RF Co-Designed Bandpass Filters/Isolators Using Nonreciprocal Resonant Stages and Microwave Resonators

This article reports on the RF design and practical development of RF co-designed bandpass filters/isolators (BPFIs). They are based on series-cascaded nonreciprocal resonant stages, microwave resonators, and multiresonant cells. The nonreciprocal stages are shaped by an in-parallel cascaded transistor-based path and a transmission line (TL) that result in a zero-phase resonance in the forward direction and high isolation (IS) in the reversed one. The operating principles of the nonreciprocal resonant stage are presented through circuit analysis and coupled-resonator diagrams (CRDs) that facilitate their design with coupled-resonator-based filter design methods. Various CRD-synthesized examples of low- and high-order transfer functions with and without transmission zeros are shown. They demonstrate the applicability of the proposed method for alternative-type of filtering responses with and without gain in the forward direction and high levels of IS in the reversed one. For proof-of-concept demonstration purposes, five prototypes were designed, manufactured, and measured at S-band.