Design of Compact Dual-Passband Filters by Parasitic Passband With Controllable Passbands

This letter presents a new method for designing dual-passband filter by parasitic passband. On the one hand, the interdigital capacitor at the openings of the hairpin resonators is used to push the parasitic passband to higher frequency. On the other hand, we can use a coupled structure (stepped impedance resonators) at the bottom of hairpin resonators to make two passbands closer. Four complementary split-ring resonators on both the sides of the tap lines are used to achieve the independent adjustment of bandwidth. When the center frequency of the first passband is fixed at 2.4 GHz, the second passband can be tuned from 3.9 to 6.2 GHz. Meanwhile, we use only one resonant structure to achieve the dual passband response by these methods. Thus, the sizes of the filters are also minimized obviously. Finally, several filters are fabricated and measured to demonstrate the design.