High isolation and common-mode suppression balanced-to-balanced microstrip diplexer with mixed coupling

Abstract

In this letter, a compact and high isolation balanced-to-balanced microstrip diplexer with mixed coupling is proposed via the combination of two high selectivity and common-mode (CM) suppression balanced bandpass filters (BPFs) without extra junction matching network. The single-ended mixed coupling BPFs are composed of two short-ended stepped impedance resonators (SIRs). The mixed coupling is modeled in a way that both magnitudes and phase differences of constituted capacitive and inductive couplings are made use of in filter synthesis. Multiple transmission zeros (TZs) can be generated by the different electric and magnetic coupling coefficients ratios and electric length ratios. Furthermore, the loading effect of a balanced filter on another balanced filter can also generate a new differential-mode (DM) TZ at the other passband to improve the isolation. Finally, the proposed diplexer for WLAN (wireless local area networks) application is fabricated and measured. The simulated and measured results exhibit a satisfactory agreement to validate the proposed configuration.