Wideband Balanced Filters With Intrinsic Common-Mode Suppression Using Coplanar Strip Double-Sided Shunt-Stub Structures

In this article, two coplanar-strip (CPS) wideband balanced bandpass filters with intrinsic common-mode (CM) suppression are proposed based on shunt-stub structures. As previously reported with higher integration and greater flexibility, the double-sided shunt-stub structures could make sufficient use of various transmission lines (TLs), such as double-sided parallel-strip line (DSPSL), microstrip line (MSL), and slot line (SL). Compared with its conventional dual circuit, the proposed structure herein is capable of realizing ideal CPS-based resonant characteristics without degraded performance. As such, a DSPSL-loaded CPS triple-mode resonator and a microstrip-to-CPS (MS-to-CPS) transition are developed as two distinctive double-sided shunt-stub element units. Meanwhile, their equivalent circuit models are generalized for analysis of these proposed shunt-stub units and the corresponding synthesis design is further theoretically derived. For experimental verification, one fifth-order and one seventh-order balanced filters are designed, fabricated, and measured. It has been well demonstrated that both the wideband differential-mode (DM) filtering property and intrinsic CM suppression are satisfactorily achieved at the same time, with a certain range of available bandwidths.