A Novel Miniaturized Bandpass Filter Using Interdigital-Coupled Spiral-Shaped Resonators

Abstract

In this paper, we present a novel miniaturized bandpass filter with high-selectivity performance and compact topology. For circuit size reduction, a spiral-shaped resonator with three branches is developed and examined. By suitably combining the resonators as the interdigital form, a special coupling scheme, which simultaneously provides electric and magnetic couplings, can be obtained readily. To investigate the new coupling scheme, the equivalent-circuit model of the proposed filter is derived and the lumped parameters are extracted. It is found that two transmission zeros can be realized close to the passband edges, resulting in high skirt selectivity. Meanwhile, the interdigital combination of the coupled-resonator pair makes the filter structure more compact. To verify the presented concept, an experimental filter operating at 1 GHz is demonstrated. The active area of the filter is only 11.6mm x 11.6 mm (0.061λg x 0.061λg), where λg is the guided wavelength at the operating frequency.