automatic gain control

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1、GSMPowerAmplifierControlLoopDesignAutomaticGainControl))1.Introduction:Poweramplifiercontrol(PAC)foraGlobalSystemforMobilecommunications(GSM)compatibleradioisoneofthemorechallengingaspectsoftheGSM-basedsystemdesign.Notonlymusttheradiomeetalloutputradiofrequency(RF)spectrum

2、specifications,butthePowerAmplifier(PA)controlloopmustalsobestableundervaryingenvironmentalconditions.Automaticgaincontrol(AGC)wasimplementedinfirstradiosforthereasonoffadingpropagation(definedasslowvariationsintheamplitudeofthereceivedsignals)whichrequiredcontinuingadjust

3、mentsinthereceiver’sgaininordertomaintainarelativeconstantoutputsignal.Suchsituationledtothedesignofcircuits,whichprimaryidealfunctionwastomaintainaconstantsignallevelattheoutput,regardlessofthesignal’svariationsattheinputofthesystem.Originally,thosecircuitsweredescribedas

4、automaticvolumecontrolcircuits,afewyearslatertheyweregeneralizedunderthenameofAutomaticGainControlcircuits(AGC).WiththehugedevelopmentofcommunicationsystemsduringthesecondhalfoftheXXcentury,theneedforselectivityandgoodcontroloftheoutputsignal’slevelbecameafundamentalissuei

5、nthedesignofanycommunicationsystem.Nowadays,AGCcircuitscanbefoundinanydeviceorsystemwherewideamplitudevariationsintheoutputsignalcouldleadtoalostofinformationortoanunacceptableperformanceofthesystem.AGCsystemsarepartofanywirelesscommunicationsystemwhereaconstantoutputsigna

6、lisdesired.ThecomplexityoftheAGCsystemisdeterminedbytherequirementsofthecommunicationsystem,thereforetheanalysis,designandimplementationcanbecomequitedifficult.AGCsystemsandcircuitswillcontinuetoevolveaslongaswirelesstechnologybecomesfaster,smallerandmorecomplex.Newdevices

7、,circuitsandtechniquesmustbestudied,developedandimplemented.2.ObjectivesofusingAGCcircuits:AutomaticGainControl(AGC)circuitsareemployedinmanysystemswheretheamplitudeofanincomingsignalcanvaryoverawidedynamicrange.TheroleoftheAGCcircuitistoproviderelativelyconstantoutputampl

8、itudesothatcircuitsfollowingtheAGCcircuitrequirelessdynamicrange.Ifthesignallevelchangesa