Frequency Tunable Impedance Matching Nonreciprocal Bandpass Filter Using Time-Modulated Quarter-Wave Resonators

In this article, we present a frequency tunable magnetless nonreciprocal bandpass filter (BPF) that uses time-modulated quarter-wave microstrip line resonators. The proposed nonreciprocal BPF can transform real-to-real, real-to-complex, and complex-to-complex termination impedances. To achieve nonreciprocity, a modulation signal is directly applied to the varactor through the transmission line. The modulation scheme in the proposed nonreciprocal BPF, while simple, nevertheless achieves an excellent nonreciprocal response. The center frequency (f0) and nonreciprocal response of the proposed BPF are tuned by changing the dc bias voltage of the varactor. The design is validated by experiments using four prototypes of nonreciprocal BPFs (filter A: 50-to-50 Ω, filter B: 20-to-50 Ω, filter C: 25+j10-to-50 Ω, and filter D: 25+j10-to-55+j10 Ω). The measured results confirm that the f0 of the nonreciprocal BPF is tuned from 1.64 to 1.97 GHz (330 MHz) with a forward insertion loss variation of 4.96 to 3.94 dB and backward isolation of 20 dB over a bandwidth of 50 MHz.