Broadband vertical transitions at millimeter-wave frequencies using microstrip-fed cavity couplers

Vertical interconnections between various signal lines in different layers are often needed in a multi-layer circuit environment. Conventionally, via holes are used as vertical transitions between different layers. As the operating frequency increases, the fabrication of via holes becomes very difficult. To circumvent this problem, non-direct-contact vertical transitions are necessary. Vertical transitions using microstrip-fed cavity couplers are proposed. Simulation results indicate that the FEM technique can provide more accurate prediction for the S-parameters of a cavity coupler compared to the MoM technique employed in earlier investigations for similar structures. A design example of cavity-coupler-based vertical transitions is presented for millimeter-wave applications. It is shown that this vertical transition has a 1 dB bandwidth ratio of up to 85% and its performance is not susceptible to various errors which frequently occur in the fabrication process.