Wideband Balanced-to-Balanced Microstrip-to-Coplanar Strip Transitions With Intrinsic Common-Mode Suppression

In this article, a class of microstrip-to-coplanar strip (MS-to-CPS) transitions with wideband balanced-type topologies, efficient synthesis design approach, and well-verified experimental de-embedding are proposed and designed. By virtue of the electromagnetic field conversion along the transitions, the unwanted common-mode (CM) signal could be intrinsically rejected while the differential-mode (DM) counterpart is capable of being converted through a very wideband. In this context, equivalent circuits of such transitions under DM operation are first developed and synthesis design is derived so as to determine each characteristic impedance in these transmission-line (TL) models. Propagation parameters for various CPS lines with varied physical dimensions are then investigated. Experimental de-embedding procedure is described and executed to extract the fundamental responses of a single transition inherently. As a result, a third-order and two different fifth-order MS-to-CPS transitions with respective fractional bandwidth (FBW) of 100%/111%/111% are finally synthesized, designed, fabricated, and measured. Measured results agree well with the theoretical, simulated, and extracted ones, strongly demonstrating and verifying that these proposed transitions feature indeed not only the intrinsic CM suppression but also the wideband DM filtering performances.