the UK report readyas input for it.

Specific issues on which the committeehas focused include finding a suitabletemporary band for T -DAB; identifyingmilitary frequencies which can be releasedto meet demand from civilian users;

Signal set to "go" for DLRWhen the Docklands Light RailwaysBeckton extension opens inOctober, it will be controlled by the

most advanced signalling system curerentlyavailable. It replaces the traditional blockworking - admitting one train at a time to a

section of track - with a software -controlled"moving block" system which maintains adistance, variable according to speed, betweentrains. When extended to the rest of the railwayit will increase capacity fourfold.

The new system, Seltrac, designed byCandian firm Alcatel, is part of an £800mupgrade designed to rid the the DLR of thestigma of failure and ridicule which has doggedit - built on a £77m shoestring in 1985 - sincedevelopment in the area overtook capacity.Reliability fell to an ignominious 66% in 1991.

Signalling is something of a misnomer, sincethe computer controls the driverless trains like alife-size model railway, commiunicating withtheir on -board computers through inductionloops laid between the rails. An independentcomputer system keeps track of train locationsby counting axle revolutions. This device hasbeen introduced to prevent a repetition ofSeltrac's worst moment, when the VancouverSky Train system went down. All trains had to bemanually driven to known points - an operationlasting four hours - before it could be restarted.

400 470MHz

Services in the band under review includebroadcasting, aviation, business radio networks,

the emergency services -a bit of almosteverything other than television and cellular

telephones

relieving the overcrowding of business radioin the private mobile radio (PMR) bands,especially in London (described by acommittee member as "a disaster area"): anddeciding how the current regulatory regimemight be improved.

Of the 800,000 or so mobile radio users,77% of licences were for systems with 10mobiles or fewer. Radio is vital to thebusiness of these smaller users, who mightbe unwilling to move to shared (trunked)systems if it meant losing control of theircommunications. But to what extent wouldthey shift to public networks over the next10 to 15 years as services such as cellularand the PCNs became cheaper?

The role of the RadiocommunicationsAgency had attracted adverse criticism, themost damning of all from the RA's mainconsultative body on mobile radio, whichaccused it of "lack of strategic direction,lack of openness, regulatory culture, lack ofavailable data and inappropriateperformance measures."

Sir Colin Field said at a seminar in Julythat there was clearly a case for having asingle spectrum management authority forthe UK, for which the RA had some of thecredentials. However, there ought to beindependent oversight of spectrummanagement, with wide consultation in theradio community.

One major problem is lack of time: theCEPT is beginning its own Euro-study ofthe VHF and UHF bands, and the DTI wantsto have the UK report ready as input for it.

Virtually every user of the band would likemore channels, even the armed services,which, despite the ending of the cold war,are sitting on no less than 55% of the band.The Ministry of Defence view is that theworld is still a turbulent place and thatBritish forces are needed to maintain a highstate of readiness.

708 ELECTRONICS WORLD + WIRELESS WORLD September 1993

UPDATE

More than half of the 28-470MHzregion is occupied by the armedservices: is it time now for apeace dividend?

Gi .iorirnent 55.1%

iCml aviation 6.2%

Amateur 1.7%.

Land mobile 13.9%

Miscellaneous 1.5%

[lInusable 2.3%[-Other 4.5%

z Passive 0.9%

jFMG 3.6%Independent radio 1.4°%_

BBC 26%

' Emergency services 6.1%

No major concession would be possiblewithout the joint agreement of the Natopowers, since much of the military spectrumwas under the control of Nato HQ inBelgium. However, there is room formanoeuvre, for example, many ancillaryradio systems are similar to civilian PMRinstallations. Sharing of frequencies couldwell be possible since civilian users wereconcentrated in the towns while militaryactivity was mainly in the country.

This may go some way towards soothingthe civil mobile radio community, who,because of the complexity of theirrequirements and their currentovercrowding, present the committee withone of its toughest jobs.

One block of channels Nato seemsprepared to concede is in the 380 to400MHz region. This is under considerationas a home for Tetra, -a new Europeandigital trunked radio technology and theHome Office's choice for future emergencyservices networks - and as a means ofsolving a long-standing and troublesomecontinental interference problem.

This nuisance, which affects users as muchas 100 miles inland, is due to a slip-up yearsago by which split -frequency duplex UHFmobile networks in England and Wales weremisaligned with Europe. Thus, police patrolscalling their control rooms are competing forattention not with faint continental mobilesbut with well -sited, high-powered,continental base stations.

The consultants hired by theRadiocommunications Agency concludedthat the only certain solution was to reversethe entire band - with immense cost anddisruption. But, given frequencies in whichto resettle existing users, the Home Officecould at last make a start.

Richard Lambley. Mobile and Cellular

The Radio Spectrum Review Secretariatcan he contacted at theRadiocommunications Agency, Room 506,Waterloo Bridge House, Waterloo Road,London SEI 8UA. Telephone 071-2/5 2157:fax 071-2/5 0992.

Mercury to launch newmobile phone service

Mercury, British Telecom's mainrival in the Britishtelecommunications market, is now

embarking on its biggest gamble yet.Mercury is launching a cellular phoneservice, called One -2 -One, to rival theestablished networks, BT's Cellnet andVodafone. Although the potential rewardsare high, Mercury is in uncharted waters.

Mercury is allowed by law to sell itsservice direct to subscribers, whereasCellnet and Vodafone must sell through athird party layer of "service providers". Thisgives Mercury a price advantage. But it willbe short-lived because the DTI plans tochange the rules for Cellnet and Vodafone.Also, while Mercury has to build acompletely new network of base stations,Cellnet and Vodafone already have thettiand can easily slash prices to undercut One -2 -One.

At the same time Mercury faces acompletely new set of technical problems,never before faced by any cellphoneoperator. Whereas the existing services useanalogue technology to carry the speechsignals, Mercury's One -2 -One service willuse new all -digital technology. By unhappycoincidence Mercury's launch comes just asthe main cellular operator in the US,Ameritech, announces the results of a longterm trial which has convinced Ameritechthat digital technology is not yet ready tooffer the public.

One -2-0112's technology was born fromthe mess of .ncompatibility between theexisting cellphone services in Europe.

Cellnet and Vodafone launched their UKservices in January 1985, using the TotalAccess Communications System. TACS wasbased on technology developed in the US.Although the control signals which set upcalls, and switch them from cell to cell, aredigital, the speech is analogue. All the otherservices in Europe are analogue, but differfrom country to country.

In the mid eighties all Europeangovernments agreed on a new pan-Europeanstandard, called Global System for Mobilecommunications. GSM is all -digital. Speechis converted into digital code beforetransmission. Eight speech channels are thensqueezed into one transmission channelusing a technique called Time DivisionMultiple Access. TDMA relies on thenatural spaces between words of humanspeech. Each digitally coded conversation ischopped into short bursts, and the codebursts interleaved. The receiver stitchesthem together again.

Both Cellnet and Vodafone are obliged, by

European Memorandum of Understanding,to co-operate in providing a pan-EuropeanGSM service, using frequencies (at around900MHz) reserved for GSM in all countries.This will eventually let travellers use theircellphones anywhere in Europe. The serviceis already behind schedule, because GSMcellphones are heavier and more expensivethan analogue phones, and consume morebattery power.

Mercury's new service will use GSMtechnology, but at a higher frequency(around 1800 MHz). It realises a dreamenjoyed by Lord Young in January 1989while Secretary of State for Trade andIndustry. In his White Paper, Phones on theMore, Lord Young proposed a new PersonalCommunication Network of small wirelessphones, providing an "office in the pocket",with freedom from wires.

The DTI granted three licences to run PCNservices, to Mercury, and two otherconsortia, Microtel and Unitel. So far onlyMercury has pursued the dream, aftermerging with Unitel. Instead of spending thebillion pounds needed to build base stationsall round the UK, Mercury is cutting costs toa third by offering a service only within theM25 ring, with around 300 base stationsites. Mercury will then push slightlyoutside the M25, to cover 24% of thepopulation by next April.

GSM/PCN technology uses a clever trickto limit power drain, and so let a phone worklonger on each battery charge. When thephone sets up a call it automatically tests thestrength of the signal coming from thenearest base station, and then adjusts thestrength of the signal it transmits to the basestation to the lowest level for reliablecommunication.

