Synthesis and Design of Dual-wideband Filter with Composite Series and Shunt Resonators

This paper presents a novel dual-wideband bandpass filter comprising of composite series and shunt resonators and its direct synthesis and design theory. The composite series and shunt resonators can produce a transmission pole (TP) as well as a transmission zero (TZ) flexibly. A shunt capacitor at input/output (I/O) port is introduced to contribute an extra TP. A dual-wideband response with high skirt-selectivity can be achieved by appropriately arranging the TPs and TZs. To synthesize the lumped element prototype of filter, the filtering function is firstly obtained by iteratively solving a deterministic linear problem and the circuit model is found by a circuit extraction procedure. As an example, a dual-wideband bandpass filter with fractional bandwidths of 40% and 20% at center frequencies of 1 GHz and 2 GHz is directly synthesized, fabricated and measured. An expected characteristic is obtained.