Home Digital Systems - EBOOK 4 FREESITEPart 2 - Digital Television Part 3 - Digital TV displays Part 4 - TV Modulation Techniques Part 5 - UHF TV Broadcasting and Reception Part 6

HomeDigitalSystems

PartOne–FundamentalsofElectricity

RACalazCEng,BSc(Eng),MIET,ACGI,MSCTE

CopyrightNoticeAllrightsreserved.Nopartofthispublicationmaybereproducedwithouttheexpresswrittenpermissionoftheauthor.Alllogosandtrademarksarethecopyrightoftheirrespectiveowners.TherightofRACalaztobeidentifiedastheauthorofthisworkhasbeenassertedbyhiminaccordancewiththeCopyrightDesignsandPatentsAct1998.

DisclaimerThispublicationisintendedtoprovideinformationregardingtheinstallationofdigitalsystemsinaresidentialenvironment.Everyefforthasbeenmadetomakeitascompleteandaccurateaspossible,butnowarrantyoffitnessisimplied.Theauthorshallhavenoresponsibilitywithrespecttolossordamagearisingfromtheinformationcontainedinthisdocument.Correctionsandcommentsshouldbeaddressedtobob@calaz.com.

AbouttheAuthorBobCalazisanacknowledgedexpertinthefieldofTV,satelliteandmultimediainstallationshavingbeenacharteredelectricalengineerformorethan40years.For26ofthoseyears,hewaswiththeRediffusiongroupofcompanies,includingasecondmenttoSouthAfricafor15yearsaschiefengineerofRediffusionSouthAfrica.HewasamemberofagovernmentcommitteesetuptodefinethetechnicalstandardsforTVsystemsinmulti-dwellingunits,andwasresponsibleforthedesignandinstallationofa200channelTVdistributionsystemfortheheadquartersoftheSouthAfricanBroadcastingCorporation.ReturninghometotheUKin1985,hefoundedRaceCommunicationsLtd(basedinBerkshire)toservicethegrowingTVindustry.Hehassinceconductedmanytechnicaltrainingcoursesforcommercialandmilitarypersonnel.

Bobhaspresentednumeroustechnicalarticlesandpapersthroughouttheworldandhasbeenregularlyindemandasanadvisortoavarietyofcommercial,industryandgovernmentalbodies.HisrecentpublicationsincludetworeferencebooksonDigitalTV,SatelliteandMultimedia.

Thecomplete“InsiderGuide”setofpublicationsThispublicationisoneofaseriescoveringthedesign,installation,operationandmaintenanceofdigitalreceptionanddistributionsystemsinthehome.Thecompleteseriesissub-dividedasfollows:Part1-FundamentalsofElectricityPart2-DigitalTelevisionPart3-DigitalTVdisplaysPart4-TVModulationTechniquesPart5-UHFTVBroadcastingandReceptionPart6-RadioandTVAerialInstallationsPart7-UHFTVSignalDistributionPart8-SatelliteTVReceptionPart9-SatelliteTVDistributionPart10-SatelliteIFNetworkPlanningPart11-TestEquipmentPart12-FibreOpticDistributionPart13-DistributionofVoiceandDataSignalsPart14-DigitalHomeTechnologiesPart15-StructuredCableNetworksPart16-VSATSystemsPart17-Abbreviations/GlossaryofTerms

IntroductionThisseriesofpublicationsintroducesthereadertotheapplicationofdigitalsystemsinthehome.Itissuitableforthosewithnopriorknowledgeofthesubject,whilstatthesametimeprovidingasourceofreferenceforexperiencedinstallerswhowishtoknowmoreaboutthesubject.Thispartservesasanintroductiontotheconceptofelectricityflowinginawire,anditsunitsofmeasurement.Theresistancetocurrentflowisthendescribed,leadingtotheinterrelationshipbetweenvoltage,currentandpower.Sincevoltagesareusedtomeasurethestrengthofatelevisionsignal,theunitofvoltagemeasurementisthendefined.Thethreetypesofbasicelectroniccomponentsareintroduced,leadingtotheconceptofa“filter”.Partstwototwelvecoverradio,TVandsatellitereceptionanddistribution;partsthirteentosixteencoverdatasystemsandpartseventeenisareferenceofabbreviationsandaglossaryofterms.

