A Parallel Doubly Coupled Dual-Band Bandpass Filter

A parallel doubly coupled stepped impedance resonators (SIRs) structure is proposed in this paper to build a 2.4/5.2 GHz dual-band bandpass filter for a WLAN application. The prototype filter is a three-stage design composed by four SIRs in conjunction with the tapped-line used in the I/O ports to create zeros by the passband skirts. The proposed filter has the advantages of a much wider fractional bandwidth in both the passbands without sacrificing the passband's insertion losses. The conventional parallel-coupled microstrip line model is applicable in the proposed filter design. The filter has obtained almost twice wider bandwidth compared to that possessed by the counterpart filter of a conventional parallel-coupled line configuration. It is convinced that this filter design can be apply to the 2.4/5-GHz dual-band operation that needs a wider second passband. The experiment is conducted for filter performance validation. A very good agreement is observed between the simulation and the measurements