Synthesis design of dual‐path coupled line circular ring resonator filter with tunable characteristic impedances and transmission zeros

• Main Authors: Siti Aminah Nordin, Muhammad Asraf Hairuddin, Nur Dalila Khirul Ashar, Zuhani Ismail Khan
• Format: Article
• Language: English
• Published: Wiley 2021-09-01
• Series: IET Microwaves, Antennas & Propagation
• Online Access: https://doi.org/10.1049/mia2.12132

Abstract A dual‐path coupled‐line circular ring resonator filter synthesis is presented. The topology comprises identical coupled lines in series duplicated to form dual‐path coupled‐line interconnections. A bandpass response of nth order equal to (n/2) + 1, corresponds to controllable transmission zeros (TZs) found on both sides of the passband by varying the values of the even‐ and odd‐mode impedances of the coupled lines, denoted Zoe and Zoo, respectively. Concerning the efficiency of controllable electrical characteristics of the resonator filter, such as in‐band matching level, high selective response, and wide bandwidth, and fixing the position of the TZ frequencies, the proposed method using asymmetrical quarter‐wavelength coupled lines in a circular ring resonator filter is superior to existing methods. Ultimately, the proposed bandpass filter is implemented on contrast dielectric substrate FR‐4 and Taconic TRF‐45 substrates using a planar form of microstrip technology to verify the synthesis equation. The measurement results indicate that the filter achieved more than 40% fractional bandwidth with excellent in‐band matching levels and a controllable filter order. Both measurement and simulation results are in good agreement regarding return and insertion loss values, thus validating the proposed designs.