Since May, Mercury's engineers havingbeen touring the M25 coverage area, testingthe consistency of signal strength from thebase stations. Mercury now feels confidentto launch, but dares advertise only forbusiness customers because it does not yethave enough phones from suppliersMotorola, Siemens and NEC to meet thedemand which a consumer campaign mightgenerate.

GSM/PCN uses smart cards (credit cardswith built-in computer chips). As sold, thephones are useless. They have a slot for acard which makes them work. This lets highstreet stores sell the phones like hi-fi orvideo.

As the first company to introduce a highprofile digital cellphone service, Mercuryknows it must cope with questions about theelectrical interference which digital phones


UPDATE

will produce, with new and thus potentiallypuzzling results for sufferers. The squarewave pulses that convey the digitally codedspeech inevitably generate high frequencyharmonics. These will break through into thecircuits of a nearby tv set, hearing aid or carradio, to create mysterious buzzing noises orblips on the picture.

Nokia of Finland, Europe's leader inGSM, has studied the problem and foundthat it will be worst indoors or in a car,where the phone is partially shielded fromthe base station and must thus work at highpower to communicate. The interferencewill also be worst whenever the phone isswitched on, and goes through the automaticprocedure of testing local signal strengths.

Nokia's engineers have also tested digitalphones with car air bag safety devices,following scare stories in Scandinavia ofbags triggering by interference from digitalphones. The scare has been hardened bywarnings issued by car firms not to usedigital phones in cars with air bags."Extremely unlikely" says Nokia, whichmakes air bag electronics as well as digitalphones."So far it is all just supposition". Totry and dispel rumours, Mercury is nowrunning tests with the British carmanufacturers.

Both Cellnet and Vodafone already havecontingency plans to cut prices if One -2 -Onelooks likely to be a commercial success.Mercury's publicity will in reply remindpotential customers for One -2 -One that theircalls will be completely secure against theeavesdropping which is so easy withanalogue phones. This will also createproblems for Mercury.

Because PCN uses the same technology asGSM, it uses the same encryption system asGSM. This is so secure that the Britishgovernment has banned its export to mostcountries outside Europe, for fear of givingunfriendly armies the chance to use it formilitary communication. The USgovernment has rejected it, for fear of givingterrorists, criminals and drug dealers thechance to talk without being overheard bythe police and security services. Mercurycan thus expect to be criticised for offering aservice which is ideal for the underworld.

The digital system proposed for use in theUS uses much weaker encryption, but relieson the same TDMA technology. Leadingcellphone operator Ameritech now says thatTDMA is not yet ready for commercial use.The company loaned TDMA phones tocustomers, provided a trial service and askedfor comments. Over 40% were not satisfiedwith performance. Mercury argues thatalthough both the US and Europeantechnologies use TDMA, Europe'sGSM/PCN technology is better and readyfor use. Barry Fox

Woolies pick 'n' mixMultimedia inretailing parlanceusually means

nothing more exciting thanan electronic catalogue, buta more rewarding interactivesystem, now on trial in ahandful of Woolworthsstores, uses digital video andaudio to allow customers tosample movie and CD titles.

Now being evaluated in ahandful of Woolworthsstores, it is claimed to be thefirst public -accessmultimedia system in theUK High Steet, and the firstfully -digitised hard -disksystem in the world.

The touch -screen systemhas been described by aWoolworths manager as"easier to use than a hank'scash machine - and morefun." The same cannot be

said of its appearance,which calls to mind anarcade game designed by theEast German government. Itis eventually planned to holddetails of every CD, tapeand video produced inBritain. Titles not in stock inthe store (the vast majority,presumably) can be orderedfor mailing to home.

The system - whichrecently won a Silver Awardfrom the BritishInternational MultimediaAssociation was designedfor Woolworths by JuliaSchofield Consultants, asmall (18 staff) outfit basedin Richmond -on -Thameswhich specialises ininteractive systems.

Some 16,000 titles arestored, with clips for about aquarter of them. Part- screen

full -motion video clips,entered using a VideoLogicMediaSpace card, lastaround 15 seconds, inaddition to a 4.5 -minuteintro sequence.

Based on a 486 PC with32 megabytes of RAM and3 gigabytes of disk storage,the system uses aVideoLogic MediaSpacecard.

Apple cuts to Newton licenceThe news in July that Apple was sacking2500 of its 16,000 employeesworldwide, contained a coded message.

The one division spared the axe is the divisionwhich is ploughing a new furrow on licensingpolicy, and thereby proving that Apple nowaccepts that its current predicament is bornfrom past strategic blunders. SignificantlyApple clammed up, ducking the simplequestion of how many people work in itsPersonal Interactive Electronics division, soonafter letting it slip that PIE would be untouchedby the cuts.

Officially Apple blames its job cuts on thebitter price war between Apple's Macintoshrange and the wide variety of rival IBM -formatmodels which have dropped in price by 40%over the last year. But there is now widespreadrecognition inside Apple's workforce that theyare paying the price for a basic error made bytheir company ten years ago.

When Apple launched the Mac in 1984 itrefused to licence the technology which madethe computer so easy to use. Apple hascontinued to refuse Mac licences every since.This has created the competition from IBM -format personal computers which is nowcrippling Apple.

Apple has now changed its policy and willlicence others to use the new technology forNewton, the new hand-held personalcommunicator which works with a pen andpressure -sensitive screen instead of a keyboard.PIE developed Newton and is actively licensingit to third parties.

In the early 80s Apple developed a computercalled the Lisa which was very easy to use, andthen refined it into the Mac. When Applerefused even to consider licensing thetechnology, IBM chose another US softwarecompany, Microsoft, to provide the controlsoftware or "operating system". Microsoftdeveloped the MS-DOS opeating system whichwas very awkward to use, and has spent the lastten yars perfecting Windows, a refinementwhich now makes an IBM-PC look and feellike an Apple Mac. PIE has already licensedfour companies, Motorola, Siemens, Sharp andMatsushita (under its brand name Panasonic) touse the Newton technology and make rivalmodel communicators which adhere to theNewton standard. PIE is now talking with otherelectronics companies in Europe and Japan,including Philips and Sony, in an effort makeNewton a new de facto standard. Apple willthen collect royalties under patents, softwarecopyright and a trade mark logo depicting asylised light bulb and the word Newton.

Licences will not be required to use the Applelogo, though.

"This is a completely new experience for us"says Subra Iyer, in charge of licencing Newton."In the past we had complete control of allhardware and software. But the old way wasproven wrong. Now we have to find a new wayof ensuring that anything with the Newton trademark is compatible with anything else,regardless of who makes it and where. But wedon't want licencing control to be a bottleneck".


I

I

EASY FAST & POWERFULCAD SOFTWARE THAT GIVES YOU THE EDGE

ISIS - SCHEMATIC CAPTURE

Easy to use yet extremelypowerful schematic entry systemwith all the features you need tocreate input for ARES or otherCAD software. Now available in asuper -fast 32 bit version capableof handling huge designs even onAO -sized sheets.

Graphical User Interface givesexceptional ease of use - two mouseclicks will place & route a wire.

Automatic wire routing, dotplacement, label generation.

2D drawing capability with symbollibrary.

Comprehensive device libraries. Heterogeneous devices (e.g. relay

and coil) allowed in different placeson the schematic.

Special support for connector pinsput each pin just where you want it.

Output to printers, plotters,Postscript.

Export designs to DTP and WPpackages.Netlist formats for most popular PCB& simulation software.Bill of Materials and Electrical RulesCheck reports.Multi -sheet and hierarchical designsupport.

Automatic annotation/packaging. ASCII data import database facility

from

£275

ARES - PCB DESIGN

Advanced netlist based PCBlayou- software newly updatedto version 2.5. Major newfeatures include SMT ibrary,real time snap (for those trickySMT spacings), thermal reliefpowe- planes and eni-ancedautorouting.

Graphical User Interface.a Real tine snap. Auto track necking. Curved, 45/90 or any angle :racks Extensive through hole and SMT

package libraries as standard.le 2D &awing capability with symbol

library. Conr ectivity highlight. Output to printers, plotters,

Postscript, Gerber and NC trill. Gerber View facility. Graphics export for DTP etc Advanced netlist management with

forwerc design modification. Corn J e n t renumber and

back annotate to ISIS.Full physical and electrical designrule checks.

