60-GHz rat-race coupler using LG-CPW transmission lines in IPD technology

A reduced size rat-race coupler is proposed and designed for operation in the 60-GHz band (57-64 GHz) and is implemented in a glass-substrate IPD (Integrated Passive Devices) technology. A size reduction of 75% compared to that of a conventional rat-race coupler has been achieved by using high impedance lower-ground coplanar waveguide (LG-CPW) transmission lines that are loaded at their ends with microstrip open-circuited stubs. The experimental results of the coupler exhibited an amplitude imbalance of less than 0.5 dB and a phase variation of 5-degrees over the 57-64 GHz band. The achieved measured return loss is greater than 15 dB and the isolation is greater than 24 dB over the 57-64 GHz band. The measured results confirm the potential of the proposed rat- race coupler for use in low-cost, high performance radio front ends for 60-GHz band system-on-package radio-over-fiber communication systems.