| Summary: | 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. © 2021 The Authors. IET Microwaves, Antennas & Propagation published by John Wiley & Sons Ltd on behalf of The Institution of Engineering and Technology.
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