Autoiouter handles single, dpuble ormulti -layer boards.

Power plane generator with -eliefs. Strategy & DRC information

loadable from ISIS. Gerber import utility available.

from

£275

cocen.Electronics

ISIS ILLUSTRATOR

Sc-iematic capture for MSWidows 3.1 - produces highquality schematics like you seein the magazines with yourchoice of line thicknesses, fillsty es, fonts, colours etc. Onceentered, drawings can becopied to most Windowssoftware through the clipboard.

At9w version 2 includesnetlisting, bill of materials,hierarchy, and much more.A.1 ideal front end forWindows Simulation andP38 design.

new low prices

£99from

w

CACPAK - BUDGET PRICE CAD

TwoSU °ERSKETCHfor

CADPAKneeddiagramsPCto use.

superb

programs

he price of

hasto produce

andand is exceptionally

value

£79

- ISISand PCB II -

one.

everything youcircuit

PCBs on youreasy

at only

Cell us today on 0274 542868 orfax 9274 481078 for a demo pack.

Combination, multi -copy and edwationaldiscounts available. Prices exc P&P and VAT

14 Marriner's Drive. Bradford, BD9 4JT.

CIRCLE %O. 101, ON REPLY CARD

MICROWAVE CONTROL PANEL. Mains operated, with touchswitches. Complete with 4 digit display, digital clock. and 2 relayoutputs one for power and one for pulsed power (programmable).Ideal for all sorts of precision timer applications etc. Now only E4.00ref 4P151. Good experimenters board.FIBRE OPTIC CABLE Stranded optical fibres sheathed in blackPVC. Five metre length £7.00 ref 7P29R or E2 a metre.12V SOLAR CELL. 200mA output ideal for trickle charging etc. 300mm square. Our price £15.00 ref 15P42R. Gives upto 15v.PASSIVE INFRA -RED MOTION SENSOR.Complete with daylight sensor, adjustable lights ontimer (8 secs -15 mins), 50' range with a 90 degcoverage. Manual ovende facility. Complete with -wall brackets, bulb holders etc Brand new and "guaranteed. Now only E19.00 ref 19P29Pack of two PAR38 bulbs for above unit £12.00 ref12P43RVIDEO SENDER UNIT. Transmit both audio and video signalsfrom either a video camera, video recorder or computer to anystandard TV set within a 100' range! (tune TV to a spare channel).12v DC op. £15.00 ref 15P39R Suitable mains adaptor £5.00 ref5P 191R. Turn your camcorder into a cordless camera!FM TRANSMITTER Housed in a standard working 13A adapter(bug is mains driven). E26.00 ref 26P2R. Good range.MINAT LIRE RADIO TRANSCEIVERS. A pair of walkietalkies with a range of up to 2 kilometres. Units measure

,

22x52x155mm. Complete w ith cases a nd earpieces. £30.00ref 30P12RFM CORDLESS MICROPHONE. Small hand held unitwith a 500' range! 2 transmit power levels. Reqs PP3 battery.Tuneable to any FM receiver. Our price £15 ref 15P42AR.12 BAND COMMUNICATIONS RECEIVER. 9 short bands.FM, AM and LW DX/local switch, tuning 'eye' mains or batteryComplete with shoulder strap and mains lead. £19 ref 19P 14R.Ideal for listening all over the world.CAR STEREO AND FM RADIO. Low cost stereo system gi5 watts per channel. Signal to noise ratio better than 45db,wow and flutter less than .35%. Neg earth. E19.00ref 19P30LOW COST WALIKIE TALKIES. Pair of battery operatedunits with a range of about 200'. Our price £8.00 a pair ref8P50R. Ideal for garden use or as an educational toy.

11E1

7 CHANNEL GRAPHIC EQUALIZER plus a 60 watt power amp!20-21KHZ 4-8R 12-14v DC negative earth. Cased. £25 ref 25P14R.NICAD BATTERIES. Brand new tap quality. 4 x AA's £4.00 ref4P44R. 2 x Cs E4.00 ref 4P73R, 4 x D's E9.00 ref 9P 12R, 1 x PP3£6.00 ref 6P35R Pack of 10 AAA,s £4.00 ref 4P92R.TOWERS INTERNATIONAL TRANSISTOR SELECTORGUIDE. The ultimate equivalents book. New ed. E20.00 ref 20P32R.GEIGER COUNTER KIT. Complete with tube, PCB and all compo-nents to build a battery operated geiger counter. £39.00 ref 39P1RFM BUG KIT. New design with PCB embedded coil. Transmits toany FM radio. 9v battery recfd. £5.00 ref 5P158R. 35mm square.FM BUG Built and tested superior 9v operation £14.00 ref 14P3RCOMPOSITE VIDEO KITS. These convert composite video intoseparate H sync, V sync and video. 12v DC. E800 ref 8P39R.SINCLAIR CS MOTORS. 12v 29A (full load) 3300 rpm 6N4' 1/4'0/P shaft. New. E22.00 ref 20P22R. Limited stocks.As above but with fitted 4 to 1 inline reduction box (800rpm) andtoothed nylon belt drive cog £45.00 ref 40P8R. 800 rpm.ELECTRONIC SPEED CONTROL KIT .for C5 motor. PCB and allcomponents to build a speed controller (0-95% of speed). Usespulse width modulation. £17.00 ref 17P3R. Potentiometer control.SOLAR POWERED NICAD CHARGER. Charges 4 AAnicads in 8 hours. Brand new and cased £6.00 ref 6P3R.2xCcell model E6.00.ACORN DATA RECORDER ALF503. Made for BBCcomputer but suitable for others. Includes mains adapter, leads andbook. £15.00 ref 15P43RVIDEO TAPES. Three hour superior quality tapes made underlicence from the famous JVC company. Pack of 10 tapes New lowprice £15.00 ref J15P4PHILIPS LASER.2MW HELIUM NEON LASER TUBE. BRANDNEW FULL SPEC E40.00 REF 40P1OR. MAINS POWERSUPPLY KIT £20.00 REF 20P33R READY BUILT ANDTESTED LASER IN ONE CASE E75.00 REF 75P4R.12 TO 220V INVERTER KIT. As supplied it will handle up to about15w at 220v but with a la rge r tra nsfonner it will handle 80 watts. Basickit E12.00 ref 12P17R. Larger transformer £12.00 ref 12P41R.WIND UP SOLAR POWERED RADIO! FM/AM Radio takesrechargeable batteries. Complete with hand charger and solarpanel. £14.00 REF 14P200RA.BARGAIN NICADS AAA SIZE 200MAH 1.2V PACK OF 10£4.00 REF 4P92R, PACK OF 100 £30.00 REF 30P16R.FRESNEL MAGNIFYING LENS 83 x 52mm E1.00 ref BD827R.12V 19A TRANSFORMER Ex equipment £20 but OKPOWER SUPPLIES Made for the Spectrum plus 3 give +5 @2A, +12 ©700mA & -12 @I 50mA. £8 ref Q8P3UNIVERSAL BATTERY CHARGER. Takes AA's, Cs, Ds andPP3 nicads. Holds up to 5 batteries at once. New and cased, mainsoperated. £6.00 ref 6P36R.IN CAR POWER SUPPLY. Plugs into cigar socket and gives3.4,5,6,7.5.9. and 12v outputs at 800mA. Complete with universalspider plug. E5.00 ref 5P167R.QUICK CUPPA? 12v immersion heater with lead and cigar lighterplug E3.00 ref 3P92R. Ideal for tea on the move!LED PACK . 50 red. 50 green, 50 yellow all 5mm £8.00 ref 8P52

MINIMUM GOODS ORDER £5.00. TRADE ORDERSFROM GOVERNMENT, SCHOOLS, UNIVERSITIES& LOCAL AUTHORITIES WELCOME. ALL GOODSSUPPLIED SUBJECT TO OUR CONDITIONS OFSALE AND UNLESS OTHERWISE STATEDGUARANTEED FOR 30 DAYS.RIGHTS RESERVED TO CHANGE PRICES &

SPECIFICATIONS WITHOUT PRIOR NOTICE.ORDERS SUBJECT TO STOCK. QUOTATIONSWILLINGLY GIVEN FOR QUANTITIES HIGHERTHAN THOSE STATED.

