 The <unk> were designed by Campbell for the purpose of separating <unk> telephone channels on transmission lines , but their subsequent use has been much more widespread than that . The design techniques used by Campbell have largely been superseded . However , the ladder <unk> used by Campbell with the constant k is still in use today with <unk> of modern filter designs such as the <unk> filter . Campbell gave constant k designs for low @-@ pass , high @-@ pass and band @-@ pass <unk> . Band @-@ stop and multiple band <unk> are also possible . 
