▁decre ases ▁and ▁their ▁R F ▁imped ance ▁increases . ▁Although ▁they ▁are ▁still ▁slightly ▁conduct ive ▁to ▁the ▁R F ▁path , ▁the ▁at ten u ation ▁they ▁introduce ▁decre ases ▁the ▁unw anted ▁signal ▁to ▁an ▁accept ably ▁low ▁level . ▁They ▁are ▁often ▁used ▁in ▁pairs ▁to ▁switch ▁between ▁two ▁different ▁R F ▁sources ▁such ▁as ▁the ▁V H F ▁and ▁U H F ▁bands ▁in ▁a ▁television ▁tun er ▁by ▁supp lying ▁them ▁with ▁complement ary ▁bias ▁volt ages . ▁ ▁Harm onic ▁multiplication ▁In ▁some ▁applications , ▁such ▁as ▁harm onic ▁multiplication , ▁a ▁large ▁signal ▁amplitude ▁altern ating ▁voltage ▁is ▁applied ▁across ▁a ▁var ic ap ▁to ▁deliber ately ▁vary ▁the ▁capac it ance ▁at ▁signal ▁rate ▁to ▁generate ▁higher ▁harm on ics , ▁which ▁are ▁extracted ▁through ▁filtering . ▁ ▁If ▁a ▁s ine ▁wave ▁current ▁of ▁sufficient ▁amplitude ▁is ▁applied ▁driven ▁through ▁a ▁var ic ap , ▁the ▁result ant ▁voltage ▁gets ▁" pe aked " ▁into ▁a ▁more ▁tri angular ▁shape , ▁and ▁odd ▁harm on ics ▁are ▁generated . ▁ ▁This ▁was ▁one ▁early ▁method ▁used ▁to ▁generate ▁mic row ave ▁frequencies ▁of ▁moder ate ▁power , ▁ 1 – 2   G Hz ▁at ▁ 1 – 5 ▁w att s , ▁from ▁about ▁ 2 0 ▁w att s ▁at ▁a ▁frequency ▁of ▁ 3 – 4 0 0   M Hz ▁before ▁ade qu ate ▁trans ist ors ▁had ▁been ▁developed ▁to ▁operate ▁at ▁this ▁higher ▁frequency . ▁This ▁technique