电子信息与通信工程专业英语孙桂芝chapter3 Electronic Circuit Design课件

电子信息与通信工程专业英语Chapter3ElectronicCircuitDesign3.1TheProcessofDesign1Theprocessofdesignusuallystartswithaproblemstatement.Forexample,supposeyouwanttodesignanewelectriccarthatcangofartherononechargeandbelessexpensivetoproducethanthosecurrentlyavailable.Developingaclearstatementofthedesignrequirementsisnotalwaysasimpletask.Sometimes,alotofeffortmustbeputintothisfirststepinthedesignprocess.Itisneveragoodideatoproceedwithadesignwhentheobjectivesarenotclearlyunderstood.Onceaclearproblemstatementiscompleted,ateamofdesignersmustdevelopaplanofaction.Theyusuallystartbyenumeratingthegoalsoftheprojectinmoredetail,perhapsincludingprioritiessothatanytrade-offsthatmustbemadelatercanbeproperlyevaluated(e.g.,howimportantiscostrelativetorangeononecharge).Thenextstepistoidentifypossibleapproachestoachievingthestatedgoals.Often,thisstepisfacilitatedbyabrainstorming2session.Afterthepossibilitieshavebeenlisted,eachiscriticallyreviewedtoseeifthereareanyobviousreasonsfordismissingitfromfurtherconsideration.Theresulting—usuallyshorter—lististhensubjectedtoevencloserscrutiny.Atthispoint3,thedesignwillusuallyneedtobebrokenupintodifferentsubsystems(e.g.,motors,body),andsubgroupsofthedesignerswillrepeattheprocessforeachparticularsubsystem.Thesubsystemsmaythenbebrokenupfurther.Ateachstepthereisafour-stepprocesstobefollowed:(1)

First,thegoalsofthedesignareenumeratedand,perhaps,prioritiesareestablished.(2)

Second,alistisgeneratedcontainingasmanywaysofachievingthestatedgoalsaspossible.(3)

Third,thisinitiallistofalternativesissubjectedtocriticalreviewtoselectchoicesthatappeartobefeasible.(4)

Fourth,theremainingoptionsaresubjectedtolongerandmorethoughtfulanalysis;notethatthedesignmaybepartitionedintosmallersubsystemsatthispoint.Theprocessmayberepeatedmanytimes,withthesubgroupscomingbacktogetherandmodifyingthepartitioningordealingwithtrade-offsinthedifferentsubassemblies(e.g.,ifthebodyismademoreaerodynamicallyefficient,themotordoesn'tneedtobeaspowerful).Eventually,oneortwoapproachesaresettledon,andworkbeginsinearnest.However,bythistime,eachofthegroupshasdoneasubstantialamountofdesign,sincetoalargeextentisanexplorationofalternatives,followedbyanalysis,followedbychoosingthealternativethatseemsbestsuitedtothetaskathand4.Sometimestheprocessofdesignincludeswhatmostpeoplethinkofwhenyoumentiondesign,namely,synthesis.Synthesisistheprocessofcreatingsomethingnew,atleasttothepersonwhocreatesit.Althoughonecancertainlygetbetteratsynthesisthroughpracticeandbycriticallyexaminingtheprocessused,thereisn'tmuchthatcanbedonetodirectlyteachyouhowtosynthesizeentirelynewelectroniccircuits.Ingeneral,theprocessissimilartodesigninthatalternativesarelistedandthensubjectedtodifferentlevelsofanalysis.However,thedifferenceisinthelistingofalternatives.Ratherthanjustenumeratingalltheknownwaysofperformingafunction,youareinterestedinfiguringouthowtoperformafunctioninanentirelynewway,eitherbecausenowaycurrentlyexistsorbecauseallexistingtechniquesareforsomereasoninadequatefortheapplication.Althoughsynthesisislesssystematicthandesignandarguablynotsomethingthatcanbetaught,muchofwhatyoulearnabouttheprocessofdesignwillhelpyouwithsynthesisaswell5.Tobegoodatsynthesis,youmustfirstbefamiliarwithasmanydifferentcircuitsandfunctionsaspossible.Youmustalsolearntothinkaboutcircuitsatseveraldifferentlevelsofabstractionsothatyoucanseehowtomodifycircuitstoimprovethemoraltertheirperformance.Thestudyofcircuitdesign,aspresentedinthistext,shouldhelpyougainknowledgeandskillthatwillhelpyouwithsynthesisaswellasdesign.Asafinalnote,synthesisismorelikelytoconsumealargeamountoftimetocompleteandisalsoriskierthanusingknownmethodsinthatyourgreatnewideamayneverworkproperly!Therefore,agooddesignerwillcarefullyresearchpreviouseffortsatsolvingthesameandsimilarproblemstodiscoverasmanyapproachesaspossible.Fromabusinessperspective,itusuallyisn'tagoodideatorepeattheworkofothers.Newwordsandphrases

