Microstrip-to-stripline planar transitions on LTCC

This paper presents the design of microstrip to stripline transitions built on the same layer of substrate of a single LTCC wafer. To achieve this geometry a cavity is opened on the substrate layer just above the stripline removing the upper ground plane, morphing the stripline into a microstrip line. The main interest of these transitions is the ability to excite stripline circuits directly from the same layer, as well as use them as 50Ω matched interconnection between buried lines and MMIC's. Two transitions of this kind are realized and measured. Comparison of measured circuits and simulations provide insight on what are the effects of the fabrication tolerance of critical structures, such as substrate cavities and ground planes.