Design of a compact microstrip balanced‐to‐balanced filtering power divider with real impedance‐transformation functionality

• Published in: IET microwaves, antennas & propagation antennas & propagation, 2021-04, Vol.15 (5), p.481-494
• Main Authors: Wang, Xuedao, Wang, Min, Zhang, Gang, Zhu, Lei, Choi, Wai‐Wa, Wang, Jianpeng
• Format: Article
• Language: English
• ISSN: 1751-8725; 1751-8733
• DOI: 10.1049/mia2.12059

A microstrip balanced‐to‐balanced filtering power divider (FPD) design with impedance transformation via a coupled three‐line balun structure is proposed. The even/odd mode‐analysis method is used to construct a three‐port balun bandpass filter (BPF) with stub‐loaded resonators and a three‐line output structure after analysis and design to prescribe the filtering response of a proposed balanced‐to‐balanced FPD. To realize good isolation between the differential output ports, a complex impedance‐isolation network is loaded by means of coupled lines in the input‐feeding section. Theoretical formulas characterized by the coupling factors are provided with a specified source and load impedance ratio under the condition of perfect impedance matching at the centre frequency. According to the calculated S‐parameters, design rules are summarized to determine the overall circuit size according to desired bandwidth and return loss. For validation, two balanced‐to‐balanced FPDs centred at 2.4 GHz with 50‐to‐50 Ω matching and 50‐to‐100 Ω transformation are designed, fabricated and tested. Good agreement between the simulated and tested results of the designed balanced‐to‐balanced impedance‐transforming FPDs indicates not only high common‐mode rejection but also good port‐to‐port isolation and good matching at all ports.