标签: max1479

Inaremotekeylessentry(RKE)systemacarisunlockedfromadistancebyaradiocodesentfromakeyfobtoareceiverinthecar.RKEsystemsusuallyoperateatfrequenciesintheunlicensedISMbandthatincludes315MHzand433.92MHz.Withtheadventofremotestartingfeaturesandtwo-wayRKEwithverification,designerswanttoincreasetheusablerangeoftheseshort-rangedevices.ReplacingSAWresonatortransmitterswithhigh-frequency,tolerancephase-locked-looptransmittersallowsreceiverswithnarrowerbandwidthtobeused.RFICreceiverstakeadvantageofthesenarrowerbandwidthstoimprovesensitivity,whichinturnincreasestheusablerange.Oncethetransmittedpowerandthereceiversensitivityareestablished,thepathlossofthesignalfromtransmittertoreceiveristhemaindeterminationofusablerange.Thisapplicationnotedescribeshowpathlossisaffectedbythegroundbounceoftheradiosignalandgivesasimplifiedapproximateequationforpathloss.Graphspredictpathlossasafunctionofdistanceinanemptyparkinglot.Finally,thenoteoffersguidelinesforpredictingtheeffectsofmultipathsignalsandblockage.……