THIS MONTHSSPECIAL OFFERS

AMSTRAD MfP3 #19.00VHF/UHF TV Receiver,converts RGB orcomposite monitor into colour TV. Brand newand cased.#19.00 each. REF: EW19P1

EMERGENCY LIGHTING #5.00Fully cased units with two swivel lamps mountedon the top. Just add a 6V sealed lead acid batteryand you have a complete automatic system.Gives about 3 hours of light as soon as the mainsfail.Our Price #5.00. REF: EW5P1

NS CABLE WITH PLUG fro30p eachYou can have a 2 Mtr length of mains cable with afitted 13A plug (with sleeved pins) for just 30p!Minimum quantity: PKT 10 #3.00 REF: EWor you can have PKT 100 #27.00 REF 27P2

CORD S (non working)#30.00 for 3These are BT Curlews. They are not new andhave been stored in sunlight so the plastic casesare discoloured. They also have handset aerialmissing. This is a standard telescopic aerial -piece of wire will do! These are NONRETURNABLE.Our Price PKT 3 for #30.00. REF: EW3OPI

286 PC PACKAGEyou get

286 PC with lmb RAMMONO MONITOR102 KEYBOARD

1 FLOPPY DRIVE40MB HARD DRIVE

DESKTOP CASE

COMPLETE SYSTEMREADY TO GO

OUR PRICE JUST

£299.00 !

ANSWER PHONES from only#18.00,Yes its true BT approved push button dialling.These are customer returned units and have twofaults. We will supply you with a chip to cure oneof the faults then you have to sort out the otherproblem! NON RETURNABLE.Price #18.00 each REF: EW18P2or #60.00 PKT 4 REF: EW60P2

ANSWER PHONES - HIGH SPECLCD DISPLAYOnce again, BT approved push button diallinganswerphones. LCD display, digital speech chip.fast rewind, memories plus all the features foundon a BT Response 400 machine. Again, we willsupply you with a chip to cure one of the faultsthen you have to sort out the other problem!NON -RETURNABLE.Price #28.00 each REF: EW28P2or #100.00 PKT 4 REF: EW100P1

POWER SUPPLIES for ANSWERPHONES aboveIf you need a DC plug in power supply for theabove answer phones they are #4.00 each.Price #4.00 each REF: EW4P5

SOME OF OUR PRODUCTS MAY BE UNLICENSABLE IN THE UK

BULL ELECTRICAL250 PORTLAND ROAD HOVE SUSSEX

BN3 5QTMAIL ORDER TERMS: CASH PO OR CHEQUEWITH ORDER PLUS e3.00 POST PLUS VAT.

PLEASE ALLOW 7 -10 DAYS FOR DELIVERYTELEPHONE ORDERS

TEL: 0273 203500FAX: 0273 323077 - "

PC CASES Desktop case +psu E51.60 ref BPCC1, Deluxe slimlinecase +psu 0000 ref BPCC2, Minitower case +psu E51.60 refBPCC3, Deluxe midi case +psu £90.00 ret BPCC4.MONITORS Mitac 14' SVGA .39DP £174 ref BPCM02, Mitac 14'SVGA .280P £202 ref BPCM01.MEMORY 256K Simm 7Ons £8.40 ref BPCMI 1, 1MB Simm 7Ons£26.40 ref BPCM12, 4MB Simm 7Ons £96 ref BPCMI3MICE 2 button serial mouse with 3.5' sAvare. £840 ref BPCMI6,3button serial mouse with 3.5' s/ware £9.60 ref BPCMI7.KEYBOARDS 102 AT UK standard keyboard £18.60 ref BPCMI4,Deluxe keyboard 102 AT UK £26.40 ref BPCM15.SOFTWARE MS DOS V5 OEM version. £39.60 ref BPCMI8, MSWINDOWS V3.1 OEM version. £42 ref BPCMI9.MOT HERBOARDS 286-16 Headland chip setE46.80 ref BPCMB1,386SX-33 Acer chipset £82.80 ref BPCMB2. 386SX-40 UMC with64K cache £110 ref BPCMB3, 486SX-25 UMC with 64k cache £191ref BPCMB4, 486DX-33 UMC with 256k cache £378 ref BPCMB5,486DX-66 UMC with 256k cache E515 ref BPCM B6.FLOPPY DRIVES 1.44mb 3.5' drive £32.34 ref BPCDD05, 1.2MB525' drive £38.40. 3.5' mounting kit £5 ref BPCDD07.HARD DRIVES 42MB IDE 17ms £99 ref BPCOD01. 89MB IDE16ms ref BPCDD02, 130MB IDE 15ms £215 ref BPCDD03, 213MBIDE 14ms £238 ref BPCDD04.VIDEO CARDS 256k C&T 8 bit SVGA card E19.20 ref BPCVC01,512k Trident 9000 16 bit SVGA card £31.20 ref BPCVCO2. 1MBTrident 8900 16 bit SVGA card £45 ref BPCVC03, 1MB CirrusAVGA3 167M colours £48 ref BPCVC04, 1MB Tseng multimediaE82.80 ref BPCVC05.ADD ON CARDS Multi1/0 card 2 serial, 1 parralel, 1 game, 2 floppy,2 IDE hard drives. E11 ref BPCA0001, ADLIB sound card withspeakers E37 ref BPCA00O2, Orchid sound card with speakers £63ref BPCA00O3.

EXAMPLES OF COMPLETE SYSTEMS386SX-33 SYSTEM386SX-33 board at £82.80, use £51.60, 2MB ram E52.80, 42MBdrive £99, 512SVGA card E31.20, FDD E32.34, multi I/O card£11 SVGA colour monitor £174. 102 k/board, £25 build fee if re-quired. Total £579.34486DX-33 SYSTEM486Dx-33 board £378, case £51.60, 2MB ram E52.80. 89MB driveE166, 512 SVGA card £31.20, 3.5' FDD E32.34, multi I/o card £11,SVGA monitor £174, 102 k/board £18.60, £25 build fee if required.Total £939.84.ALL PC PARTS AND SYSTEMS ARE GUARANTEED FOR 1YEAR PARTS AND LABOUR.

IN SUSSEX? CALL IN AND SEE US!

TURN YOUR SURPLUS STOCK INTOCASH. IMMEDIATE SETTLEMENT.WE WILL ALSO QUOTE FORCOMPLETE FACTORY CLEARANCE.

MUCH MUCH MORE IN OUR 1993CATALOGUE. PLEASE SEND 41P , A4SIZED SAE FOR YOUR FREE COPY.

CIRCLE NO. 107 ON REPLY CARD

RESEARCH NOTES

Gate opens for molecular logicAteam in the Department of Chemistry atQueen's University, Belfast, has

developed the world's first molecular AND gate.It is a derivative of anthracene and is able toproduce a photonic output in response to twoseparate ionic inputs.

Dr A P de Silva and his colleagues say(Nature, Vol 364, No 6432) that the gate wasdeveloped from a variety of single -inputmolecular switches known for some years.These are essentially molecules that fluoresce inthe visible part of the spectrum in the presenceof a particular ionic species, eg hydrogen ions.The "power supply" is usually a beam ofultraviolet light.

When powered by UV, the molecular gatenormally provides no visible light output. Noris there any output in the presence of justhydrogen ions (protons) or just sodium ions.But when both ions are present, the anthracene-based molecule will emit blue fluorescence. Atruly "wire -less" AND gate!

The molecule itself contains two recognitionsites, corresponding to the two inputs of asilicon AND gate. When both recognition sitesare satisfied, the output part of the molecule -a

Soft electrons fora smoother etch

An electron assisted etching techniquebeing developed may allow routinefabrication of nanometre scale

microelectronic devices without the surfacedamage caused by current etching systems.

GeorgiaTech researchers hope thatnanometre devices will fuel the next wave ofdevelopment in the microelectronics indus-try. The technology may also find use inelectro-optic devices, optical processing, andradiation devices.

