A Slow-Wave Multilayer Synthesized Coplanar Waveguide and Its Applications to Rat-Race Coupler and Dual-Mode Filter

We investigate a novel slow-wave structure, namely, the multilayer synthesized coplanar waveguide (CPW), in this paper. This new slow-wave structure is developed on a double-layer printed circuit board with the quasi-lumped inductors and capacitors placed on the opposite side of the substrate. The new design has a high slow-wave factor of 9–10 and moderate unloaded quality factor of 20–35. When compared with previous designs, the synthesis procedure of this slow-wave structure is simplified owing to the reduced parasitic coupling between the quasi-lumped elements. To demonstrate its outstanding miniaturization capability, a miniaturized rat-race coupler and a miniaturized dual-mode ring bandpass filter are experimentally verified. The coupler is merely 6.4% the size of a conventional CPW design, while the dual-mode filter is only two-thirds the size of the second smallest one reported in the literature using printed circuit board technology. The circuit responses, in the meantime, remain comparable with their conventional counterparts. The design methodology, synthesis procedure, and simulated and experimental results are discussed in detail in this paper.