Novel design of microstrip bandpass filters with a controllable dual-passband response: Description and implementation

Novel microstrip dual-band bandpass filters with controllable fractional bandwidths and good in-between isolation are presented and implemented. A half-wavelength stepped-impedance resonator is firstly characterized, aiming at producing the two resonant frequencies at 2.4 and 5.2GHz. Two types of coupled microstrip lines in the parallel and anti-parallel formats are then investigated in terms of unified equivalent J-inverter network. Extensive results are derived to quantitatively show their distinctive frequency-distributed coupling performances under different coupling lengths. The coupling degrees of these two coupled lines at the two resonances are properly adjusted to achieve the dual-passband response with varied or tunable bandwidths. In addition, the parallel coupled line is modeled to bring out a transmission zero between the two resonances so as to achieve the good in-between isolation. The three two-stage bandpass filters are initially designed to exhibit their dual-band response with changeable dual-band bandwidths. A three-stage dual-band filter is in final optimally designed and its predicted performance is confirmed in experiment.