Conventional ion -beam etching can dam-age surfaces altering optical and electronicproperties and potentially limiting how thedevices can be used. Because of their massand high kinetic energy, the ion particles candisrupt the crystalline structure of the semi-conductor surface and introduce unwantedmaterials.

fluorophore - generates the logical response.Dr de Silva emphasises that the AND gate and

other related molecules provide only arudimentary foundation for future molecularphotonic devices. A molecular computer is"well on the horizon, if not over it".Nevertheless, the fact that a useful logicfunction can be performed at the level of asingle molecule must offer considerableimpetus in this area. Experiments to implementa molecular OR function are already advanced.

Practical obstacles to the further developmentof molecular computing systems include theneed to make electrical interfaces withswitching devices - creating some form ofmolecular "wire". Alternatively it might bepossible to operate in the wet chemical domainas at present: much human brain logic clearlyoperates very satisfactorily in this way.

In the short term, de Silva and his co-workershave found some very practical applications fortheir molecular logic. Interestingly enough theygive a foretaste of how such devices couldinterface with the human body, either in thetrue thinking sense, or as biochemical sensorslooking for malfunctions.

The new process, however, uses low ener-gy electrons (10 to 500eV) with reactivehydrogen to cut the electronic featuresthrough the patterning process. Because the

F

F

ultr.ccmici 11.

121

ultraviolet

ultraviolcr

1 112 01

Chemical AND gate: Only when the two ions areboth present does the and molecule emit bluelight

De Silva envisages his AND gate being usedas an agent to spot the simultaneous presence oftwo ions that might, for example, be diagnosticof a particular illness. A sort of intelligentthermometer?

electrons are lighter and carry less energy,there is less damage to the surface.

Dr HP Gills, an associate professor work-ing on the project said: "If we can make thisprocess work commercially, it will helpenable the routine fabrication of these quan-tum scale devices."

He added: "The impact on the microelec-tronics industry is tied to the ultimate impactof these quantum well devices, which will bequite important in the future."

According to Gills, it is easy to disorderthe surface of a material so that it no longerfunctions properly as a transistor junction.

But one wony is the potential surfaceeffects from the reactive hydrogen.

Gills explained: "It remains to be seenwhether hydrogen has any detrimentaleffects in our process. But hydrogen isattractive because the chemistry is simplecompared to the species used in the conven-tional technique."

He estimated that there is at least two yearsof work to go before the research produces apractical process that can be used routinely.


RESEARCH NOTES

Magnetic fields thatupset the brain

Nerve activity in brains ofpatients with epilepsy has

been shown by an internationalteam of scientists to be triggeredby weak external magneticfields. And it is the presence ofmagnetic particles in the brainthat could be what makes theorgan sensitive to magnetism.

Investigations into the safetyof electromagnetic fields haverightly concentrated on theinteractions of AC fields withbiological tissue, whether in test -tubes or in people. If theamplitude is big enough and thefrequency high enough, an ACfield can cook: but the effects ofsmaller signals are much lessclear.

Static fields, whether electricor magnetic, have not beenaddressed to anything like thesame extent. Such fields are apermanent feature of the Earth'satmosphere; they rarely dropbelow 100V/m even in settledweather conditions. Animalstudies suggest that exposures ashigh as 340kV/m have no effecton physiology or reproductivecapability. The only adverseeffects on human health appearto be the micro -shocks than canresult from surface charges.

As for static magnetic fields,we are all constantly exposed tothe Earth's magnetic field,varying from 30-70oT over thesurface of the planet. Even largestatic fields of 1-2T seem to haveno adverse effects on health - atleast not in the short term. In areport published last year, theNational Radiological ProtectionBoard concluded that althoughstatic magnetic fields might haveeffects on biological enzymereactions, any healthimplications have yet to beestablished.

But the new twist to the storycomes with publication of aUS/Swiss study showing thatexternal magnetic fields cantrigger nerve activity in brains ofpatients with epilepsy. A teamfrom the University ofCalifornia, Santa Barbara, theInstitut ftir Geophysik in Zurichand the University Hospital inZurich have shown that fieldsonly 100 times stronger than theEarth's can trigger brain celldischarges associated withepileptic seizures. At a meetingof the American GeophysicalUnion, evidence was alsoproduced for the presence ofmagnetic particles in the same

8

7

6

4

a. 3"ift

2E

z 1

0-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10-4 Before application of field After application of field

Time (s)

Anomalous activity in the 10s periods prior to and following the fieldcharge. Random activity is represented by the incidence of events in the10s intervals prior to field applications.

region of the brain. Althoughthere is no proven link, theseparticles would be a plausiblemeans by which the magneticfield exerts its effect.

Not only do these discoveriesshed new light on the extent ofhuman sensitivity to magneticfields, they may also provide anew tool that neurosurgeonscould use in the treatment ofdrug -resistant epileptics. Beingable to induce epileptic firing toorder will make sit easier toidentify the precise area affected.That in turn would greatlyincrease the precision of anysubsequent surgery.

The question remains as towhether smaller fields, such asthose generated bycommonplace consumer devices

could trigger an epilepticseizure. The Swiss and US teamsay no. They argue that suchdevices generate fields about tentimes too small to bring aboutthe effect observed. Yet thereremains strong evidence fromanimal and human studies thatbiological systems can detectmagnetic fields as small as theEarth's - that may be howpigeons (and some humans) cannavigate.

Being able to detect somethingand being affected by it are twovery different things. But if abiological system can respond inany way at all to an externalstimulus, it must at least beworth asking a few morequestions about the healthimplications of regular exposure.

-rhe thought of being able to magnifyelectrons enough to see them is mind -

boggling in the extreme. But that in effect iswhat has been done by an IBM team at theirAlmaden Research Division in California.

The team has produced a computergenerated picture (Nature, Vol 363, No6429) showing waves and ripples, togetherwith what look like ranges of mountainpeaks and flights of steps. The interveningflat surfaces represent a "sea" of electrons

STM that puts electrons in the pictureflowing from one atom to another, theelectrons in the conduction bands of theatoms.

Don Eigler, one of the IBM team, saysthat the ripples on this surface are createdwhen electrons bounce off surface features.

The electron ripples - to give some idea ofthe scale - are about a thousand timessmaller than a single atom. They are also,without doubt, the smallest things that haveever been "seen". Better still, they provide adirect visual demonstration of the wavenature of the electron. IBM has long been atthe forefront of atomic imaging. It was anIBM team in Zurich who won the NobelPrize for their invention in 1981 of the toolthat made it all possible - the scanningtunnelling microscope or STM. This now -universal atomic probe works by suspendinga metal needle a few atomic diameters abovethe surface to be scanned. If a small potentiElis then applied between the two, a tiny

current tunnels its way across the gap, acurrent critically dependent on the spacing.

To make the STM into a practicalinstrument, the vertical drive of the needle isincorporated into a feedback loop that seeksto preserve a constant tunnelling current. Ifthe needle is then moved horizontally acrossthe surface of a sample, the feedback loopgenerates an error signal corresponding tothe surface topography. In practice thesedata are fed into a computer that generatesthe spectacular pictures we now see.

IBM says this apparently "blue -skies"research is essential because the ability toresolve quantum mechanical interferencepatterns of surface electrons is the mostpowerful means yet of studying defects onmetallic surfaces. Understanding suchdefects, which are important in regulatingfilm growth and surface catalysis, will be avital step in the development of newquantum devices, especially sensors.


RESEARCH NOTES

Computer has designs on its own patent rightsA computer program has submitted a patent applicat on forA some of its own inventions, according to the June .sate of

Chemistry In Britain. The program called Invention is named asthe primary applicant, along with its developer Todd Wipke of theUniversity of California, Santa Cruz.

Zany perhaps, but Invention has now designed a whc le series ofmorphine analogues that have the same overall chemical structureas the natural substance. The fact that none of these compoundsexists and none has yet been synthesised is immaterial: they couldwell form the basis of some of tomorrow's pharmaceutical drugs.

Invention took a whole day to churn out hundreds of morphine

look-alices and has chosen the three best ones for the. patentdisclosure. The University has yet to decide on whether to submita full patent application.

Wiplce says that in future, computer programs are far morelikely tkan human chemists to invent new chemicals. He believesthat, like their white -coated counterparts, computer programs aremore able to make inventive leaps and are less constrained bypreconceived ideas. More importantly still, they have no problemsvisualising complex three-dimensional chemical structures.

