Novel Second-Order Dual-Mode Dual-Band Filters Using Capacitance Loaded Square Loop Resonator

This paper presents a novel approach for designing dual-mode dual-band bandpass filters using capacitance loaded square loop resonators (CLSLR). The CLSLR features compactness and spurious response suppression because of the loaded capacitance. A dual-band response is obtained via an extremely large perturbation in a single resonator and second-order dual-mode dual-band filters are realized by a new cascading principle. Coupling coefficients between two resonators in both bands can be controlled independently and external quality factors are also controlled by a particularly-designed coplanar waveguide (CPW) feed line. Three types of filters are designed to validate the analysis. They are direct coupling dual-mode dual-band filter, source-load coupling dual-mode dual-band filter with a better band-to-band isolation and CPW-feed dual-mode dual-band filter with a wider bandwidth. Measured results show good agreement with the design specifications.