electric[iˈlektrik]adj.电的；电动的；发电的；导电的；令人震惊的n.电；电气electriccar电动车enumerate[iˈnju:məreit]vt.列举；枚举；计算review[riˈvju:]vt.回顾；检查；复审n.回顾；复习；评论；检讨；检阅brainstorming[ˈbreinˌstɔ:miŋ]n.头脑风暴法，集体研讨；发表独创性意见scrutiny[ˈskru:tini]n.详细审查；监视；细看；选票复查subsystem[ˈsʌbsistəm]n.子系统；次要系统subgroup[ˈsʌbɡru:p]n.小群；隶属的小组织thoughtful[ˈθɔ:tfʊl;-f(ə)l]adj.深思的；体贴的；关切的subassembly[ˌsʌbəˈsembli]

n.组件，部件synthesis[ˈsinθisis]n.综合，【化学】合成；综合体electroniccircuits电子电路analysis[əˈnælisis]n.分析；分解；验定systematic[ˌsistəˈmætik]adj.系统的；体系的；有系统的【图情】分类的arguably[ˈɑːɡjʊəbli]adv.可论证地；可争辩地；正如可提出证据加以证明的那样地Notes1.本节涉及电子电路设计中的设计流程部分。2.

brainstorming头脑风暴法。头脑风暴法是指将所有可能解决问题的方案都列出来，而不考虑这些方案是否切实可行。3.

atthispoint此时。此句可翻译为：此时，通常需要将设计工作分割成若干个不同的子系统(如发动机、车身等)的设计，各小组的设计师针对每个子系统重复该方案审核工作。4.句中的since引导原因状语从句，省略了从句的主语design。此句可翻译为：然而至此为止，各个小组已经做了大量的设计工作，因为设计在很大程度上就是对各种选择方案的探索，然后进行分析，最终选定一套适合于相应任务的最佳方案。5.句中although引导让步状语从句，而主句中的主语由what引导的主语从句充当。此句可翻译为：虽然综合的系统性不如设计那么强，而且似乎没有什么东西可以拿来直接使用，但是所学的有关设计步骤的许多内容同样有助于综合的学习。

Figure3.1Anamplifiercircuit

3.2AnalysisforDesign1Howdoesthemethodofanalysisyouarefamiliarwithdifferfromthetypeofanalysisthatisusedinthedesignprocess?Aexampleswillhelptoexplainthedifference.Wewillanalyzecircuitscontainingonlyresistorsandsources.First,considerthecircuitshowninFigure3.1;itistypicalofthekindsofcircuits.Thecircuitusesamodeloftheinputsourceandacontrolled,ordependent,source.ThecircuitinFigure3.1canbeanalyzedusingeitherloopornodalanalysis.Wechoosenodalanalysisandnotethatthereareonlytwoindependentnodesinthecircuit:theinputandtheoutput.Ourchoiceofusingnodalanalysisisnotasarbitraryasitmightseem,althoughforthiscircuititturnsouttonotmakeaverybigdifference.UsingKirchhoff'scurrentlaw(KCL)2atinFigure3.1leadsto(3-1)WritingKCLfor，resultsin(3-2)Solvingeq.(3-1)foryields(3-3)andpluggingthisresultintoeq.(3-2)produces(3-4)fromwhichthefinaltransferfunctionisobtained:(3-5)Wehavedeliberatelyincludedmostofthestepsinthederivationofeq.(3-5)toillustratethatthisbrute-forceapproachtoanalyzingthecircuit,whilecorrect,tendstofocusconcentrationonthealgebra,ratherthan3thecircuit.Analternateanalysistechnique—andtheonethatallexperienceddesignerswoulduse—istobreaktheproblemintoasequenceofsmallerproblemsanduseafewwell-knownresultstowritetheanswerbyinspection.Themethodstartsbynoticingthattheoveralltransferfunctioncanbewrittenastheproductoftwosimplertransferfunctions:

