Study on The Approaches of Enhancing Common-Mode Rejection Ratio of Balanced Bandpass Filters

• Main Authors: Chien-Feng Kao, 高健峰
• Other Authors: Shih-Cheng Lin
• Format: Others
• Language: zh-TW
• Online Access: http://ndltd.ncl.edu.tw/handle/53437777555213510796

碩士 === 國立嘉義大學 === 電機工程學系研究所 === 105 === In the first part of this thesis, a microstrip balanced filter using interlocked half-wave (λ/2) and quarter-wave (λ/4) resonators has been proposed. Specifically, a balanced filter with quasi-elliptic response is designed and implemented, achieving the inherent transmission zeros (TZs). By attaching properly chosen capacitors along the bisected symmetric line of the filter structure, the common-mode transmission can be significantly reduced thus achieving satisfactory CMRR among passband. The selection guideline of the attached capacitors is described according to the current density distribution of the balanced filter under common-mode excitation. By using the method, balanced filter with improved CMRR has been proposed. In the second part of this thesis, the multiple half-wave and quarter-wave resonators (form the differential-mode viewpoint) are utilized to design 2nd-, 3rd-, and 4th-order balanced filters. The uniform-impedance resonators are adopted for simplifying the filter structure. Several balanced filters with different configurations are considered. We compare the simulated results obtained by electromagnetic simulator and arrive some conclusions. For a balanced filter with low CM transmission, by observing the coupling stages among its even-mode half-circuit, it is found that cascading multiple capacitive or inductive coupling may easily cause mismatch thus reduce the CM transmission (improve the CMRR). Furthermore, since the CM coupling coefficients corresponding to the structure are determined once the DM response has been well-designed, a virtual CM filter response can be selected/assigned based on the extracted CM coupling coefficients, providing a virtual CM external equality factor. It is found that once the true quality factors extracted from the true structure differ more from the virtual ones, the lower CM transmission (better CMRR) can be achieved. Based on the concluded design guideline, a balanced filter structure with great CMRR is designed by using stepped-impedance resonator for size miniaturization.