Interconnects and Transitions Across Dissimilar Transmission Lines: Enabling System Integration and Applications

This presentation reviews the historical background, current advances, and future directions of transitions and interconnects between dissimilar transmission lines in the design of RF, microwave, and mm-wave circuits and systems. These developments stem from the fundamental reality that no single transmission-line technology can fulfill all the requirements of a complete system architecture. Design considerations such as transmission loss, bandwidth, mode control, integration format, power handling, component interfacing, and signal crosstalk must all be balanced to achieve optimal performance. The talk covers a wide range of planar and non-planar structures such as substrate integrated waveguides, encompassing both TEM and non-TEM modes, over frequencies extending from MHz to THz. Particular emphasis is placed on interconnects such as vias and feedthroughs, which are increasingly critical in technology-driven interposers, advanced packaging solutions, integrated passive devices (IPDs), and chiplet-based ecosystems. Practical design examples — including horizontal and vertical waveguide-to-planar transitions — will be presented to illustrate implementation challenges and solutions. Finally, emerging trends in high-speed wireline and wireless interconnects will be discussed, with a focus on their role in future datacenter links and short-range communication applications.