Design Method for Quasi-Coaxial Transition

Abstract

Quasi-coaxial transition can be used between substrate integrated coaxial line (SICL) and ground coplanar waveguide (GCPW) for high-speed interconnection. In this work, a design method for the quasi-coaxial transition is proposed for high bandwidth and high-speed transmission rate. The propagation modes of SICL, GCPW and the transition are analyzed to show that the bandwidth of the total interconnect structure is limited by the cutoff frequency of the first higherorder mode of the quasi-coaxial transition. Hence, the transition should be designed to be small enough for wide bandwidth. On the other hand, the transition is usually designed to be $50 \Omega$ for the lowest return loss, which limits the size of the transition. Finally, the design method for the quasi-coaxial transition is proposed to make tradeoff between its characteristic impedance and the cutoff frequency for the highest bandwidth. Simulation results show that the largest 3 dB bandwidth of the total interconnect structure can be obtained when the characteristic impedance of the transition is $47 \Omega$ instead of $50 \Omega$ . The largest bandwidth is 130 GHz, which is 26% larger than that when the quasi-coaxial transition is $50 \Omega$ . Therefore, the design method can significantly improve bandwidth of the interconnect structure including SICL, GCPW and the quasi-coaxial transition.