On miniaturization and performance improvement of planar wideband bandpass filters without coupled lines

Two classical approaches are compared in various respects for the design of wideband planar BPF´s without parallel coupled lines, namely, BPF´s designed by pole placement with contributing unit elements (UE) and inverter coupled resonator filters designed from LP prototypes. It is shown that the circuit obtained by direct BP synthesis is a special case of inverter coupled filter with inverters converted into single unit elements. It is also shown that if the inverters are converted into odd number of UE´s then they become contributing UE´s which can be used to improve skirt slopes considerably. For transversal size reduction and reshaping the stopband response, equivalent circuits of all-stub resonators are derived by starting from distributed element resonator models. The techniques are applied on a unit element coupled all-stub resonator BP filters resulting in wide spurious free stopbands with reduced size.