Invention will now be harnessed to the task of trying to think uppotential new AIDS drugs.

Optical switch that needs no powerNon Linear

Crystal

Probe

Pump

Optical mixer: non-linearcrystal produces angulardiffraction proportional tophase difference betweentwo or more input signals.

-4-- Photo -inducedgrating

Researchersin the Department

of Electrical and ElectronicEngineering at King's CollegeLondon has demonstrated an all -optical switch, at a recent summerexhibition of the Royal Society. Itis a novel method of routingoptical signals to differentdestinations without the need formechanical or electronicswitching. At the moment it isstill very much at the laboratorystage, but the new devicedemonstrates clearly the potentialfor locating optical data switchesaway from the main switchingcentres traditionally employed inelectronic data networks. JeremyEverard, one of the team, saysthere is no reason in principlewhy an optical routing switchcould not be located in the middleof the Atlantic.

Secret of the optical switch isthat it dispenses with thetraditional "three -terminal"approach; there is no externalsignal to control the switch.Instead, the switching signal iscoded in the data itself. So whena stream of data reaches theswitch, the switch is able torecognise where, of maybe tendifferent destinations, it is meantto go.

The switch itself relies on the

beam that is phase -coherent withthe input beam. The angle ofrouting is determined in practiceby superimposing on the inputsignal a slightly delayed versionof the same signal.

In the experimental system,demonstrated at the RoyalSociety, routing to two differentoutputs was achieved by creatingthe delayed signal using a secondset of mirrors. Jeremy Everard is

Experimental system, demonstrated at the Royal Society,switching to different outputs, by changing the phase betweentwo samples of a laser beam.

fact that when two laser beamsare applied to an optically non-linear crystal (barium titanate),they give rise to a three-dimensional diffraction grating.What happens then is complex.The "pump" beam is created, notby a second laser, but by aprocess of phase conjugationwithin the crystal. This, togetherwith the addition of semi -reflecting mirrors at varyingdistances from the non-linearcrystal enables the system toroute an input signal only to (andthrough) the mirror that reflects a

Movablemirror

confident that in a future practicalsystem, the destination addresscould be coded opto-_Semi -reflecting mirrors, atdifferent optical distances fromthe non-linear crystal, reflect lightback to the crystal. The opticalinput signal will only route to themirror that is phase coherent withthe crystal.

Non -LinearCrystal

electronically at source. In thisway the switch itself could beburied under the streets, maybemiles away from the source.Instead of being a bench -toparrangement of lasers, mirrorsand crystals, it would beminiaturised inside an integratedoptic package only a fewmillimetres across.

Looking even further ahead, DrEverard sees the possibility ofcascading these switches in sucha way that it might be possible tocode an input signal so that itwould route itself automaticallyto any one of hundreds ofthousands of differentdestinations.

At the moment this remains adream; the experimental switch isstill cumbersome and slow. Butwith new crystal materials andintegration technologies, the dayis not too far off when thousandsof tv or data channels could berouted almost anywhere withtotal reliability and without theoverheads of electrically switchednetworks.

Research Notes is written by JohnWilson of the BBC World Service.

L4

L3

0/P2

Semi -reflectingMirrors

i% 0/P1

Input PumpBeam

September 1993ELECTRONICS WORLD + WIRELESS WORLD 715

IMF ENGINEERING

Until quite recently spreadspectrum techniques werealmost exclusively in themilitary domain. Their usein GPS and the latestcellular phones will befollowed by many othercivil applications. Thisarticle, the first of threeparts, examines thetechnology by describingan experimental directsequence voicetransmission system as aworked example.By James Vincent*.

Voice link over spreadspectrum radio1: basic principles

*James Vincent G1PVZ works for a majoraerospace company.

Most Communication Engineersare used to minimising transmis-sion bandwidths. The trend hasbeen to use narrower bandwidths,

as with the transition from double sideband tosingle sideband modulation. It is quite obviousthat narrower bandwidths permit more com-munication channels to be packed into adefined frequency band.

However the rationale of using the verywide bandwidths required by Spread Spectrumsystems needs explanation. Claude Shannonproduced a ground breaking paper on themathematical theory of communication in

1949. Shannon's resulting theorem can beexpressed as:

C = W log2[1+ -SJ

bits

where C = data rate in bits per second, W =bandwidth (Hz), S = average signal power(W), N = mean white gaussian noise power(W). It can be seen from the equation that theonly options available to increase a channel'scapacity are to increase either the bandwidth(W) or the signal to noise ratio (S/N).

An increase in the signal to noise ratiorequires an increase in transmitter power as


RF ENGINEERING

the noise within the channel' is beyond ourcontrol! Thus we can either trade power orbandwidth to achieve a specified channel datarate. Because of the logarithmic relationship,increasing the power output is often unrealis-tic. However if frequency allocation con-straints permit, the bandwidth can beincreased. An appreciable increase in datacapacity or signal to noise ratio (for a fixeddata rate) can then be achieved.

Spread spectrum systems utilise very widebandwidths and low signal to noise ratios.From Shannon's theorem:

C = W log2[1+ -S

= log2[1+ -NS

changing bases

As logo P =logb P

loge a

log, P = logb a log Pll

= log2 e x loge [1+ -NS

Now logo b = 1logo a

C 1

2

x log IW = loge

-=1.44log,[1+ -S

By logarithmic expansion

loge [-NS -12 ÷ 131

-4I_N]+5I NJ etc

In a spread spectrum system the signal tonoise ratio (S/N) is typically small, much lessthan 0.1

C 1.44S-W N

thus WNC

From the derived relationship it can beclearly seen that a desired signal to noise ratiofor a fixed data rate C, can be achieved byincreasing the transmission bandwidth.

For example, assume a data rate of32Kbits-I and a signal to noise ratio of 0.001(-30dB)

CNW

I . 44S

thus W -32 x103 x1000 - 22MHz

1.44

So for a data rate of 32Kbits-I, operation atthe very low S/N ratio of -30db is achievableby spreading the signal over a bandwidth of22MHz. By using a very much wider band-width than that of the original data it is possi-ble to maintain data capacity without increas-ing the transmitter output power. It is anextreme example of a power -bandwidth tradeoff.

Frequency

12= 1=1IL=

1=1

V

1=1

==11=11==Til

ZZZZ Z Z Z

1=1=21

=21

Z

Spreadsignal

Unspreadsignal

TimeFig. 1. Frequency hopping spread spectrum provides anti -jam communications. The transmissionfrequencies change periodically in discrete, pseudorandom steps, the sequence of which is knownonly by the intended receiver. Multiple users interleave transmissions.

Frequency

Spreadsignal

Unspreadsignal

Time

Fig. 2. Direct sequence spread spectrum. An RF carrier modulated with this pseudorandom code willbe spread into a wide bandwidth continuous spectrum signal which looks like noise. At the receiver,the same pseudorandom code is used to correlate and hence despread the signal. Manytransmissions may use the same frequency simultaneously; signals using different pseudorandomcodes do not correlate and hence look like an addition to the background noise.

Two criteria (see Dixon) for a spread spec-trum system are: that the transmitted bandwidth is muchgreater than the bandwidth or rate of theinformation being sent; and that some function other than the informa-tion being sent determines the resulting radiofrequency bandwidth.

The two major techniques used in spreadspectrum systems are frequency hopping (fh)and direct sequence (ds). Of the two, fre-quency hopping is perhaps the easiest to visu-alise. In a frequency hopping system the fre-quency or channel in use is changed rapidly.The transmitter hops from channel to channelin a pre -determined but pseudo -randomsequence (see Fig. 1). The receiver has anidentical list of channels to use (the hop set)and an identical pseudo -random sequence gen-erator to that of the transmitter. A synchro-nising circuit in the receiver ensures that thepseudo -random code generator in the receiversynchronises to the one in the transmitter.When the transmitter and receiver are syn-chronised the user is unaware that the trans-mitter and receiver are rapidly changing fre-quency.

