Chapter 1 Basic Electronics.docx
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Chapter 1 Basic Electronics.docx
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Chapter1BasicElectronics
Chapter1BasicElectronics
(基础电子学)
1.1ReceiverCircuits
(接收电路)
Thepurposeofareceiveristoselectadesiredgroupoffrequenciesfromonetransmitter,getridofallunwantedsignalsandnoise,andthendemodulatethesignaltoobtainthemodulatinginformation.Thebetterthereceiverdoesitsjob,thecloserthedemodulatedsignalwillresembletheoriginalsignalfromthetransmitter.Regardlessofthetypeofdemodulationrequired,themainfunctionsperformedbyareceiverarefilteringandamplifying.Thesuperheterodynereceiveristhelogicalchoiceforthejob.
(接收器的目的是从一台发送器中选择一要求的频率的组,除去全部不需要的信号和噪音,以及demodulate获得调节的信息的信号。
接收器做它的工作越好,demodulated信号将从发送器象原先的信号越近。
不管要求的这类型解调,被接收者执行的主要功能正透过和放大。
超外差接收器是工作的逻辑选择。
)
NEWWORDSANDPHRASES
biasn..偏置
bulkn..大多数
ceramicn.陶瓷
deficiencyn.不足
demodulatev.解调,检波
dioden.二极管
distinguishablea.可分辨的
distortionn.失真
driftn.漂移
ferriten.铁酸盐
filamentn.灯丝
filtern.滤波器
frequencyn.频率
harmonicn.谐波
impedancen.阻抗
inevitablyadv.不可避免
intermodulatev.互调
mixern.混频器
nonlinearityn.非线性
obscureda.模糊的
octaven.倍频程
overloadingv.过载
padn.板,垫
passbandn.通带
phasen.相位
potentiometern.电位计
quartzn.石英
radiatev.辐射
resemblev.像,类似
RFabbr.射频
sensitivityn.灵敏度
skirtn.边缘
specificationn.指标,规格
spuriousa.寄生的,虚假的
superheterodynea.超外差的
tendencyn.趋向
transmittern.发射机
atmospheric大气噪声
balancedmixer平衡式混频器
FM-stereobroadcasting调频立体声广播
imagesignals镜像信号
incandescentlamp白炽灯
intermediatefrequency中频
localoscillator本地振荡器
negativefeedback负反馈
noisefigure噪声系数
phaselockedloop锁相环
tunedcircuits调谐回路
two-terminaldevice双端口器件
1.1.1SuperheterodyneReceiver
(超外差接收器)
Sinceitiseasiertodesignnarrow-band,steep-skinfiltersandobtainhighgainsatlowerfrequencies,the“superhet”receiverisanefficientdesign.Allincomingsignalsaremixedwiththeoutputofalocaloscillatorandthedifferencefrequencyisselectedandamplifiedbytheintermediatefrequencyamplifiers.ThebigbenefitisthattheseamplifiersremainatafixedfrequencyandonlytheRFamplifierandlocaloscillatorneedbetunable.Fig.1.1isablockdiagramofatypicalsuperhetreceiver.Onefurtherbenefitisthefacethatthegainisconcentratedattwoorsometimesthreedifferentfrequencies.Thisreducesthegainrequiredatanyonefrequencyandleadstomorestableamplifiers.Whenover120dBofRFgainisinvolved.everylittlebithelps.
(因为设计窄带,陡皮的过滤器并且在更低的频率获得高的增加是容易的,"superhet"接收器是一种有效率的设计。
全部进来的信号被与一个本地振荡器的产量混合,差别频率被中频放大器选择并且放大。
大的好处是这些放大器在固定频率保持并且只RF放大器和本地振荡器需要是tunable.Fig。
1.1是典型的superhet接收者的方框图。
一更进一步的利润是收获被两点集中或者有时的脸3不同的频率。
这降低在任何一频率需要的收获并且导致更稳定的放大器。
当涉及的射频增益超过120分贝,每一点都有用。
)
ThefunctionofeachiteminFig.1.1canbeexplainedasfollows:
(在图1.1的每个项目的功能可以被解释如下)
1.RFamplifier
(RF放大器)
Itshouldhavejustenoughgain,usuallyabout10dB,toestablishtheoverallnoisefigureofthereceiver.Thetunedcircuitsattheinputandoutputneedonlybeselectiveenoughtorejectimagesignalsandotherspurioussignalsthatcouldintermodulateandappearattheintermediatefrequency.SomeAGCmaybeneededtopreventoverloadingonstrongsignals.TheRFamplifiermayalsobecalledontosuppressanytendencyforthelocaloscillatortoradiateouttotheantennaandinterferewithotherlisteners.
