New patch resonator bandpass filter with wideband and tunable operation

Abstract

Microstrip bandpass filters play important roles in a variety of microwave circuits and systems. Recently, the increasing demand of wireless communication applications necessitates radio frequency (RF) transceivers operating at dual or multi-frequency bands in order that users can access various services with a single handset, so the dual-band filters which are applied in a dual-band wireless communication system and the multi-band filters are key circuit block in present communications. According to the reported works, dual-band filter is designed by directly cascading the two individual filters with two specified single passbands, and using stepped-impedance-resonator (SIR) structure which controls the second passband by adjusting the impedance ratio and electric lengths of SIRs, or using patch resonators to implement a dual-mode dual-band bandpass filter, by comparison, single patch resonator dual-band filter is seldom reported. Patch loop resonator filter can be seen as a regular resonator with fractal-shaped defection, where, fractal can change the current pattern of filter, and make it distributes along the flexual conductor surface instead of the original simple patch surface, so the electric length increased. On the other hand, fractal-shaped defection can act as perturbation of filter and shift the resonant frequencies, induce dual-mode or multi-mode, and suppress the higher order parasitical harmonics to implement multi-band or wide band as well as miniaturization. In our report, a new type of microstrip bandpass filter using single isosceles trapezoidal resonator with fractal-shaped defection is proposed to implement multi-transmission zeros, wide stopband, and an extraordinarily required dual-band performance, and the second passband is tunable which can be suppressed when it is not required and a ultra-wide stopband property can be obtained. Compared with an isosceles triangular patch filter, the trapezoidal one has more parameter to control, and especially dual-passband and wide band can be implemented, and simultaneously, the filter size is reduced because of the top triangle cut. The designed filter has compact size without coupling gaps, miniaturization and nicer performances.