Tunable Multi-Band Non-Reciprocal Bandpass Filters

Multi-band non-reciprocal bandpass filters (BPFs) with reconfigurable transfer function characteristics are reported. They are based on $N$ in-series cascaded multi-resonant cells (MRCs) that each comprise two non-resonating junctions and $K$ frequency-tunable resonators resulting in an $N^{\text{th}}$-order $K$-band BPF response. Non-reciprocity is introduced by modulating the resonators' resonant frequencies in both time and space. This is achieved by modulating the capacitance of constituent resonators with DC and AC signals, where the AC signals are progressively phase-shifted from one MRC to the next. The BPF can be tuned in frequency and in the direction of propagation. For practical demonstration purposes, a 3rd-order dual-band lumped-element prototype was designed and tested for center frequencies of 198 MHz and 245 MHz. It demonstrated maximum isolation levels of 26.1 and 35.9 dB and center frequency tuning ranges up to 1.1:1.