AMD_highspeed_board_design

High-SpeedBoardDesignTechniques高速电路板设计技术INTRODUCTION简介Themostimportantfactorinthedesignofmanysystemstodayisspeed.66-MHzthru200-MHzprocessorsarecommon;233and266-MHzprocessorsarebecomingreadilyavailable.Thedemandforhighspeedresultsfrom:a)therequirementthatsystemsperformcomplextasksinatimeframeconsideredcomfortablebyhumans;andb)theabilityofcomponentmanufacturerstoproducehigh-speeddevices.Anexampleofa)isthelargeamountofinformationthatmustbeprocessedtoperformeventhemostrudimentarycomputeranimation.Currently,ProgrammableArrayLogic(PAL®)devicesareavailablewithpropagationdelaysof4.5ns,andcomplexPLDssuchasMACH®havepropagationdelaysof5ns.Whilethismightseemfast,itisnotthepropagationdelaythatcreatesthepotentialforproblems,butratherthefastedgeratesneededtoobtainthefastpropagationdelays.Inthefuture,muchfasterdeviceswillbecomeavailable,withcorrespondinglyfasteredgerates.今天，速度成为许多系统设计中最为重要的因素。100MHz到上GHz的处理器已经非常普及，将来，会有更高速度的器件出现，以适应人们对于大量数据的处理，如图形处理、音视频处理等等。Designinghigh-speedsystemsrequiresnotonlyfastcomponents,butalsointelligentandcarefuldesign.Theanalogaspectofthedevicesisasimportantasthedigital.Inhigh-speedsystems,noisegenerationisaprimeconcern.Thehighfrequenciescanradiateandcauseinterference.Thecorrespondingfastedgeratescanresultinringing,reflections,andcrosstalk.Ifunchecked,thisnoisecanseriouslydegradesystemperformance.高速系统的设计不仅需要有高速的器件，还需要设计师的智慧与仔细的工作。这对于数字电路和模拟电路都一样重要。在高速系统中，噪声的产生是一个最值得关注的焦点。高频信号很容易由于辐射而产生干扰，高速变化的(highskewrate)数字信号会导致振铃(ringing)、反射(reflection)、串扰(crosstalk)等，如果没有经过认真的检查，这些噪声将严重降低系统的性能。Thisapplicationnotepresentsanoverviewofthedesignofhigh-speedsystemsusingaPC-boardlayout.Itcovers:‹thepowerdistributionsystemanditseffectonboardinghousegeneration,‹transmissionlinesandtheirassociateddesignrules,‹crosstalkanditselimination,and‹electromagneticinterference.本文讲述高速系统设计的技术，包括‹电源分配系统(powerdistributionsystem)及其影响;‹传输线及其相关的设计规则;‹串扰及其消除;‹电磁干扰。1.POWERDISTRIBUTION1.电源分配(PowerDistribution)Themostimportantconsiderationinhigh-speedboarddesignisthepowerdistributionnetwork.Foranoise-freeboard,itisnecessarytohaveanoise-freepowerdistributionnetwork.NotethatitisjustasimportanttodevelopacleanVCCasitistogetacleanground.ForACpurposes,whichiswhatthisapplicationnotemainlydiscusses,VCCisground.在高速电路板设计中最重要的考虑就是电源的分配网络。电源分配网络必须为低噪声的电路板上各部分的电路提供一个低噪声的电源，包活Vcc和地。注意:一个干净的VCC和一个干净的地是一样重要的，因为对于交流信号，Vcc就是地。Thepowerdistributionnetworkalsomustprovideareturnpathforallsignalsgeneratedorreceivedontheboard.Thisisoftenoverlookedbecausetheeffectofthereturnpathislessapparentatlowerfrequencies.Manydesignsworkevenwhenthenatureofthereturnpathisignored.电源分配网络同时还得为电路板上所有产生或接收的信号提供一个信号回路。这点经常被设计师们忽视，因为在低频电路中，信号回路的影响并不明显，许多设计在被忽视信号回路时也能工作得不错。1.1PowerDistributionNetworkasaPowerSource1.1电源分配网络作为电源1.1.1TheEffectofImpedance1.1.1阻抗的影响Considera5x5boardwithdigitalICsandapowersupplyof+5.0V.Thegoalistodeliverexactly+5.0Vtothepowerpinsofeverydeviceontheboard,regardlessofitspositionrelativetothepowersource.Furthermore,thevoltageatthepinsshouldbefreeoflinenoise.Apowersourcewiththesecharacteristicswouldbeschematicallyrepresentedasanidealvoltagesource(Figure1a),whichhaszeroimpedance.Zeroimpedancewouldensurethattheloadandsourcevoltageswouldbethesame.Italsowouldmeanthatnoisesignalswouldbeabsorbedbecausethenoisegeneratorshavefinitesourceimpedance.Unfortunately,thisisonlyanideal.假如有一块电路板，上面有多个数字系IC和+5V电源，忽略IC相对于电源的位置和线路噪声，那么+5V电源传送到各个IC勤电源管脚，并且保持+5V不变。具有这些特性的电源，可原理性地代表一个理想的电源，它的阻抗是0.。0阻抗保证了电源端的电压与负载端的电压一致，因为，噪声源的源阻抗相对于电源的器阻抗为无穷大，所有的噪声都将被这个理想的电源所吸收。遗憾的是，这仅仅是一个理想而已。理想电源特性如图1a所示。Figure1billustratesarealpowersourcewithassociatedimpedancesintheformofresistance,inductance,andcapacitance.Thesearedistributedoverthepowerdistributionnetwork.Becauseofthenetwork’simpedance,noisesignalscanaddtothevoltage.Thedesigngoalistoreducethepowerdistributionnetworkimpedancesasmuchaspossible.Therearetwoapproaches:powerbusesandpowerplanes.Powerplanesgenerallyhavebetterimpedancecharacteristicsthanpowerbuses;however,practicalconsiderationsmightfavorbuses.图1.b所示为一个实际电源，它具有一定的阻抗，图中以电阻、电感、电容网络的形式表示。事实上，阻抗是分布在整个电源分配网络中。因此，噪声将叠加在电源上。设计的目标就是要尽可能地减小电源分配网络的阻抗。电源分配网络的形式可分为总线式(powerbuses)和电源层式(powerplanes)。通常电源层式比总线式有更好阻抗特性，但实际上，有的情况下，总线会更好。1.1.2PowerBusesvsPowerPlanes1.1.2电源总线和电源层Twopower-distributionschemesareshowninFigure2.Abussystem(Figure2a)iscomposedofagroupoftraceswiththevariousvoltagelevelsrequiredbythesystemdevices.Forlogic,thesearetypically+5Vandground.Thenumberoftracesrequiredforeachvoltagelevelvariesfromsystemtosystem.Apower-planesystem(Figure2b)iscomposedofentirelayers(orsectionsoflayers)coveredwithmetal.Eachvoltagelevelrequiresaseparatelayer.Theonlygapsinthemetalarethoseneededforplacingpinsandsignalfeed-throughs.两种电源分配方案如图二所示。总线系统由一组具有电路板所需的各个电压的电源线组成。电源层系统则是由一个或多个电源层或多个层的电源部分组成，其中因需与元件管脚和过孔隔离有一些通孔。Earlydesignsfavoredbusesbecauseoftheexpenseofdevotingentirelevelstopowerdistribution.Thepowerbusshareslayerswiththesignallines.Thebusmustsupplypowertoalldevices,whileleavingroomforthesignaltraces;therefore,busestendtobelong,narrowribbons.Thisresultsinarelativelysmallcross-sectionalareawithasmallre