Design of lumped rat-race coupler in multilayer LTCC

This paper proposes a compact lumped rat-race coupler configuration. Combining the advantages of lumped elements - low spurious problem and predictable response - with multilayer low-temperature co-fired ceramic technology, circuit layouts can be integrated vertically to realize a miniaturized rat-race coupler. The proposed coupler uses high-pass and low-pass lumped models to replace the transmission line sections of the conventional design. A new structure with enhanced bandwidth is achieved by increasing the order of the 270° transmission line section. Measurement results are shown to validate the design and theoretical prediction, demonstrating 98.63% area reduction compared to conventional rat-race coupler and a 50% fractional bandwidth.