Integrated Nonreciprocal Components Based on Linear Time-Varying Circuits

Lorentz reciprocity is a fundamental property of linear time-invariant passive circuits and systems constructed from conventional materials. However, nonreciprocal components, such as circulators, enable new wireless communication paradigms, such as full duplex wireless. Conventionally, nonreciprocal circulators have been realized using ferrite materials that exhibit the magneto-optic Faraday effect, and are consequently incompatible with CMOS, bulky, and expensive. Recent research has shown that reciprocity can be broken and nonreciprocal circulators can be built in CMOS using linear periodically time-varying (LPTV) circuits. This represents an interesting and unique property of LPTV circuits beyond the traditionally exploited tunable high-quality filtering in the so-called “N-path filters”. We will describe the fundamental physical principles, as well as four generations of CMOS circulators and circulator-based wireless systems that target emerging full-duplex and 5G mm-wave applications.