Differential Dual-Band Filters With Flexible Frequency Ratio Using Asymmetrical Shunt Branches for Wideband CM Suppression

In this paper, a group of differential dual-band bandpass filters based on a single quad-mode stub-loaded twin-ring resonator (TRR) are presented, which utilize asymmetrical shunt branches for common-mode (CM) suppression. Under differential-mode (DM) operation, the stub-loaded TRR manifests quad-mode characteristic for dual-band design. The frequency ratio of the two passbands can be flexibly selected according to the form of the loaded stubs. Different from the inline topology, differential multimode structures inevitably suffer from the hardship in CM suppression, especially for dual-band designs. Despite previous work fulfilled moderate CM suppression via embedded defected ground structure units, but the CM noise in the DM lower band can just be shifted away rather than suppressed. Instead, a novel CM-suppression approach is devised attributing to the asymmetrical short-terminated branches along the bisection line of the TRR. The transmission of the CM noise near the DM passbands can be blocked effectively. Furthermore, multiple CM notches can be obtained by the cooperation of the asymmetrical branches and source-load coupling, leading to the favorable CM suppression in a wide frequency range. For verification, a prototype differential dual-band filter based on the open stub-loaded TRR has been realized with frequency ratio which is larger than two. Similarly, a differential dual-band filter using the short-circuited stub-loaded TRR is also presented as an extension for frequency ratio which is smaller than two. The simulation and experiment results of the two demonstrated filters are given, showcasing a good agreement.