Microstrip Line Non-Reciprocal Bandpass Filter With Tunable Center Frequency

In this paper, we present a design of a magnet-less microstrip line non-reciprocal bandpass filter (BPF) with tunable center frequencies. The proposed non-reciprocal BPF uses quarter-wavelength time-modulated resonators. To achieve a non-reciprocal (vertical bar S-21 vertical bar not equal vertical bar S-12 vertical bar) filter response, a progressive phase shift AC modulation signal is applied to varactors through transmission line, which simplifies the modulation scheme. The center frequency is tuned by changing dc-bias voltage of varactor diode. For experimental validation, a prototype has been designed and fabricated. The measured results confirmed that center frequency is tuned from 1.45 GHz to 1.55 GHz with forward insertion loss variation of 3.71 dB to 3.40 dB and reserve isolation is higher than 10 dB.