Analysis and Design of mmWave Passive CMOS Circulators With Unequal Port Impedances

This paper presents a new theoretical framework for integrated linearly periodically time-varying circulators, with a focus on non-50Ω port impedance scenarios. The current literature predominantly addresses 50Ω port impedance cases, thereby missing opportunities to enhance system features with favorable impedance levels on each port. In particular, at microwave frequencies, the proper choice of a low impedance for the transmitter output and an intermediate-to-high impedance for the receiver input is beneficial. Furthermore, our method boosts antenna performance since an optimum feed impedance can be chosen. We present a theoretical analysis to establish the general conditions for non-reciprocal operation in these unconventional impedance scenarios, and derive a formula that can be used to design the optimal operation. The analysis is validated through simulations of three circulator designs: an ideal S-parameter-based circulator, a switched transmission line circulator, and a switched all-pass network circulator. These designs incorporate non-50Ω port impedances for demonstration purposes and are designed using the proposed methodology for 25GHz operation. The simulations demonstrate that the sample circulators achieve competitive performance in terms of both loss and isolation.