Multi-functional Bandpass Filters with Frequency Tunability and RF Co-designed Isolator Functionality

This paper reports on the RF design and practical validation of a multi-functional bandpass filtering (BPF) concept with center frequency tunability and RF co-designed isolator functionality. It is based on a hybrid implementation scheme where frequency-tunable passive resonators and non-reciprocal frequency-selective stages (NFSs) are combined to create highorder BPFs with center frequency tunability and fully-directional transfer functions. An NFS is proposed to facilitate the realization of a tunable non-reciprocal resonator with frequency-selective transmission response in the forward direction and high RF signal cancellation in the reverse direction. Tunability in the NFS is achieved by combining a transistor-based network with a tunable capacitively-loaded coupled-line section. For experimental validation purposes, an NFS and a three-stage BPF/isolator were designed, manufactured, and measured at L band. They exhibit the following characteristics: i) NFS: center frequency tuning between 1.54-2 GHz with maximum directivity between 26.8-42 dB. ii) Three-stage BPF/isolator: center frequency tuning between 1.52-2 GHz with maximum directivity between 22-44dB.