icted ▁at ▁the ▁right , ▁a ▁single ▁dual - package ▁var ic ap ▁di ode ▁adjust s ▁both ▁the ▁pass band ▁of ▁the ▁tank ▁circuit ▁( the ▁main ▁station ▁selector ), ▁and ▁the ▁local ▁oscill ator ▁with ▁a ▁single ▁var ic ap ▁for ▁each . ▁This ▁is ▁done ▁to ▁keep ▁costs ▁down ▁– ▁two ▁dual ▁packages ▁could ▁have ▁been ▁used , ▁one ▁for ▁the ▁tank ▁and ▁one ▁for ▁the ▁oscill ator , ▁four ▁di odes ▁in ▁all , ▁and ▁this ▁is ▁what ▁was ▁dep icted ▁in ▁the ▁application ▁data ▁for ▁the ▁LA 1 8 5 1 N ▁AM ▁radio ▁chip . ▁Two ▁lower - cap ac it ance ▁dual ▁var act ors ▁used ▁in ▁the ▁FM ▁section ▁( which ▁oper ates ▁at ▁a ▁frequency ▁about ▁one ▁hundred ▁times ▁greater ) ▁are ▁highlight ed ▁by ▁red ▁ar rows . ▁In ▁this ▁case ▁four ▁di odes ▁are ▁used , ▁via ▁a ▁dual ▁package ▁for ▁the ▁tank ▁/ ▁band pass ▁filter ▁and ▁a ▁dual ▁package ▁for ▁ ▁the ▁local ▁oscill ator . ▁ ▁Switch ing ▁ ▁Special ▁types ▁of ▁var ic ap ▁di ode ▁exhib iting ▁an ▁ab rupt ▁change ▁in ▁capac it ance ▁can ▁often ▁be ▁found ▁in ▁consumer ▁equipment ▁such ▁as ▁television ▁tun ers , ▁which ▁are ▁used ▁to ▁switch ▁radio ▁frequency ▁signal ▁paths . ▁When ▁in ▁the ▁high ▁capac it ance ▁state , ▁usually ▁with ▁low ▁or ▁no ▁bias , ▁they ▁present ▁a ▁low ▁imped ance ▁path ▁to ▁R F , ▁whereas ▁when ▁reverse ▁bi ased ▁their ▁capac it ance ▁ab rupt ly