Broadband and Compact Double Stepped-Impedance CPS Filters with Coupled-Resonance Enhanced Selectivity

Three broadband and compact double stepped-impedance coplanar stripline (CPS) filters are proposed and studied. These filters are based on cross-coupled resonances within a high-impedance transmission line section sandwiched between two lower-impedance sections. The initial filter is composed of two high-impedance resonators coupled by a capacitor. In a second design, the upper cutoff selectivity of this filter is improved by an additional attenuation pole (triplet cross-coupled system), which is provided by strips coupled to the high-impedance line. Finally, in the third design the selectivity of both the upper and lower cutoffs is enhanced by the introduction of a shorting strip inductance (quadruplet cross-coupled system). The proposed filters are significantly more compact than typical conventional bandpass filters. The designed filters exhibit a fractional bandwidth of 63%, 70% and 83%, respectively, and a pass-band insertion loss of less than 1 dB.