by Yongzhong Zhu, Dan Li, Yongjun Xie and Rui Yang, Xidian University, Xian Shaanxi, China
Microwave filters are essential components in modern wireless communications systems. It is important to reduce their size and weight in order to integrate them with other components in a single chip system.1 A main trend in filter design is a multilayer filter, which has a much better quality than a planar one.2 At microwave frequencies, a negative permeability over a given frequency range can be realized in the magnetic resonance frequency of a periodic array of SRRs.3 In this article, a multilayer technology and an SRR structure are combined for the design of a novel multilayer filter. Compared with the conventional microstrip filter, the proposed filter shows a significant size reduction and improved selectivity. The measured and simulated results are in good agreement.
Theory
Shown in Figure 1, the SRR is made of two concentric rings separated by a gap, both having splits at opposite sides. The geometrical parameters, such as the split width, gap distance, metal width and radius are represented by d, t, w, and r, respectively. The subscripts i and o denote the inner and outer rings. Besides the electric and magnetic coupling, the incident field also induces the magnetoelectric coupling. An SRR not only has an electric resonance, but also has a magnetic resonance. In addition, the magnetic resonance frequency is lower than the electric resonance frequency.4 Using the magnetic resonance of the SRR in the filter design, a significant size reduction can be obtained.
Figure 1 Split-ring resonator schematic.
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