(应该有刚刚足够大约10分贝通常获得,对建立总噪声接收者的身材。
调谐电路在输入和输出需要只选择得足以拒绝图像信号和其他假的信号,那罐装intermodulate并且在中频出现。
一些AGC可能被需要防止在强信号上超载。
RF放大器可能也要求压制适合地方振荡器给天线在外散发出并且干扰其他倾听者的任何趋势。
)
2.Mixerandlocaloscillator
(混和器和本地振荡器)
Themixerhastwoinputs,onefromtheRFamplifierandonefromthelocaloscillator.Thenonlinearitiesofthemixerwillcreatenumerousintermodulationproducts,andoneofthese,thesumordifferencefrequency,willoccurattheIFfrequency.Usually,therewillbeasecondfrequency,theimage,thatcanalsomixwiththeoscillatorfrequencyandproduceanoutputantheIF.Dependingonthetypeofmixerused,conversiongainfrom-10dBto+30dBarecommon.Thelocaloscillatormustbetunable,yethavealowdriftrateandrelativelylowsidebandnoise,sincethiscouldincreasethenoiselevelofthereceiver.
(混和器有二输入端门,来自本地振荡器的RF放大器和一减去一。
那些nonlinearities的那些交际家创造许多互调产品和这些中之1将,那些总数或者差别频率,发生将在如果频率。
通常,将有第2频率,这幅图像,那与也能混合那些振荡器频率并且生产产量一如果。
取决于类型交际家使用,变换从-10dB加快到+30dB是普通的。
本地振荡器一定是可调的,然而低漂流比率和相对低边带niose,这增加那些噪声电平的那些接收者能。
)
3.IFfiltersandamplifiers
(IF过滤器和放大器)
Thissectionestablishestheoverallbandwidthandadjacentchannelselectivityofthereceiver.Thebulkofthereceiver’sgainwillbeconcentratedhereandsometypeofautomaticgaincontrolwillbeincludedtoadjustforvariationsinreceivedsignalstrength.TheIFisusuallyatalowerfrequencythantheRF,but,insomespecialcases,theIFmaybehighertoreducespuriousintermodulationandimageproblems.
(这个部分建立接收者的总的带宽和相邻信道选择性。
接收者的收获的大部分将在这里被集中,一些类型自动增益控制将被包括调整以求在收到的sighal强度方面的变化。
IF通常在更低的频率那些RF,但是,在一些特例内,IF可能高降低假互调和图像问题。
)
4.Demodulators
Foreachtypeofmodulationused(i.e.,AM,FM,SSB,PM),anumberofdifferentcircuitsexist.Somewillhavegain,othersaloss,Somewillrequireareferenceinput(i.e.,SSBandphasemodulation),otherswon’t.ThedemodulationmayalsoberequiredtoproduceoutputstoAGCorAFCcircuits.Therecoveredaudiolevel(orvideo,etc.)willdeterminetheamountofgainrequiredinthefollowingaudioorvideoamplifiers.
1.1.2Specifications
Beforebeginningthedesignofareceiver,itisnecessarytoconsiderthespecificationsrequiredofthefinalresult.Inmostcasesthisendsupasacompromisebetweenwhatthedesignerwouldlikeandwhatispossible.Thedeterminingfactorwillusuallybefinanciallimitations.Thefollowingshouldthenbeconsideredbeforeproceeding:
1.Tuningrange
Whatrangeoffrequenciesmustbetunedandwillitbetunedcontinuouslyorindiscretechannels?
