Miniature wideband rat-race coupler in silicon-based integrated passive device technology

In this paper, a novel miniature wideband rat-race coupler design is proposed. The proposed rat-race coupler employs differential bridged-T coils and a phase inverter as building blocks, such that both compact size and wide bandwidth can be achieved at the same time. Specifically, a miniature rat-race coupler for 2.5 GHz application is implemented in silicon-based integrated passive device technology. The circuit size is only 3.275 mm×2.247 mm. The measured to-dB return loss bandwidth for the difference port is larger than 93% while the 15-dB return loss bandwidth is larger than 63%. Very small amplitude and phase errors as well as good isolation performance are also achieved. To the best of our knowledge, the proposed one is the smallest ever reported rat-race coupler with a bandwidth larger than 30%.