Compact Wide-Stopband Dual-Band Balanced Filter Using an Electromagnetically Coupled SIR Pair With Controllable Transmission Zeros and Bandwidths

Abstract

This brief presents a dual-band balanced filter with wide stopband, controllable transmission zeros and bandwidths, and compact size based on a newly designed electromagnetically coupled stepped impedance resonator (SIR) pair. The proposed SIR contains three different impedance lines, which can lead to more degrees of design freedom for resonant frequency control. Two electrical length ratios of the SIR can be individually tuned to obtain two desired differential-mode (DM) resonant frequencies for dual-band design, while keeping the first spurious frequency far away for wide-stopband design. Moreover, an inherent common-mode (CM) suppression can also be obtained without adding extra structures due to the discriminating CM resonant frequencies. The mechanism of transmission zeros (TZs) generation is also investigated by means of phases and admittance analysis. It reveals that four TZs around the two operating bands can be flexibly controlled via adjusting the ratio of magnetic and electric coupling coefficients of the SIR pair as well as the coupling space of the feeding lines, which have greatly enhanced the passband selectivities and the band-to-band isolation. Subsequently, the graphs of coupling coefficients and external quality factors for the filter design are extracted, showing sufficient degrees of design freedom for bandwidth control. The proposed dual-band balanced filter is finally fabricated and measured at 1.75 GHz and 3.64 GHz, respectively. Measured results are in good agreement with the simulated ones. Compared with the state-of-the-art works, the proposed circuit performs a deeper and wider stopband of $4.5{f}_{1}^{d}$ with 40dB attenuation (where ${f}_{1}^{d}$ is the fundamental frequency), and more than 50-/45-dB in-band CM suppressions with compact size ( $0.14\lambda _{g} \times 0.27{\lambda }_{g}$ , where ${\lambda }_{g}$ is the guided wavelength of the fundamental frequency).