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

This paper presents the design and practical validation of a Ku-band multi-functional RF component that integrates quasi-elliptic bandpass filter and RF isolator (BPFI). It is based on cascaded: i) unilateral frequency-selective stages (UFSs) shaped by a pHEMT transistor-based path and a coupled-line feedback and ii) passive resonators whose number and performance can control the selectivity, directivity and power transmission response of the BPFI. Parametric design studies are discussed for UFS characteristic optimization, including gain, bandwidth, NF, and IIP3, while considering the 0.15 µm InGaAs process capabilities. A comprehensive design methodology using coupling routing diagrams facilitate the BPFI design using coupled-resonator synthesis. For experimental verification, a UFS and four-stage BPFI prototypes were manufactured and tested using a 0.15 µm GaAs MMIC process. The four-stage BPFI demonstrated a quasi-elliptic transfer function shaped by four poles and three transmission zeros, with -1.7-dB gain, 30-dB directivity, 9.2-dB NF, and 0.2-dBm IIP3 at 13 GHz.