However should the receiver not be syn-chronised to the transmitter or a conventionalreceiver be used, nothing will be heard unlessthe transmitter hops onto the receiver's tunedfrequency. As a frequency hopping transmittertypically hops over tens to thousands of fre-quencies per second (the hop rate), the time itstays on a particular channel (the dwell time)is very snort and as a result the signal wouldappear as a burst of interference.

The other major spread spectrum techniqueis known as direct sequence or pseudo -noise.In this technique a pseudo -random code

directly phase shift keys the carrier increasingits bandwidth (see Fig. 2). In a typical directsequence system a double -balanced mixer(DBM) is driven by the pn code to switch acarrier's phase between 0° and 180°. This isknown as biphase shift keying (BPSK) orsometimes phase reversal keying (PRK).Unlike a frequency hopping transmitter wherethe pseudo -random sequence commands asynthesiser to change frequency, the directsequence signal is directly generated by thepseudo -random sequence.

The receiver despreads this wideband signalby using an identical synchronised pseudo-random code to that in the transmitter. As withthe frequency hopper, the receiver must use acircuit to adjust its clock rate so that thereceiver's pseudo -random code is at the samepoint in the code as the transmitter. A trackingcircuit is necessary to maintain synchronismonce it has been attained.

Sending data with spread spectrumSpread spectrum signals (whether directsequence, frequency hopping or their hybrids)can support any conventional analogue or dig-ital modulation scheme to impress data ontothe spread spectrum carrier.

Obviously some modulation formats are lesssuitable than others. Amplitude modulationand its derivatives are the least desirable astheir use will destroy the signal's uniformpower spectral density. This constant carrierenvelope is very desirable for spread spectrumsystems designed for covert usage.

Frequency modulation (frequency shift key-ing for data) is often used in frequency hop-ping systems, but is infrequently used in directsequence systems. This is because when adirect sequence signal passes through a squar-

September 1993ELECTRONICS WORLD + WIRELESS WORLD 717

ing or frequency doubling circuit, a carrier attwice the signal's centre frequency is pro-duced. This twice frequency narrowband car-rier will contain any modulation impressed onthe direct sequence signal. Thus with analoguemodulation it is possible for the signal to bedemodulated without any prior knowledge ofthe pseudo -random spreading code.

One of the commonest modulation tech -

Voice in

Encoderout

Go up"

r

"Go down

Decoder out

Voice in

niques used in conjunction with directsequence is known as code inversion or mod-ification. The digitised voice or digital data isexclusive ORed with the pn spreading code.This will invert the pn code sequence if thedata is a "1" or pass the pn code unmodified ifit is a "0". Provided that the data stream issynchronised with the pn code, the correlationproperties of the code are unaffected.

I U

Compressor

1\i\J 7\AJ

Encoder

Prototype direct sequence spread spectrumexciter and receiver system for 435MHz.Detailed circuitry will be appear in the next twoissues of Electronics World

Assuming synchronisation at the receiver,the unmodified code despreads the directsequence signal. This produces a narrowbandsignal which is still biphase shift modulated,but this time with the data or digitised speech.This signal can then be demodulated by a con-ventional biphase shift demodulator such as asquaring or Costas loop demodulator.

This code modification modulation is simpleto implement in the transmitter and relativelyeasy to demodulate in the receiver. It also hasthe advantage of providing message privacywhich the analogue modulated direct sequencesignal does not have. It should be noted that it

Fig. 3. Delta modulation provides a digital bitstream equivalent of the audio modulationsignal which will be used to modify thespreading code. A practical system uses a

compressor/expander systemto optimiseperformance.

Digital Decode' Expander

Voice out


RF ENGINEERING

is possible to directly demodulate uncorrelat-ed spectral components of an analogue mod-ulated direct sequence signal should thedemodulating receiver be very close to thetransmitter. In addition the code modificationtechnique preserves the constant power enve-lope of the direct sequence signal.

One disadvantage of code modification isthat voice or other analogue signals requiredigitisation. As in any system design, theselection of the digitisation technique is veryimportant. The technique selected must usethe lowest possible data rate as data rate isinversely proportional to the process gain ofthe system. The technique selected for the sys-tem described uses a enhanced form of deltamodulation to digitally encode the voice into aserial data stream.

Delta modulationDelta modulation is a variation of pulse -codemodulation. It compares successive signalsamples and transmits only their differences,rather than the actual amplitude as in PCM.This reduces the number of bits required tocode the speech. The continuous audio signalis sampled at periodic intervals. The sampledvalue is then compared with a staircaseapproximation of the output signal. If thesampled waveform exceeds the staircaseapproximation, a positive pulse is generated. Ifthe sampled waveform is less than the stair-case approximation, a negative pulse is gen-erated. This output pulse, positive or negative,forms the next step in the staircase approxi-mation, i.e. the sum of the binary pulse train atthe output of the encoder produces the delta -modulated waveform (see Fig. 3).

At the receiver, the transmitted pulses areintegrated and passed through a lowpass filterto remove unwanted high frequency compo-nents. The output consists of the original ana-logue signal together with some additionalnoise somewhat similar to quantisation noise.

Continuously variable slope delta modula-tion (CVSD) takes advantage of the fact thatvoice signals do not change abruptly and thatthere is only a small change from one sampleto the next. A reasonably good reproductioncan be obtained by transmitting in a giveninterval whether the output signal shouldincrease or decrease. A linear delta modulatedsystem has the undesirable feature that there isone input level which maximises the signal tonoise ratio. In CVSD this is overcome bycompressing the large amplitude in the signalsrelative to the smaller ones prior to encodingusing a compressor circuit. In this way theinput level to the encoder can be maintainedclose to the value which gives the maximumsignal to noise ratio.

The receiver decodes the delta modulatedbinary stream and passes the analogue signalthrough an expander to counteract the effectsof the transmitter compressor. Companding isoptimised for the human voice. CVSD is con-siderably more effective than standard deltamodulation and also exhibits less serioussound degradation in the presence of digitalnoise interference than PCM.

Auto -Correlation and Cross -Correlation

The main basis of spread spectrum communications is the correlation function, a measureof the similarity between functions. For the autocorrelation function:

+co

11-1, ( r) = f(t)x f(t - r)dt-O

A time dependent function (such as Snub° is compared with an identical replica timeshifted by a magnitude and summed (integrated) for all values oft. This function has amaximum at r= 0 which shows that (obviously) a function is most similar to itself when ithas not been time -shifted. For periodic functions, further maxima appear for a multiple ofthis period.

The response of the correlation function at other values than I= 0 determines how wellthe original function j(t) can be found again by variation of the time shift r. It is alsopossible to compare various fumetionsf(t) and g(t) using the cross -correlation function:

+00

LYK(r)= (t)xg(t- r)dt

This cross -correlation function is a measure of the degree of agreement betweenfunctions. Since the functions to be compared are different `l/K(r) may never achieve themaximum value of 'PA(r). It is an indication that the functions are different when a certainthreshold (-1 in the case of a binary code) is not exceeded.

In the correlation of binary code sequences, the result for cross -correlation will be +1 ifthe functional values coincide and -1 if they do not. The integration then forms a summingof all bits of the code. The correlation value for a certain phase -shift can therefore besimply calculated by placing the bits over another and comparing them bit by bit. Thecorrelation rate is the sum of agreements and disagreements.

For example, the maximal code sequence 1110010 is compared with itself in the sevenpossible phase -shifts.

shift

0

sequence

1110010

agreements disagreements agreementsminus

disagreements

1 0111001 3 4 -12 1011100 3 43 0101110 3 4 -14 0010111 3 4 -15 1001011 3 4 -16 1100101 3 4 -17 1110010 7 0 +7

As can be seen the auto -correlation function value is always -1, except for the case ofcoincidence, where it is a maximum. The greater the length of the code, the higher theauto -correlation amplitude and the greater the code discrimination or cross -correlationresponse. The auto -correlation function for maximal and non -maximal codes are shown inthe drawing below. As shown in the figure, maximal codes have only one auto -correlation

0 shift (magnitude=2"-1)

Maximal code

side maximum

0

-1 bit +lbit

Non -maximal code

degree ofdiscrimination

maximum, whereas non -maximal codes have side maxima as well.When non -maximal codes are used it is important to ensure that a sufficiently large

spacing exists between the main and side maxima. Despite these disadvantages, non -maximal codes are used to exploit their main advantages of rapid synchronisation andmessage security.


