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APRIL 2023 I DESIGN007 MAGAZINE 33 e CPW offers several advantages over a conventional microstrip transmission line: • Simplifies fabrication • Facilitates easy shunt as well as series surface mounting of active and passive devices • Eliminates the need for via holes and wraparound (ground plating on the edge of a substrate to provide a low inductance path) • Reduces radiation loss at very high microwave frequencies e impedance of a CPW is determined by the ratio of trace width to clearance, so size reduction is possible without limit, the only penalty being higher losses. In addition, a virtual ground plane exists between any two adjacent lines, as there is no field at that point. Hence crosstalk effects, between parallel trace segments, are extremely weak. Despite the efforts to evolve and improve the existing transmission line structures, it still remains a technological challenge, which necessitates the emergence of a revolutionary concept. As digital transmission frequencies head toward 100 GHz and beyond, the current main- stream PCB technology—the copper intercon- nect—is reaching its performance threshold. Ultimately, dielectric loss, copper roughness, and data transfer capacity are the culprits. However, the biggest performance restric- tion for PCB interconnects is the size of the conductor. Metallic waveguides, on the other hand, are a better option compared to tradi- tional transmission lines, but they are bulky, expensive, and non-planar in nature. Recently, substrate integrated waveguide (SIW) struc- tures have emerged as a viable alternative and are ideally suited to the high-speed transmis- sion of electromagnetic waves. SIWs are planar structures fabricated using two periodic rows of PTH vias or plated slots connecting adjacent copper ground planes of a dielectric substrate as shown in Figure 2 (le). Several types of transition from SIWs to microstrip or CPW structures are possible but most are challenging to implement. ey can be roughly divided into single-substrate or multilayer substrate applications. Dual-layered SIW transitions to microstrip or CPW struc- tures have been successfully applied. But mul- tilayer SIW circuits oen suffer from alignment issues. Z-axis alignment of the multilayer lami- nate book has always been a major limitation of implementing any broadside-coupled applica- tion. However, due to the similarity between the traditional waveguide and microstrip modes, the dual-layer microstrip-to-SIW transition is undoubtedly the simplest to implement. Figure 3 illustrates the transition from a microstrip transmission line to a SIW. e propagating electromagnetic wave, guided by the microstrip trace, travels through the dielec- tric between layers 1 and 2 and radiates from the solder mask into the surrounding volume. However, as the wave enters the SIW, it begins to tunnel between the ground planes and as such, the dispersion losses are solely based on the losses of the substrate material. e simula- tion of the electric field shows how the losses Figure 2: The SIW (left) features similar properties to the metallic waveguide (right).