(3-6)Theoveralltransferfunctionofacomplexnetworkcanalwaysbebrokendownasshownineq.(3-6),butdoingsoisusefulonlyiftheindividualtransferfunctionscanbewrittenbyinspection,oratleastwithsignificantlylesswork4.Forexample,iftheindividualnetworksareallunilateral,theirtransferfunctionscanusuallybederivedwithminimaleffort.Aunilateralnetworkisanetworkinwhichsignalsflowinonlyonedirection.Inotherwords,forthisexample,doesnotdependon.Eachoftheindividualtransferfunctionsontheright-handsideofeq.(3-6)canbewrittenbyinspection,sincetheyarebothresistivevoltagedividers.Examiningtheoutputofthecircuit,weget(3-7)or(3-8)Examiningtheinputside,weget(3-9)or(3-10)Substitutingthesetwotransferfunctionsbackintoeq.(3-6),orsubstitutingeq.(3-9)intoeq.(3-7)directly,leadstoeq.(3-5)again,butwithlesseffortandmoreimportantly,aclearrealizationonourpartofwhereeachtermintheequationcomesfrom.Wehavechosenaverysimpleexampletobeginwith,sothebenefitsmaynotbeobviousatthisstage.However,inmorecomplicatedcircuits,thedifferenceinthedifficultyofsolvingsimultaneousequationsversuswritingasequenceofsimplertransferfunctionscanbequitelarge.Wewanttointroduceanotationthatwillbeusedthroughoutthistextandthathelpstocultivatetheproperstyleofanalysis.Inanalyzingacircuitlikethis,itisoftenconvenienttodefinean(3-11)andan(3-12)Theseattenuationfactorswillbedifferentfordifferentcircuits,buttheconceptisalwaysthesameandleadstosimplifiedanalyses.Indesigningamplifiersandmanyothercircuits,weoftendesiretomaketheattenuationfactorsasclosetooneaspossible.Iftheinputattenuationfactoristoosmall,thesignalisreducedpriortoanyamplificationorprocessing.Additivenoise,whichisalwayspresent,canthenbeasignificantproblem.Ontheotherhand,iftheoutputattenuationistoosmall,itmeanswearewastingsomeofoursignalindrivingtheload,andwehavetoincreasethegaininthesystemtomakeupfortheloss5.Notethatitisusuallytheattenuationfactor,notthepowertransfer,thatshouldbemaximized.

Newwordsandphrasesresistor[riˈzɪstə]n.【电工学】电阻器source[sɔ:s]n.电源controlledsource受控源dependentsource关联源inputsource输入源loopanalysis环路分析法nodalanalysis节点分析法Kirchhoff'scurrentlaw(KCL)基尔霍夫电流定律unilateral[ˌju:niˈlæt(ə)r(ə)l]adj.单向的；【植】单侧的；单方面的；单边音voltagedividers分压器attenuationfactor衰减因子concept[ˈkɔnsept]n.概念，观念load[ləʊd]n.负载，负荷；工作量；装载量vi.【力】加载；装载；装货vt.使担负makeupfor补偿，弥补powertransfer能量转移Notes1.本节涉及电子电路设计中的电路分析部分。2.

Kirchhoff'scurrentlaw(KCL)基尔霍夫电流定律。定义：对于任意一个集中参数电路中的任意一个节点或闭合面，在任何时刻，通过该节点或闭合面的所有支路电流代数和等于零。3.

ratherthan而不是。此句可翻译为：以上详细介绍了式(3-5)的大部分推导步骤，目的是为了说明这种粗略的分析方法虽然正确，但是更加注重代数推导而不是电路本身。4.

onlyif只有……才。此句可翻译为：一个复杂网络的传输函数总是能分解成式(3-6)所示的形式，但是只有当每个传输函数可根据观察写出或只需做较少工作就可写出时这种方法才有用。5.