TableofContentsPart1–FundamentalsofElectricity1.Electricity2.Voltage3.Current4.Resistance5.OhmsLaw6.Power7.Prefixes8.Voltages9.UHFandSatelliteTVSignalLevels10.Impedance

1.Electricity

Electricitycanbecomparedtothewaterdistributionsysteminyourhouse.Witheverythingturnedoff,thewaterpressureexists,butnowaterisused.Whenatapisturnedon,waterflowsinthepipe,thequantitydependingonthepressureinthemains,thesize(orresistance)ofthepipeandtheamountthetapisturnedon.Asmoretapsareopened,morewaterisused.Iftoomanytapsareturnedonatthesametime,thepressuredropsandthereisverylittlewatercomingfromeachtap.

Adomesticelectricitysupplyworksinmuchthesameway.Theequivalentofwaterpressureisthemains“voltage”.Withalltheswitchesturnedoff,noenergyflowsthroughthecables.Whenaswitchisturnedon,“current”flowsthroughthem,theamountdependingonthevoltageapplied,thesizeofthecablesandthe“resistance”oftheload.Asmoreswitchesareturnedon,morecurrentisused.

Inadomesticenvironment,lightingcablesarerelativelythin,withatypicalcross

sectionalareaof1mm2,becausethecurrentrequirementsaresmallandacircuitbreakerlimitsthemaximumcurrenttosixamps.Powercablestowallsocketsarethicker(2.5mm2)andthetotalcurrentconsumptionislimitedtothirtyamps.Akitchencookerutilises4mm2cablestocaterforcurrentsofuptofortyamps.Thisiswhyanelectricityconsumerunitincorporatescircuitbreakersofdifferentvalues.Allelectricitycableshavea“resistance”tocurrentflowandthepurposeofacircuitbreakeristolimittheamountofcurrentandthustheamountofheatbeinggeneratedalongthecable.

2.VoltageVoltage(V)canberegardedasthesourceofenergyandismeasuredinVolts.ThemainssupplyinBritainisnominally230Volts.RadiotransmittersoftenworkatmuchhighervoltageswhilstTVaerialsreceivesmallvoltages–tinyfractionsofavolt.Voltageisusuallythemostimportantcharacteristicandallsignallevelmetersmeasurevoltages.A“multimeter”canbeusedtomeasurelowvoltagessuchasthosegeneratedbyatorchbattery.

Forhighervoltages,anelectrician’smultimetershouldbeusedwithproperlyshroudedleadsconformingtotherelevantBritishStandard.

3.CurrentCurrent(I)istheenergyflowingthroughacableandismeasuredinAmperesorAmps.Allactiveelectroniccircuitsusecurrentprovidedbyapowersupplyunit(PSU)andthetotalcurrentrequirementcanbecalculatedbyaddingtogetheralltheindividualcurrentrequirements.Currentcanalsobemeasuredusingamultimeter.

Notethatthecurrentconsumedbyalightbulbwillvary,dependingonitswattage.

4.ResistanceResistance(R)isdefinedinOhms,andbythesymbol“Ω”onacircuitdiagramA“resistor”isusuallythemostcommoncomponentinelectroniccircuitsanditsvaluecanbedeterminedfromthecolouredstripesonitsbodyasshown.

Thefirstandsecondstripesindicatethefirstandseconddigitsofthevalueinohms,andthethirdstripeindicatesthenumberofsubsequentzeros.Forexample:RedRedOrange:

Thevalueofthisresistoris22000ohms.

Theonlyresistorvaluelikelytobeencounteredbyinstallersis75ohms.Sucharesistorwouldhavestripescolouredviolet/green/black:

Thereissometimesafourthstripetoindicateitsaccuracy:Gold+/-5%Silver+/-10%Nofourthstripe+/-20%

Someconnectorshave75ohmresistorsbuiltintothem–theyareknownas75ohmterminationsandareusedattheendofasystem“spur”line,oronanunusedhighoutputofadistributionamplifier.Resistancevaluescanalsobemeasuredwithamultimeter.

