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1、Deliyannis,TheodoreL.etal"Frontmatter"Continuous-TimeActiveFilterDesignBocaRaton:CRCPressLLC,1999TheodoreL.DeliyannisUniversityofPatras,GreeceYichuangSunUniversityofHertfordshire,U.K.J.KelFidlerUniversityofYork,U.K.Continuous-TimeActiveFilterDesign©1999CRCPressLLCPreface“Inthisdigital
2、age,whoneedscontinuous-timefilters?”Suchanobviousquestion,andonewhichdeservesanimmediateresponse.True,wedoliveinadigitalage—digitalcom-puters,digitalcommunications,digitalbroadcasting.But,muchthoughdigitaltechnologymaybringusadvantagesoveranalogsystems,attheendofthedayadigitalsystemmus
3、tinterfacewiththerealworld—theanalogworld.Forexample,togaintheadvantagesthatdigitalsignalprocessingcanoffer,thatprocessingmusttakeplaceonbandlimitedsignals,ifunwantedaliasingeffectsarenottobeintroduced.Aftertheprocessing,thesignalsarereturnedtotherealanalogworldafterpassingthrougharec
4、onstructionfilter.Bothband-limitingandreconstructionfiltersareanalogfilters,operatingincontinuoustime.Thisisbutoneexample—butanysystemthatinterfaceswiththerealworldwillfinduseforcontinu-ous-timefilters.Thetermcontinuous-timeperhapsneedssomeexplanation.Therewasthetimewhenana-logfilterswereju
5、stthat—theyprocessedanalogsignalsinrealtime,incontrasttodigitalfilterswhichperformedfilteringoperationsondigitalrepresentationsofsamplesofsig-nals,oftennotinrealtime.Theninthe1970s,alongcamesampleddatafilters.Sampleddatafiltersdidnotworkwithdigitalrepresentationsofthesampledsignal,butoper
6、atedonthesamplesthemselves.Perhapsthebestknownexampleofsuchanapproachisthatofswitched-capacitorfilters,whichasthenamesuggests,useswitches(usuallytransistorswitches)togetherwithcapacitorsandactivedevicestoprovidefilterfunctions.Notethatthesefiltersarediscontinuousintimeasaresultoftheswitc
7、hingwhichtakesplacewithinthecircuits;indeedcontinuousbandlimitingandreconstructionfiltersareneededasaresult.Muchresearchtookplaceinthe1970sand1980sonswitchedcapacitorfiltersasaresultoftheadvantagesforintegratedcircuitrealizationthattheypromised.Therewassomuchstressonresearchinthisareath
8、atdev