Ontheotherhand另一方面。此句可翻译为：从另一方面来说，如果输出衰减因子太小，那就意味着在驱动负载时损失了一部分信号，因此必须提高系统增益以弥补损失的信号。

3.3ElectronicSystems1Havingpresentedabasicideaofwhatconstitutesdesignandofhowanalysisforthesakeofdesigndiffersfromanalysisforthesakeofdeterminingperformance,wenextdiscusselectronicsystems.Therearethreereasonsforthisdiscussion:First,itwillprovidemotivationforthematerialthatfollows;second,itwillhelptopresentanoverallframeworkfortheupcomingmaterialsothatitiseasiertoseehowitfitsintoabroaderpictureofthefieldofelectricalengineering;andfinally,itwillprovideanopportunitytoindicateclearlywhatitisaboutthismaterialthatmakesitdifferentfromthematerialpresentedinthelinearcircuitsandsignalscoursesyouhavehadbefore.3.3.1ElectronicversusElectricCircuitsStrictlyspeaking,anelectriccircuitisacompleteloopinwhichelectronscanflowandthatincludescircuitscontainingtransistors.Nevertheless,mostpeople(butnotall)woulddistinguishbetweenanelectriccircuitandanelectroniccircuit—hencethetitleofthistext.Anelectriccircuitisacircuitthatcontainsresistors,capacitors,inductors,transformers,switchesandthelike,butnotransistors,integratedcircuits,ortubes(yes,theydostillgetusedonceinawhile).Ignoringtubes,andrecognizingthatintegratedcircuitscontaintransistors,resistors,capacitors,and,occasionallyinductors,therealdistinctionbetweenanelectriccircuitandanelectroniccircuitboilsdowntowhetherornotitcontainstransistors.Sowhatisitabouttransistorsthatmakethemspecial?Theshortansweristhattheyareactivedevices,whileresistors,capacitors,andsimilarcomponentsarepassivedevices.Apassivedeviceisadevicethatcanneitherprovideenergynordirecttheflowofenergy.Anactivedevicecancontroltheflowofenergyorcansupplyenergy.Figure3.2Mechanicalanalogyofpassiveandactivedevices

