Non-Reciprocal Bandpass Filter with Tunable Center Frequency and Constant Fractional Bandwidth

This paper presents a novel frequency reconfigurable microstrip non-reciprocal bandpass filter (BPF) with constant fractional bandwidth (FBW) based on time modulated λg/4 stepped impedance coupled-line resonators (SICLRs). The modulation is achieved by loading both ends of the λg/4 resonators with varactor diodes. By modulating the resonant frequencies of the filter’s resonators with phase shifted AC signals, the forward and backward waves can be controlled. In this manner, RF propagation is enabled in one direction (from port 1 to port 2), having the backward wave sufficiently attenuated. In addition, changing the resonant frequencies of the filter’s resonators, the center frequency can be tuned keeping the FBW constant. A two-pole BPF was designed, fabricated and measured demonstrating 1.34:1 center frequency tuning in the non-reciprocal state whereas the reverse isolation was measured up to 29 dB.