Non-Reciprocal RF-Bandpass Filters Using Transistor-Based Microwave Resonators

This paper reports on the RF design of nonreciprocal microwave bandpass filters (BPFs) in which the function of a BPF and an RF isolator are simultaneously obtained within the volume of a single RF component. The proposed configuration uses as a basis non-reciprocal resonant stages that are shaped by a pHEMT transistor-based network whose drain and gate are connected through a transmission-line (TL)-based element. By appropriately selecting the TL characteristics, a frequency-selective enhanced signal transmission is obtained in the forward direction - from port 1-to-2 - and full RF signal cancellation in the reverse one - from port 2-to-1 -. For proof-of-concept validation purposes, a two-stage non-reciprocal BPF was designed, manufactured and measured. It exhibited a passband centered at 2.2 GHz, a bandwidth (BW) of 210 MHz and a gain of 5.6 dB. Within its 3 dB-BW passband frequencies its isolation levels were measured between 33.4 - 44 dB.