Compact Balanced-to-Unbalanced Filter Design with High Selectivity

• Main Authors: Chieh-ling Chen, 陳捷寧
• Other Authors: Tsen-chieh Chiu
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/48854905912222794411

碩士 === 國立中央大學 === 電機工程研究所 === 97 === 　　In the thesis, the design of the Balanced-to-unbalanced filters with high selectivity is presented. The proposed circuit configuration is built by combining a balun and bandpass filters in which the selectivity and bandwidth can be adjusted by controlling the position of transmission zeros. The proposed filters are designed in L-band (1452 MHz~1492 MHz). By adding lumped elements in the balun circuit, the electric length of coupled lines can be shorter than λ/4. Moreover, a transmission zero which is controlled by a series capacitor between the coupled lines of the balun can be applied to restrain the undesired second harmonic. 　　In order to improve the selectivity, the filters will be integrated into the above-mentioned balun. Using circuit analysis, the designer can set transmission zeros at anywhere to get great selectivity. The balun and the bandpass filters will be merged together by using common coupled lines, so the miniaturization could be achieved. The effect of combining the balun and bandpass filters, and the characteristic of asymmetrical balun will be discussed. The proposed balanced-to-unbalanced filter has advantages regarding selectivity, circuit compactness, and design flexibility. All circuits are fabricated on substrate, ARLON 25N. The relative permittivity, loss tangent, and thickness of the substrate are 3.38, 0.0025, and 0.46 mm, respectively. The proposed balanced-to-unbalanced filters achieved 0.5 dB amplitude imbalance and 6° phase imbalance. The measured results are in good agreement with the simulation.