Amechanicalanalogymaybeuseful.Ifyouwanttoliftaheavybox,youcanusealever,asshowninFigure3.2(a),oryoucanuseaforklift,asshowninpart(b)ofthefigure.Theleverisapassivedevice:Youaredirectlysupplyingtherequiredenergy.Theleversimplyletsyouexertasmallforceoveralargedistancetoproducealargerforceoverashorterdistance.Ontheotherhand,theforkliftisanactivedevice:Youdirectitsmotion,butitusesanothersourceofpowertoaccomplishthetask.Anelectricanalogtotheleverisatransformer,asshowninFigure3.3(a):Thetransformercanboostthevoltagesuppliedtoagivenload(analogoustoforceinFigure3.2),butthecurrentsuppliedtotheloadisproportionatelylowerthanthecurrentdrawnfromthesource(analogoustodistanceinFigure3.2).TheelectricanalogoftheforkliftisthetransistorcircuitshowninFigure3.3(b):Itusesaninputvoltagetocontrolanoutputvoltage,buttheinputsourcedoesnotneedtosupplythepowertotheoutput.Thecircuitincludestheinputsource,thetransistor,theload,andaseparatesupplyofenergy—abatteryinthiscase.Thebatteryisanalogoustothemotorintheforklift.Thetransistorisadevicethathasacurrentflowingfromtheoutputtogroundthatiscontrolledbythevoltageappearingfromtheinputtoground2.Thevoltageacrosstheloadisequaltothecurrenttimestheloadresistance,andthisvoltagecanbemuchlargerthanthesourcevoltage.Figure3.3ElectricalexamplesofpassiveandactivevoltagegainUnlikethetransformer,however,theoutputcurrentfromthetransistorcanalsobemuchlargerthantheinputcurrent,whichisideallyzero.Atthispointanimportantdifferencebetweenactiveandpassivecircuitscanbepointedout.Sincemechanicalpowerisforcetimesdistanceperunittime(integratediftheforceisnotconstant),thepowerdeliveredtotheloadinFigure3.2(a)isnomorethanthepowerobtainedfromthesource(i.e.,thepersonpushingonthelever).Inreality,theoutputpowerislowerbecauseoffrictionlossesandothereffects.ButinFigure3.2(b),thepowerdeliveredtotheloadcanbesignificantlylargerthanthatprovidedbythesource(i.e.,thedriver),whichisnegligibleinthiscase,sinceallthe"source"hastodoispushthelever.ThepowerinFigure3.3(b)issuppliedbythemotorintheforklift.Intheelectricalcase,powerisequaltovoltagetimescurrent.FortheidealtransformerinFigure3.3(a),thepoweroutisexactlyequaltothepowerin.However,theidealtransformercanonlybeapproximatedinpractice.Therewillalwaysbesomelosssothatthepoweroutwillactuallybelowerthanthepowerinandthepowergainofthesystemisstrictlylessthanone.ALLpassivecircuitshavepowergainslessthanone.ForthetransistorcircuitshowninFigure3.3(b),however,thesignalpoweroutcanbemuchlargerthanthepowerdeliveredbythesignalsource,sothepowergainforthesignalcanbegreaterthanone.Energyisnotproducedinthecircuit,ofcourse;itsimplyusestheenergyprovidedbythepowersupply(thebatteryinFigure3.3)todrivetheload.Inotherwords,thetransistorusestheinputsignalasatemplatetoproduceanoutputthatlooksliketheinput,butmayhavegreaterpower.Becausetheycontrolordirecttheflowofenergyfromsomepowersupply(e.g.,abattery),activecircuitscanhavepowergainsgreaterthanone.3.3.2AnalogandDigitalElectronicCircuitsTogainabetterunderstandingofelectronicsystems,let'sbrieflyexaminetwocommonelectronicsystems:anFMreceiverandacomputer.Beforewegointoanydetail,however,weneedtounderstandafundamentaldifferencebetweenthesetwosystems:TheFMreceiverisananalogsystem(althoughmodernreceiverscertainlycontaindigitalcomponentsaswell,thetransmittedsignalisanalog),whereasthecomputerisadigitalsystem(computersdocontainanalogcircuitsaswell,mostnotablyinthedisplay,powersupply,soundreproduction,andmassstoragedevices).TheinputtoanFMreceiverisafrequency-modulated(FM)signal.Thatis,thefrequencyofahigh-frequencycarrierischangedinresponsetotheinformationtobetransmitted(themusicinthiscase).Therearetwomainreasonsfornottransmittingtheaudiosignalitself:First,therewouldn'tbeanywaytoseparateonestationfromanother,and,second,theantennaswouldneedtobehuge.Antennasneedtobeareasonablefractionofawavelengthtoworkwelland,asanexample,thewavelengthofa1kHzwavepropagatinginairisabout186miles.Tohavepracticalsystems,therefore,weusehigh-frequencycarriers,commonlycalledradio-frequency(RF)waves.Totransmitthesignal,wemodulate(i.e.,change)someparameterofthecarrierwave(e.g.,amplitudeorfrequency)withthesignal.Thereceivermustthendemodulatethecarriertorecovertheoriginalsignal.Inthecaseofastereoreceiver,thesituationiscomplicatedfurtherbythedesiretosendbothleft-andright-channelsignalsandtohaveold-fashionedmonauralreceiversstillwork(notimportantnow,butitwaswhenFMwasfirstintroduced).Therefore,ifyousimplydemodulateanFMsignal,yougettheleft-andtheright-channelsignalscombined,denoted(L+R);thisiswhatamonauralreceiveruses.Inaddition,thereisaseparatesubcarriermodulatingtheRFwavethatcontainsthedifferencebetweentheleft-andright-channelsignals(L-R).Thissubcarrierisignoredbymonauralreceiversbutisdemodulatedbystereoreceivers.Byaddingandsubtractingthetwodemodulatedsignals,theoriginalstereosignalcanberecovered;thatis,(L+R)+(L-R)=2Land(L+R)-(L-R)=2R.Wewillignorethiscomplicationandshowonlyamonauralreceiverinthediscussionthatfollows.AblockdiagramofamonauralFMreceiverisshowninFigure3.4.Thisblockdiagramshowsasuperheterodynereceiver,whichsimplymeansthattheRFwaveisfirstbroughtdowntoanintermediatefrequency(IF),goesthroughsomesignalprocessingthere,andisthendemodulatedorconvertedtotheaudiosignal3.Heterodyningistheprocessofcombiningtwosignalstoproduceabeattoneandisaccomplishedbymultiplyingthesignals.Figure3.4BlockdiagramofamonauralreceiverReturningnowtotheblockdiagram,wenotethatthereceiverfirstfilterstheRFsignalcomingfromtheantennatochoosetheonestationyouwanttolistento.ThediagonalarrowthroughtheRFfilterblockisacommonwayofshowingthattheblockisinsomewayvariable;inthiscase,youcantunethefiltertoselectdifferentstations.Thissignalisamplifiedandsenttoamixer,whichmultipliesthesignalbyasinewavethatisgeneratedlocallybythelocaloscillator(LO).Tounderstandtheoperationofthemixer,considerasinglesinusoidalinput.Multiplyingtwosinewavesproducessinewavesatthesumofthetwofrequenciesandatthedifferenceofthetwofrequencies.Ifthetwofrequenciesareclose,thedifferencewillbeatamuchlowerfrequencywhereitislessexpensivetoperformfurthersignalprocessing.ItisalsopossibletouseanLOfrequencyequaltothenominalRFfrequency,inwhichcasetheresultingreceiveriscalledadirect-conversion,orhomodyne,receiver4.Inthecaseofbroadcast-bandFM,theIFisat10.7MHz,sotoreceiveastationat100.1MHz,theLOissetto110.8MHz.Theoutputofthemixer,whichistheintermediatefrequency,isthensenttotheIFfilterandamplifier.MostofthefilteringisdoneintheIFsection,sinceitislessexpensivetoimplementgoodfiltersatthatfrequency.Thesefiltersoftenusecrystals.TheRFfilterisonlyapreselectfilter.TheLOfrequencyischangedtomixthedesiredsignalfrequencydowntotheIFfrequency.TheIFamplifierisoftenanautomaticgaincontrol(AGC)amplifierthatchangesitsgainsothattheoutputamplitudeisheldconstant.TheoutputoftheIFsectionthengoesintoademodulatorthatconvertsthefrequencychangesintovoltagechangestoproducetheaudiooutput5.AblockdiagramforabasiccomputerisshowninFigure3.5.Themainelementinthecomputeristhecentralprocessingunit(CPU).Whenthecomputeristurnedon,theCPUinitiallyreadssomebasicinstructionsstoredinthenonvolatileread-onlymemory(ROM).Usingthesebasicinstructions,andperhapsautomaticallyloadingadditionalinformationfromnonvolatilemassstorage(e.g.,diskdrive),thecomputerisreadytorun.Informationthatmustbeaccessedquicklyisstoredinrandom-accessmemory(RAM),andinformationthatisnotneededimmediatelyisstoredonsomeslowermassstoragedevice,mostcommonlyadiskdrive6.Inputfromtheuserisacceptedfromakeyboard,amouse,andperhapsotherdevices;outputcanbedirectedtothescreen,aprinter,orotherperipheraldevices.Figure3.5BlockdiagramofabasiccomputerItisbeyondthescopeofthepresentdiscussiontogointomoredetail.Forexample,therearemanywaysoforganizingtheCPU,andthecomputercanincludemanymoredevicesthanhavebeenshownhereandcanbeconstructedusingmanydifferentcircuittechnologies.Forourpurposes,itwillsufficeatthispointtosaythatthecircuitsinthecomputeraremostlydigital,andtheinformationismanipulatedinbinarywordscomposedofsequencesofbinarydigits.Theprocessofdesignrevisited.Wearenowreadytorevisitthetopicofdesignwithreferencetothetwospecificexamplesystemsdescribedabove.TheprocessofdesignproceedsatmanydifferentlevelsofabstractionasnotedinSection3.1.Inthecaseofacommunicationsystem(i.e.,transmitterandreceiver),onemustfirstdecidewhatformofmodulationtouse—forexample,AMorFM.Thecharacteristicsofthemodulationmustalsobedetermined.Forexample,howfarwillweallowthefrequencyofourcarriertobechangedintheFMcase?Thesedecisionsweremadelongagoforstandardbroadcastsystems,buteveniftheyarealreadydetermined,manychoicesremainforthedesignerofthereceiveritself.Tostartwith,wehaveshownonlyonetypeofreceiver;itispossibletousedifferentapproaches(e.g.,thedirectconversionreceivermentionedbrieflybefore).Assumingourblockdiagram,however,therearestillplentyofchoicestomake.HowmuchfilteringshouldbedoneintheRFsection?HowlargemusttheAGCcontrolrangebe?DoweevenwanttouseanAGCamplifier?(Theperformancewillbedegradedwithsimplerschemeslikeamplitudelimiting,buttheyarecheaper.)Doweusealocaloscillatorthatisanalogandtunedbyvaryingsomecomponentvalue,ordoweuseasingle-frequencyoscillatorandaprogrammabledigitaldividercircuittomakea"digital"FMreceiver?HowmanystagesofIFfilteringdoweuseandwhattype?Whatkindofdemodulatordoweuse?Weseetherearemanyoptionsattheblockdiagramlevelalone.Similarchoicesexistforthecomputer.Howlargeadigitalwordshouldbeused(e.g.,16bit,32bit)fordata?Howlargeadigitalwordshouldbeusedformemoryaddressing?WhatkindofCPUarchitectureshouldbeused?Shouldweuseparallelprocessors?Shouldtherebealocalcachememory?Thereareagainmanymorechoicesthatcouldbelisted,evenattheblockdiagramlevel.Allofthesechoicesinvolvetrade-offsbetweencostandperformance,oroneperformancespecificationoveranother.Someofthesetrade-offsarequitecomplex,andmanydependonthedetailsofhoweachblockisimplemented.Therearesimilarlistsofchoicesandtrade-offstobeevaluatedinthedesignofeachindividualblockaswell;thereare,forexample,severalgoodwaystomakeanAGCamplifier.Thatiswhythedesignprocessbouncesbackandforthbetweendifferentlevelsofabstraction,fromsystem-levelblockdiagramstotheimplementationoftheindividualblocksandbackagain.Theseindividualblocksarethemselvesoftenmadeofmanyidentifiableblocks,andthesamekindofdiscussionappliestothemindividually.Itisimportanttorealizethatoptimumchoicesareveryrarelyidentifiable.Designersshoulddothebesttheycantoconsiderthealternativesandmakeaninformedchoice,butsometimesthechoiceboilsdowntoanarbitraryone.Theimportantpointistomakesurethatwhereclear-cutdifferencesexist,weunderstandthemandmakeinformedchoices.Anyarbitrarychoicesthataremadeshouldnothaveanimpactontheoverallsystemperformanceorcost.(Iftheydo,itreallycan'tbeconsideredarbitrary.)Notethatneitherofthesystemsdiscussedisentirelyanalogorentirelydigital.Therefore,itisimportantforacircuitdesignertoknowsomethingaboutbothkindsofdesign,eventhoughmostdesignerswillspecializeinoneortheother.Itisalsoimportantforthecircuitdesignertounderstandsomeofthesystemissuesinvolvedinthedesign.Bothofthesesystemsaretoocomplicatedforanyonepersontoknoweverythingfromthesystemleveldownthroughthetransistor-levelcircuitdesignandontothesolid-statedevicedesignandfabrication.Nevertheless,fortheoveralldesignteamtobeabletogothroughthekindofprocessdescribed,eachmembermustknowsomethingaboutthedisciplinesthatareadjacenttotheirown.