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.