A Simple Method to Design Wideband Electronically Tunable Combline Filters

Abstract—A new systematic approach for designing wide-band tunable combline filters is presented. New results on tunable combline filter theory are proposed and explicit de-sign formulas, to obtain the filter design parameters from specifications, are included. These design parameters are: center frequency, resonator electrical length, instantaneous bandwidth and tuning capacitance. The proposed design technique is used to construct an X-band wideband mi-crostrip tunable filter from 8.0 GHz to 12.0 GHz with com-mercial GaAs FETs as tuning elements. Parasitic effects and simulation problems are also discussed