5.OhmsLawThisstatestherelationshipbetweenthesethreevalues:Volts=AmpsxOhmsorV=IxRIftwoofthesevaluesareknown,thethirdonecanbecalculated.Theformulaisoftenwrittenasfollowstoillustratetheserelationships:

Thus,V=IxRI=V/RR=V/I

Forinstance,ifalightbulbisconnectedtothe230Vmainspowersupplyandconsumes0.5ampsofcurrent,itsresistancecanbecalculatedasfollows:Resistance=Voltage/Current=230/0.5=460ohms

6.PowerPower(W)ismeasuredinWatts.ItsrelationshiptoVoltsandAmpsisasfollows:Watts=AmpsxVoltsorW=IxVor,insideatriangle:

Thepowerofthelampintheaboveexamplecanthereforebecalculatedasfollows:Power=CurrentxVoltage=0.5x230=115watts

7.PrefixesInordertosimplifytherecordingofhighandlowvoltages,prefixesareused,asfollows:

Onemillionthofavolt=1microvolt( V)

Onethousandthofavolt=1millivolt(mV)Onevolt=1volt(V)Onethousandvolts=1kilovolt(kV)Onemillionvolts=1megavolt(MV)Thesameprefixesareusedforamps,ohmsandwatts(andalsoforotherparameters,aswillbeseenlater).Forexample:Onethousandthofavolt=1millivolt(mV)Onethousandthofanamp=1milliamp(mA)Onethousandthofanohm=1milliohm(mΩ)Onethousandthofawatt=1milliwatt(mW)

8.VoltagesSincethevoltagelevelsattheoutputofaTVorsatelliteamplifiercanbeuptoonethousandtimeshigherthantheinputlevel,ourindustryusesamoreconvenientunitofmeasurement,calledadecibelordB.

MostsignalmetersdisplaylevelsindBwithreferenceto1 V(abbreviatedtodB V)andtheconversionfromVoltstodB Visasfollows:

OthermetersdisplaylevelsindBmV(dBwithreferenceto1mV)asshownbelow:

Notethat,toconvertfromdBmVtodB V,onesimplyadds60tothereading.

9.UHFandSatelliteTVSignalLevels

DomesticaerialandsatelliteinstallersalwaysmeasuresignallevelsindB VordBmVbecausethemathematicsislimitedtosimpleadditionandsubtraction.Lossesinasystemaresubtractedandamplifiergainsareadded.Forinstance,ifthesignallevelfromanaerialis55dB VandthelossonthecoaxialcablebetweentheaerialandTVreceiveris10dB,thesignallevelattheTVis55–10=45dB V:

Ifanamplifierwithagain(oramplification)of13dBisaddedatthereceiverlocation,thesignalwillbeincreasedby13dBto58dB V:

Note:ThefiguresinsidethecirclesrepresentthesignallevelsindB V,aconventionthatwillbeusedthroughoutthisseriesofdocuments.

IftheamplifierwerefittedattheaeriallocationinsteadofbeingadjacenttotheTVreceiver,thesignallevelsonthecablewouldbedifferentbuttheendresultwouldbethesame:

Ifthesignalissplittofeedtwooutletlocations,thesplitterwouldreducethesignaltoeachlegby4dBandthelevelswouldbeasfollows:

TheseexamplesshowthattheuseofdB’smakesthecalculationofsignallevelsonasystemveryeasy!

Thesameprinciplesapplytosatellitesignals.IfthesignallevelfromtheLNBis65dBµVandthecablelossis15dB,thelevelattheinputtothesatellitereceiveris50dBµV:

Itisimportanttobeabletomeasuresignallevelsbecausethesewilldeterminethereliabilityofthesystem.Ifthelevelsaretoohighortoolow,theoverallperformancewillbeadverselyaffected.

10.ImpedanceElectricalcircuitsaremadeupofthreefundamentalcomponents:

Resistance,measuredin“ohms”Capacitance,measuredin“farads”Inductance,measuredin“henries”

Acapacitorandaninductorcanbeusedtogethertoactasa“filter”,topassorrejectabandoffrequencies,asdescribedinpartfour.Radiosignalscanberelayedon“coaxial”cableswhichhaveacentreconductorandanouterscreen,separatedbyinsulationknownasthe“dielectric”.Whenanalternatingvoltageisappliedtooneendofthecablewithnoloadontheotherend,acurrentwillflowduetoitsinherentcapacitanceandinductance.UsingOhm’slaw(R=V/I),the“resistance”isknownasthe“characteristicimpedance”ofthecable,withthesymbol“zo”.Virtuallyallcablesusedtorelayradio,TVandsatellitesignalshaveazoof75ohms.Iftheloadresistanceisalso75ohms,thecableisthensaidtobe“matched”.Iftheloadresistanceisanyothervalue,someofthereceivedsignalscanbereflectedbackalongthecable,causing“standingwaves”whichcancausecorruptionofthesignalsbeingrelayed.