Ashort-wavereceiver,forexample,mustcontinuouslytunefrom3to30MHzandwillusuallyrequiresomebandswitching.Thelocaloscillatorwillbeacontinuouslytunabletype.DemodulatorswillbeneededforAM,SBB,andCW,andIFbandwidthsshouldcorrespond.ForCB,anarrowrangeoffrequenciesfrom26.965to27.405areneededandwillbetunedas40discretechannels.Thelocaloscillatorwillthereforelikelybeaphase-lockedloopsynthesizer.DemodulationcouldbeeitherAMorSSB.
2.Sensitivity
Often,toomuchemphasisisputonsensitivitywithoutattentiontootherdetails.Forexample,a100-kHznavigationreceiverwillpickupsomuchatmosphericnoisethata100-μVdesiredsignalfromtheantennacouldbeobscuredattimes.Ontheotherhand,a0.1-μVsignalat150MHzwilloftenbereadilydistinguishablefrombackgroundnoise.
3.Bandwidth
Whenthemodulationtypeandchannelspacingareknown,itispossibletodeterminetheIFbandwidthanditsskirtcharacteristics.ForAMaircraftcommunications,abandwidthof30kHziscommon—nottoprovidewidebandwidthforhighaudio-frequencyresponsebuttoaccommodatefrequencytolerancesinthetransmittersandreceivers.Thefilter-skirtcharacteristicswillbesettorejectadjacentchannelsignalsasrequired.
4.Spurioussignals
AnotherwisegooddesigncanbeuselessifunwantedsignalscansneakintothereceiverattheIFfrequency(s),theimagefrequency,atvariousspuriousfrequencyrelatedtointermodulationproducts,andthroughcross-modulationproblems.
Typicalspecificationsforseveralgoodreceiversareasfollows:
(1)FMstereotener:
frequencyrange88—108Mhz
Sensitivity:
1.8μVacross300-Ωinputfor20dBofquieting
Selectivity:
100dBforchannels400kHzeithersideofcenterfrequency
Bandwidth:
350kHzat–6-dBpoints
Imagerejection:
90dB
Spuriousrejection:
90dB
IFrejection:
90dB
AMsuppression:
65dB
Captureratio:
1.5dB
(2)Shortwavereceiver:
frequencyrange3.0—30MHz
Sensitivity:
0.5μVfor10dBS+N/Nratio
Bandwidth:
2.3kHzat—6dB,5.5kHzat—60dB(SSBmode)
Imagerejection:
60dB
IFrejection:
75dB
(3)CBreceiver:
frequencyrange26.965—27.405MHz
Sensitivity:
0.5μVfor10dBS+N/Nratio
Bandwidth:
6kHzat—6dB
20kHzat—60dB
Imagerejection:
60dB
Oncethespecificationsarecarefullydetermined,itistimetostartthedesign.Butwhatisthebeststartingpoint?
Generally,themostsensitivepointswillbethetwononlinearcircuits,themixerandthedetector.TheIFamplifiertakesuptheslackbetweenthetwo,andtheRFamplifierpicksupthedeficienciesofthemixer.
1.1.3Mixer
ThemixersectionofthereceivershouldideallyproduceanIFoutputonlyatthedifference(orsum,forup-conversion)oftheinputfrequencies.OnceoftheseinputswillbethelocaloscillatorsignalandtheotherwillbethedesiredRFsignal.Again,ideally,noothercombinationofinputsignalsshouldproduceanIFoutput.Ifsuchfrequenciesdoexist,filtersmustbeprovidedtoremovethembeforethereachthemixer.
Theclosestthingtoanidealmixerisanycircuitwithaperfectsquare-lawtransfercharacteristic.Inadditiontotheinputsignalsandtheirsecondharmonicsappearingattheoutput,thesumandthedifferencewillalsoappear.ThedifferenceisusuallytheonesignaldesiredandsoisselectedbyIFfiltering.TheamplitudeofthedifferencesignalswillbeproportionaltotheproductoftheoriginalRFsignallevelandthelocaloscillatorlevel.AnyothertwosignalsattheinputcouldalsoproduceanoutputattheIFiftheyareseparatedbyanamountequaltothedifferencefrequency.However,theoutputleveltheyproducewillbeproportionaltotheirsignallevels.
SomediscriminationagainstunwantedmixingproductscanthereforebehadifallRFinputlevelstothemixerarekeptaslowaspossibleandthelocaloscillatorsignalkeptashighaspossible.Theonedesiredsignalwillther
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