An improved simplified composite right‐/left‐handed zeroth‐order resonator and its application to design a compact microstrip bandpass filter with enhanced upper stopband

Abstract

An improved simplified composite right‐/left‐handed (SCRLH) zeroth‐order resonator (ISZOR) is introduced, comprising two lateral parallel capacitors and a centred T‐type inductor circuit. Starting from the lumped circuit of the ISZOR, its miniature microstrip layout can be generated with on more than three times full‐wave simulations by virtue of the implicit space mapping technique. Multi‐solutions of the microstrip ISZOR with identical fundamental mode are obtained by sweeping key physical and electrical parameters, uncovering the relationship among physical sizes, resonant modes, unloaded quality factors and circuit elements. A three‐pole microstrip Chebyshev bandpass filter (BPF) with ultra‐wide upper stopband is further designed. By choosing the constituent ISZORs with identical base frequency but different higher resonant frequencies and applying the over‐coupled I/O terminals, wideband spurious response suppression in the upper stopband is achieved. The target BPF centers at 3.1 GHz with a relatively bandwidth of 5%, which is optimised through a customised aggressive space mapping iterative process. The optimal BPF layout dimensions are automatically obtained with just a few time‐consuming full‐wave simulations. A prototype measurement shows the BPF features good bandpass selectivity in a simple and easy‐to‐fabricate structure, with a transversal size reduction of more than 65% compared to a former counterpart and a rejection level better than 15 dB over 40.0 GHz.

A compact microstrip second‐order lossy bandpass filter with improved simplified composite right‐/left‐handed zeroth‐order resonator

“…However, the first parasitic resonance of the MISZOR locates around 13.03 GHz, a little lower than the preset goal, since distributed effects in the MISZOR gradually take more effects when frequency goes up, leading to the lumped equivalent circuit unable to capture the entire electromagnetic effects occurred in the MISZOR layout. 10,12 The UQF of the generated MISZOR is 89.0 at the zeroth-order mode, obtained with the eigenmode solver in Ansys HFSS.…”

“…10 Compared with the conventional composite right-/left-handed zeroth-order resonator (CZOR), 11 the series interdigital capacitance is excluded in the employed compact ISZOR, reliably avoiding the spurious resonance close to the working band. 12 Starting from the lumped circuit model of the ISZOR, the microstrip ISZOR (MISZOR) can be generated efficiently by virtue of the implicit space mapping (ISM) technique. 10,12 With the disclosed correlation among the MISZOR's physical sizes, resonant modes, UQFs, and circuit elements, a solid foundation to facilitate the LF design is established.…”

“…12 Starting from the lumped circuit model of the ISZOR, the microstrip ISZOR (MISZOR) can be generated efficiently by virtue of the implicit space mapping (ISM) technique. 10,12 With the disclosed correlation among the MISZOR's physical sizes, resonant modes, UQFs, and circuit elements, a solid foundation to facilitate the LF design is established. In the former works, the capacitively coupled microstrip stub was used to implement the terminal coupling and the realizable coupling strength can be controlled by adjusting the gap width and the stub length, but it is not easy to realize strong coupling under limited fabrication tolerance.…”

“…In the former works, the capacitively coupled microstrip stub was used to implement the terminal coupling and the realizable coupling strength can be controlled by adjusting the gap width and the stub length, but it is not easy to realize strong coupling under limited fabrication tolerance. 10,12 To this end, an inductive coupling microstrip corner is proposed in this work to implement the large terminal coupling. The integral MISZOR-based LF features a very compact and delicate layout, which is optimized through the automatically executed Nelder-Mead simplex algorithm.…”

“…The measured IL is a little larger than the preset goal, and this may be due to the extra radiation loss from the zerothorder mode, but the IL and RL levels and the IL inband flatness are far better than those of the former works without employing the LF synthesis. 10,12 Moreover, there are two additional transmission zeros (TZs) appearing at 2.25 and 5.28 GHz, respectively, promoting the LF bandedge selectivity substantially. At the two TZs, most of the surface current is concentrated around the MISZOR short-circuited stub close to the excitation port, demonstrating that they are both related to the shortcircuited stub resonance.…”