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1、TECHNOTENo.6JoeCarr'sRadioTech-NotesDealingWithAMBroadcastBandInterferencetoYourReceiverJosephJ.CarrUniversalRadioResearch6830AmericanaParkwayReynoldsburg,Ohio430681DealingWithAMBroadcastBandInterferencetoYourReceiverJosephJ.CarrIfyouliveanywhereneara
2、nAMbroadcastband(BCB)station,thenyoumighthaveseriousreceptionproblems,evenwithahighqualityreceiver.Althoughoneofthethingsyougetwhenyoupaythepremiumpriceforahighqualityreceiverissuperioroverloadprotection,thesignalfromalocalAMBCBstationmightoverwhelmit
3、sdefenses.IntheUSAweusethe530-1700KhzmediumwaveAMBCB.Moststationsoperatewith1,000to10,000wattsofRFoutputpower(althoughafew250to500wattlocalfizzlersalsoexist).Afewstationsaredesignatedclearchannelstations,andoperatewith50,000watts,24-hoursaday.Thesesta
4、tions(e.g.WSMNashville,650Khz)areonfrequenciesthatarenotassignedtootherstationsforadistanceof,Ibelieve,1300milesradius.IfyoulivewithinafewhundredyardsofanAMBCBstation,thenitspossibletoseemorethanonevoltofRFappearingatyourreceiverantennaterminals(alabo
5、ratorymeasured4voltsinonecase!).Giventhatyourreceiverlikestoseesignalsinthedozensofmicrovoltslevel,thenyoucanunderstandtheproblem.TheProblemSowhatistheproblem?Yourreceiver,nomatterwhatfrequencyitreceives,isdesignedtoacceptonlyacertainmaximumamountofra
6、diofrequencyenergyinthefront-end.Ifmoreenergyispresent,thenoneormoreofseveraloverloadconditionsresults.Theoverloadcouldresultfromadesiredstationistoostrong.Inothercases,therearesimplytoomanysignalswithinthepassbandforthereceiverfront-endtoaccommodate.
7、Instillothercases,astrongout-of-bandsignalispresent.Figure1showsseveralconditionsthatyourreceivermighthavetosurvive.Severaldifferentreceiverproblemsresultfromthevarioustypesofoverload,allofwhicharespeciesintermodulationand/orcrossmodulation.Ifyoutunea
8、crosstheshortwavebands,especiallythosebelow10or12Mhz,andnoteanAMBCBsignalthatseemslikeitishundredsofkilohertzwide,thenyouarewitnessingblanketing.IfdrivesthemixerorRFamplifierofthereceivercleanoutofitsmind,producingahugenumberofspurioussignals,
