Non-Reciprocal Balanced Bandpass Filters with Quasi-Elliptic Response

This brief reports on the RF design and practical development of a non-reciprocal balanced bandpass filter (BPF) that exhibits a highly-selective quasi-elliptic response in the forward direction of propagation that is shaped by four transmission poles and two transmission zeros (TZs). By modulating some of the filter's resonators with phase-progressed AC signals, a non-reciprocal response is obtained in the differential mode. Its common-mode is also highly suppressed due to the incorporation of a balanced network that results in two additional TZs and resistive loss that are unique to the common-mode. The filter order can be increased by cascading additional resonators. For validation purposes, a microstrip prototype centered at 725 MHz was designed, manufactured, and measured. It showed a high isolation in the differential-mode reverse transmission of up to 62.1 dB. Moreover, the common-mode was suppressed by over 45 dB in a bandwidth greater than one octave.