RF ENGINEERING

435 MHz carrier (7 dBm)

4 MHzclock

PN codegenerator

0

Diphaseencoder

Deltamodulator

Microphone

!Block diagram of the digital voice directsequence transmitter. Detailed circuitrywill be carried in the next issue.

DBM

435MHz direct sequencesignal output

435 MHzHelical BPF

5,0IRECT SEOGENCE SPREAD SPECTR,..1REF 20.0 dBm ATTER 30 OBPEAK

LOG10

CENTER 435.4 mHzRES Bw 100 kHz TBW 30 kHz

MKR 435.50 MHz7.00 dBm

SPAN 20.00 MHzSWP 20 Mel.=

Spectrum analyser plot: direct sequence spreadspectrum signal at transmitter helical filteroutput

NRZ

Diphase

0 0

-L_Fig. 4. The transmitted signal must be insensitive to phase. Digitised awn) in phase -sensitive NRZ isconverted to its diphase equivalent for transmission enabling the receiver to recover the data withthe correct polarity. This diphase code is exclusive-ORed with the pseudorandom spreading code toprovide a drive signal for the double balanced modulator of the sort shown below.

L.O.

Carrier in

D1

Pseudorandom Code in- - - +25 mA

R.F.

Output (spread)

IVAARP\

- -25 mAFig. 5. The biphase modulator. Diode pairs D1, D2 and D3, D4 alternately switch with thepseudorandom code signal causing phase reversal between the local oscillator and output ports

Circuit descriptionThe system is described in functional blocks.First, the transmitter direct sequence modula-tor. The exciter's clock frequencies are pro-vided by a master 4MHz crystal oscillator anda divider. Power -up reset (with manual over-ride) is configured around a Schmidt -trigger.

A shift register and exclusive OR gates areconfigured as a 4MHz 127 chip ( code bit)long maximal pseudo -random code generator(see section pseudo -random codes and gen-eration).

Microphone audio is amplified by the vogad(voice operated gain adjusting device) to theoptimum level for the input of the delta mod-ulator. The delta modulator converts the audiointo a 32Kbits-1 serial data stream. This serialbinary data stream must be coded into a for-mat which is polarity insensitive because thereceiver demodulator cannot recover thedespread data's absolute phase. Only datatransitions are recovered at the receiver, hencethere is no way of determining whether theoutput data stream is inverted or not.

The digitised audio is converted from a nonreturn to zero (NRZ) format into a polarityinsensitive diphase (biphase-mark) datastream. This subcircuit produces a diphase sig-nal (Fig. 4), where a logic 1 has start, mid -bitand end transitions and a logic 0 has only startand finish transitions.

In addition to providing phase insensitivedata transmission, the format also makes clockrecovery at the receiver relatively easy, asunlike NRZ even a continuous stream ofdiphase encoded 0's results in many start andfinish data cell transitions. The diphase encod-ed delta modulated digital voice signal is ex-ORed with the pseudo -random code produc-ing a code modified pn spreading code.

The data modified pn code from the outputof the exclusive -OR gate provides a balanceddrive (±24mA as an AC logic family devicehas equal sink and source currents) via a cou-pling capacitor and son matching pad, to adouble balanced mixer (DBM) configured as abiphase shift keyer.

The pn code output alternately sinks andsources current, causing the diodes in theDBM to alternately switch on and off pro-ducing 180° phase reversals in the 435MHzcarrier signal (see Fig. 5). The output spec-trum consists of a series of symmetrical side -bands which have a Sinc2x distribution due tothe many frequency components of the pseu-do -random code.

As the spreading code has a pseudo -randomcharacter, the occurrence of a particular fre-quency is pseudo -random in time and thedirect sequence output appears as noise on aspectrum analyser. The spread spectrum signalhas a main lobe bandwidth of 8MHz (twicethe pn code clock rate for BPSK). This isamplified by a MARS MMIC (monolithicmicrowave integerated circuit) and furtheramplified to around 100mW by a MotorolaCA4812 class A amplifier module. Helicalband pass filtering is used to ensure that theoutput signal is within the permitted band-width before free -space transmission.


RF ENGINEERING

Spread spectrum terminology

Process gain (Gp) is a fundamental concept inspread spectrum systems. The process gainindicates the gain or signal to noise improve-ment exhibited by a spread spectrum systemby nature of the spreading and despreadingprocess. Process gain can be estimated by thefollowing empirical relationship:

GProcess Gain = P= BWRF.

Rini'

Process Gain =10 logio[ BW".1dB

Runt-0

where

BWRF = 3dB bandwidth of the transmittedspread spectrum signal (Hz). R;,, f = data rateof the information transmitted (bits per sec-ond).

For a direct sequence signal, BWRF isassumed to be equal to the 3dB bandwidth ofthe spectrum (which is 0.88 times the pseudo-random code clock rate for a biphase shiftkeyed direct sequence system). For a fre-quency hopping system BWRF is equal to mtimes the channel bandwidth where m is thenumber of frequency channels available

Jamming Margin. Although the process gainis directly related to the interference rejectionproperties a more indicative measure of how aspread spectrum system will perform in theface of interference is the jamming margin(Mi). The process gain of a system will alwaysbe greater than its jamming margin.

Mi = G - step, (S ]dBwhere

1,,,,,e,= system implementation losses (dB);G = process gain (dB); (S/N) , = signal tonoise ratio at the information output (dB).

A spread spectrum system with a 30dB pro-cess gain, a minimum required output signalto noise of 10dB and system implementationloss of 3dB would have a jamming margin of30-(10+3)dB which is 17dB. The spread spec-trum system in this example could not beexpected to work in an environment withinterference more than 17dB above the desiredsignal.

Power spectral density. By nature of thespreading process, the output power of thespread spectrum transmitter is spread over typ-ically many megahertz of bandwidth. Thespectral density is the number of Watts ofradio frequency power present per Hertz ofbandwidth. Thus for a direct sequence trans-mitter of 1W output and a spread bandwidthof 8MHz the power spectral density is:

1

W / Hz = 125nW / Hz8, 000, 000For a conventional AM transmitter,

power spectral density is around1

W / Hz = 1660/,6000some 31dB greater

The advantage to the military user is that thesignal strength apparent to a conventional nar-rowband receiver is very, very low and wouldprobably not be recognised as a communica-tions signal, hence the expression "LowProbability of Intercept" and "Low Probabilityof Recognition".

GlossaryAntijamtning (AJ): Techniques used to min-imise the effects of jamming or unintentionalinterference.

Auto -correlation: This is a measure of sim-ilarity between a signal and a time shiftedreplica of itself. Auto -correlation is a specialcase of cross -correlation. The auto -correlationfunction is the fundamental theoretical basis ofspread spectrum communications.

Biphase Shift Keying (BPSK): A phase shiftkeying technique where the carrier phasechanges between 0° and 180° (0 and it radi-ans) under the control of a binary code. BPSKis frequently used to generate direct sequencespread spectrum signals, where the binarycode is a pseudo -random sequence.

Chip: A single element of the spreadingcode. This may be one or more of the pn codebits, depending on the modulation techniqueused. For BPSK one chip represents one codebit, whereas for quadrature phase shift keying(QPSK) one chip represents two code bits.

This is because there are four states forQPSK (0°,90°,180°and 270°) and only twostates for BPSK (0°and 90°). Obviously twobinary bits are required to represent four statesand only one bit for two states.

Code: The term code usually refers to thepseudo -random code used to control the mod-ulation technique used to spread the carrier.

Code Division Multiple Access (CDMA): Amultiplexing technique where each user isgiven a different pseudo -random spreadingcode. To communicate with a particular user,the sender must select the code assigned tothat user.

If the CDMA codes are carefully selected toensure good correlation properties, thenunwanted CDMA transmissions will not becorrelated and hence rejected as widebandinterference (up to the limit of the jammingmargin Mi of the system). This technique canpermit many users to operate simultaneouslyon the same freq

CTRON Cs tiJRLD - WorldRadioHistory.Com · Gyrator acts as electronc choke, High -power class -A...

Documents

Transcript of CTRON Cs tiJRLD - WorldRadioHistory.Com · Gyrator acts as electronc choke, High -power class -A...