Research on a Miniaturized Wide Stopband Folded Substrate Integrated Waveguide Filter

To meet the requirements of 5G/6G communication systems for miniaturization, high integration, and a wide stopband, this paper proposes a fourth-order bandpass filter based on an eighth-mode Folded Substrate Integrated Waveguide (FSIW) using High-Temperature Co-Fired Ceramic (HTCC) technology. The design combines the miniaturization characteristics of FSIW with the three-dimensional integration capability of HTCC. Size reduction is achieved through an eighth-mode FSIW cavity structure with dimensions of 0.29λg × 0.29λg, where λg denotes the waveguide wavelength at the center operating frequency (f0). Metal vias suppress high-order mode coupling, a bent 10 microstrip line introduces transmission zeros, and an L-shaped stub improves the high-frequency response. Three controllable transmission zeros are generated in the upper stopband, achieving 20 dB@3.73f0. Measurements show a center frequency of 6.4 GHz. Although slight frequency deviation and insertion loss are observed, the design provides clear advantages in miniaturization, stopband width, and the number of transmission zeros compared with reported work, indicating potential for high-density integrated communication systems.ObjectiveThe rapid development of 5G/6G communication systems increases the demand for Radio Frequency (RF) microwave devices that provide miniaturization, high integration, and wide stopband performance. As core components of RF transceiver front-ends, bandpass filters transmit useful signals and suppress interference. Conventional Substrate Integrated Waveguide (SIW) filters often show large size, limited stopband extension, and insufficient control of transmission zeros, which restrict their use in high-density integrated systems. To address these challenges, this paper presents a miniaturized wide stopband fourth-order bandpass filter based on an eighth-mode FSIW structure and HTCC technology to achieve compact size and broad stopband performance.MethodsThe filter integrates the miniaturization capability of FSIW with the three-dimensional integration characteristics of HTCC. First, an eighth-mode FSIW cavity is developed by modifying a quarter-mode FSIW cavity. A square patch is replaced with a triangular patch (eighth-mode cavity I), followed by slot etching in the triangular patch (eighth-mode cavity II). Second, a fourth-order bandpass filter is constructed by symmetrically designing two triangular metal patches for each cavity type and stacking them vertically. A common metal layer (fifth layer) containing coupling windows enables coupling between the upper and lower cavities. Three techniques are used to optimize performance: metal vias to suppress high-order mode coupling, bent microstrip lines to generate transmission zeros, and an L-shaped stub to enhance high-frequency response. Parameter scanning of key dimensions (d2, s4, s6) verifies the controllability of transmission zeros. The filter is fabricated using HTCC on an Al2O3 substrate with relative permittivity 9.8 and loss tangent 0.000 2.Results and DiscussionsMeasurements show a center frequency of 6.4 GHz. Although fabrication and assembly deviations cause slight frequency shift and additional insertion loss, the filter demonstrates strong performance compared with reported designs (Table 2). The size of 0.29λg×0.29λg is smaller than that of most SIW filters. The upper stopband extends to 20dB@3.73f0, outperforming filters of comparable size. Three controllable transmission zeros appear in the upper stopband, and parameter scanning confirms their tunability (Fig. 13).ConclusionsA miniaturized wide stopband fourth-order bandpass filter based on an eighth-mode FSIW structure is presented. The eighth-mode cavity combined with HTCC technology achieves a compact footprint of 0.29λg×0.29λg, meeting the integration requirements of 5G/6G systems. The use of metal vias, bent microstrip lines, and L-shaped stubs generates a wide stopband of 20 dB@3.73f0 and three tunable transmission zeros, strengthening interference suppression. Adjustable parameters enable flexible tuning of transmission zero frequencies without affecting the passband, improving the adaptability of the design to different interference conditions. These advances address key challenges in miniaturization, stopband extension, and design flexibility of SIW filters, offering a practical solution for RF front-ends in next-generation high-density integrated communication systems.