Highly Miniaturized Multiband Bandpass Filter Design Based on a Stacked Spiral Resonator Structure

This paper describes a stacked spiral resonator (SSR) structure for designing very compact multiband bandpass filters. The resonant frequencies of the proposed SSR structure can be determined by designing the spiral geometry and controlling the mutual coupling in a stacked structure. The multiple passband bandwidths can then be determined by the spacing of different layer patterns between two coupled SSRs. An adequately designed geometry of the input/output resonator with a tapped-line feed can achieve matching conditions for all passbands simultaneously. Moreover, multiple transmission zeros created on both sides of each passband provide high stopband roll-off rates.