Several Methods for Suppressing the Second- and Higher-Order Spurious Responses of Parallel-Coupled Microstrip Filters

• Main Authors: Meng-Jiun Jiang, 江孟駿
• Other Authors: Jen-Tsai Kuo
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/67735912599198836082

博士 === 國立交通大學 === 電信工程系所 === 94 === In this dissertation, the cause of spurious responses of microwave bandpass filters is investigated. Several methods are proposed to tackle these spurious responses in order to design microwave bandpass filters with wider upper stopband bandwidth. According to different methods and circuit structures, the dissertation is divided into three parts. In part one, the cause of spurious responses is studied, and suspended substrate is proposed to design coupled-line filters free of second harmonic resonance. Based on filter synthesis formulas for arbitrary image impedances, not only the passband symmetry can be improved, but also the spurious response at second the passband frequency (2fo) can have a suppression of nearly 40dB without any additional tuning. The upper limit of upper stopband can be extended from 2fo to 3fo, such that double upper rejection bandwidth can be obtained. In part two, the multilayered structure with a uniform dielectric overlay is proposed, and a suppression of at least 40dB to the spurious responses at 2fo can be achieved. The synthesis formulas and filter design approach in part one of the dissertation can be directly applied to the filter design of part two. However, the use of quarter-wave impedance transformer is inevitable as attempting to use overlay dielectrics with different dielectric constants or thickness. In order to overcome this problem, coupled input/output stages are replaced with tapped input/output resonators and the filter synthesis formulas are rewritten, so that the freedom of filter design can be greatly enhanced. In part three, parallel-coupled microstrip filters with over-coupled stages are proposed. An inherent zero of a microstrip coupled stage near twice the design frequency (2fo) is found tunable by varying its coupling length. It is found that applying over-coupling to the end stages, rather than to every coupled stage, is sufficient to improve the filter characteristics in the upper stopband. Also, increasing the image impedance of the filter sections can further enhance the suppression. This idea is extended to design over-coupled middle stages for suppressing the spurious |S21| peaks at 3fo and 4fo of the filter, so that the upper stopband can be greatly enhanced up to 5fo. The passband preserves a response as good as the traditional design. In each part of the dissertation, several circuits are designed and fabricated to verify the proposed ideas and the measured responses agree very well with the simulated ones.