The filters were designed by Campbell for the purpose of separating multiplexed 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 The design techniques used by Campbell have largely been superseded .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 topology used by Campbell with the constant k is still in use today with implementations of modern filter designs such as the Tchebyscheff filter . Campbell gave constant k designs for low @-@ pass , high @-@ pass and band @-@ pass filters . Band @-@ stop and multiple band filters are also possible .