Common- and differential-mode reflectionless balanced nonreciprocal bandpass filter

This paper presents the first reported design of a balanced nonreciprocal bandpass filter with both common-mode (CM) and differential-mode (DM) reflectionless characteristics. The nonreciprocal behavior is achieved using a time-modulated resonator, which isolates in-band backward interference signals, thereby protecting preceding circuits from their negative effects. To solve the negative effects of reflected waves of the reflection-based CM noise suppression and reflection-based DM stopband attenuation, CM and DM reflectionless structures are integrated at both the input and output ports, ensuring reflectionless operation for both CM and DM signals. Meanwhile, the implementation of DM reflectionless characteristics effectively addresses the issue of reflection zero degradation typically observed in time-modulated resonator-based nonreciprocal filters. The proposed filter exclusively transmits differential forward signals, which will greatly improve the anti-interference ability and stability of the balanced RF circuits. To validate the concept experimentally, a 1.5-GHz microstrip prototype is designed, simulated, fabricated, and characterized.