Analysis and Design of a Millimeter-Wave Cavity-Backed Circularly Polarized Radiator Based on Fundamental Theory of Multi-Port Oscillators

This paper presents the design and implementation of a millimeter-wave fundamental frequency circularly polarized radiator, which employs a multi-port cavity as the core of a high- $Q$ resonator, a power combiner, and an antenna. A general theory of multi-port oscillators—and in particular, the widely used circularly symmetric oscillator topologies—is presented, which quantitatively investigates and predicts all possible oscillation states, their associated modes of operation, and the oscillation stability. The outcomes of this theory are utilized to design a power-optimized eight-port rotationally symmetric radiator, excited at the oscillation state with a 45 phase difference between the adjacent ports, which, in turn, enables a high-efficiency circularly polarized radiation. Fabricated in 0.13- $\mu \text{m}$ BiCMOS process, this cavity-backed circularly polarized radiator operates at the frequency of 114 GHz and achieves 14.2-dBm equivalent-isotropic-radiated power (EIRP), 5.2% dc-to-EIRP conversion efficiency, 8.2 dBm radiated power, 1.3% dc-to-radiation efficiency, and −99.3 dBc/Hz phase noise at 1-MHz offset, without using silicon lens.