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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.