RF Co-Designed Bandpass Filter/Circulator With Tunable Center Frequency, Bandwidth, and Out-of-Band Isolation

This letter reports on a new type of miniaturized RF co-designed bandpass filter/circulator (BPFC). Miniaturization is achieved by: 1) capacitively loading a multi-mode ferrite-based disk resonator; 2) combining the function of an RF circulator and a bandpass filter (BPF) within the volume of a single microwave component; and 3) by tuning its transfer function. The proposed BPFC exhibits a quasi-elliptic power transmission response in the forward direction-passband in-between four transmission zeros (TZs)-and an all-stop response in the reverse one. Reconfigurability is achieved by tuning the capacitive loads of the ferrite disk and the mixed electromagnetic coupling elements in the RF input/output. For proof-of-concept validation purposes, an S-band tunable prototype was designed, built, and measured. It demonstrated multiple levels of tuning. These include the frequency tuning of 1.27:1, the bandwidth tuning of 1.73:1, the out-of-band isolation tuning, and intrinsic switching-off.