Design of Capacitively Loaded Coupled-Line Bandpass Filters With Compact Size and Spurious Suppression

[EN] This paper is focused on the synthesis of capacitively loaded coupled lines for the design of bandpass filters with compact size and spurious suppression. The filters consist of cascaded pairs of coupled lines periodically loaded with patch capacitors. The loading capacitances provide a slow-wave effect to the coupled lines useful for filter miniaturization, whereas periodicity introduces bandgaps that can be controlled in order to achieve spurious suppression. Filter design is achieved following an automated process based on aggressive space mapping (ASM) optimization. The two reported examples, a third-order filter with central frequency f0=1.0 GHz, 8% fractional bandwidth, and 0.1-dB ripple level and a fifth-order filter with central frequency f0=1.8 GHz, 8% fractional bandwidth, and 0.1-dB ripple level, demonstrate the potential of the proposed ASM-based design approach, as well as the effectiveness of the loading capacitances to reduce filter size and reject the spurious bands. Significant size reduction in both filters is achieved, whereas the out-of-band rejection is better than 30 and 45 dB in the third-order and fifth-order filters, respectively, up to at least 4f0 .
This work was supported in part by MINECO-Spain under Project TEC2013-47037-C5-1-R, Project TEC2013-40600-R, and Project TEC2013-49221-EXP, in part by the Generalitat de Catalunya under Project 2014SGR-157, and in part by FEDER funds. The work of F. Martin was supported by the Institucio Catalana de Recerca i Estudis Avancats. The work of M. Orellana was supported in part by the Universidad de Costa Rica, in part by MICITT, and in part by CONICIT