A Cascaded Self-Similar Rat-Race Hybrid Coupler Architecture and its Compact Ka-Band Implementation

We present a cascaded rat-race coupler architecture that performs broadband in-phase and differential signal generation while maintaining a very compact passive structure size. A proof-of-concept 24-42GHz 2nd order cascaded rat-race coupler is implemented in the Global foundries 45nm CMOS SOI process. Based on 3Dfull-wave electromagnetic EM modeling and simulations, the cascaded rat-race design achieves a less-than 5 phase error, less-than 1dB magnitude mismatch, an average insertion loss of only 2.5dB, and better-than -10dB input matching across the entire Ka-band.