Ku-Band Multi-Functional Bandpass Filtering Isolators (BPFIs) Using GaAs Coupled-Line-Based Unilateral Frequency-Selective Stages

This paper presents the design and experimental validation of a Ku-band multi-functional RF component integrating the function of a quasi-elliptic bandpass filter and an RF isolator (BPFI). The design is based on cascaded: (i) GaAs MMIC-based unilateral frequency-selective stages (UFSs), comprising a pHEMT transistor-based path and coupled-line feedback, and (ii) passive resonators, whose number and characteristics can be tailored to control the BPFI's selectivity, directivity, and power transmission response. Parametric design studies are conducted to provide insights on UFS characteristic optimization, including gain, bandwidth (BW), noise figure (NF), and input third-order intercept point (IIP3), considering the capabilities of the commercially available 0.15 µm InGaAs process. A coupling routing diagram-based design methodology is also provided, facilitating BPFI design using conventional coupled-resonator synthesis techniques. For experimental validation, a single-stage UFS and a four-stage co-designed BPFI were manufactured and tested using a 0.15 µm GaAs MMIC process. The four-stage BPFI demonstrated a quasi-elliptic transfer function with four poles and three transmission zeros, achieving a passband gain of -1.7 dB, directivity of 30 dB, an NF of 9.2 dB, and an IIP3 of 0.2 dBm at 13 GHz.