The Challenge of Active Circulators

Ferrites are the traditional materials used in conventional passive circulators [1]-[3]. One of the first ferrite microwave circulators was the Faraday rotation circulator, which is based on Faraday rotation, the phenomenon that occurs when microwaves of a specific polarization are incident upon a ferrite material subjected to a magnetic field parallel to the direction of propagation. These circulators were later replaced by resonance isolators and differential phase shift circulators, which have higher power-handling characteristics and are simpler to construct. In 1964, the lumped-element circulator [4] was brought to the attention of the microwave industry. The lumped-element Y circulator uses ferrite and has a mesh mechanism in place of the ordinary strip-line circulators center conductor. Ferrites are too expensive and bulky for practical use in integrated circuits.