Resonance characteristics of capacitively loaded CPW open‐loop resonators

Abstract

ABSTRACT:Capacitively loaded coplanar waveguide (CPW) Key words: coplanar waveguide resonators; open-loop resonators; filters INTRODUCTIONMiniaturization of microwave components is one of the most important requirements in the design of a microwave circuit. Conventional planar transmission line circuits cannot reduce the guide wavelength by more than the factor ͌ r (where r is the relative permittivity of the substrate) from the free-space wavelength. A reduction in size requires a change in the geometry of the microwave component. There have been some reports of miniaturization based on curving conventional transmission lines into coplanar meander lines [1,2]. In addition, a capacitively loaded microstrip loop resonator has been investigated by Hong and Lancaster [3]. They have also used the microstrip open-loop resonators (MOLRs) for cross-coupled planar microwave filter applications [4], and described a microstrip slow-wave OLR [5]. Yu and Chang [6] have proposed compact elliptic-function bandpass filters using MOLRs. Subsequently, Görür et al. [7,8] discussed the effect of the width of the strips of both MOLR and CPW OLR on their resonance characteristics, and showed that these resonators are not only much simpler in structure, but also have a larger slow-wave effect compared to conventional half-wavelength resonators and conventional loop resonators.On the other hand, uniplanar transmission lines such as coplanar waveguide (CPW), coplanar strips (CPS), and slot line (SL) have become preferable over the conventional microstrip lines with their increased use in monolithic or hybrid integrated circuit applications at microwave frequencies, due to their small dispersion, low radiation, easy integration with lumped elements or active devices, high circuit density, and lack of via holes. In addition, the fabrication process of uniplanar circuits is simpler than those of microstrip or double-sided circuits, because only one side of the substrate is used. Consequently, many attractive components using uniplanar structures have been developed. However, in spite of the advantages mentioned above, the uniplanar types of microwave components have not been sufficiently considered in microwave literature, as compared with microstrip ones. In this paper, we investigate the influence of the capacitive fingers between the arms of coplanar OLRs on the resonance characteristics. CAPACITIVELY LOADED CPW OPEN-LOOP RESONATORA conventional CPW rectangular OLR is shown in Figure 1 The extending parts of the open arms, whose lengths are equal to the length l f of a finger, increase the circumference of the OLR and, consequently, the resonance frequency of the OLR shift down because the guided wavelength is increasing. On the other hand, the CPW variable gap OLR (CPW VGOLR) discussed in [7] has similar resonance characteristics. The authors of [7] have shown that the CPW VGOLR, illustrated in Figure 1(c), has a significant slow-wave effect as well as a higher performance, as compared with the conventional half-wavelength CPW an...

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