Non-Reciprocal 2nd-Order Bandpass Filter by Using Time-Modulated Microstrip Quarter-Wavelength Resonators

A microstrip non-reciprocal 2nd-order bandpass filter is presented by using time-modulated resonators. Quarter-wavelength resonators are loaded with varactors to miniaturize the circuit size. The varactors are reverse-biased by dc voltages and then sinusoidal waves are applied to modulate their equivalent capacitance in time-domain. Circuit analysis shows that by properly introducing phase shift between the two modulation waves, non-reciprocal transmission can be generated. To verify the proposed concept, a microstrip 2nd-order non-reciprocal bandpass filter at 1.0 GHz is designed and simulated. Results show low insertion loss in the forward transmission and large attenuation in the backward transmission, leading to 4.7 dB non-reciprocity in the transmission magnitude.