Design And Research Of Microstrip Balanced Filter

In wireless communication system, the balanced system has excellent electromagnetic compatibility features, due to its higher immunity to environmental noise and internal active device noise, thus it has been widely used.In the balanced communication system, especially in RF front-end circuit, balanced filters are of great value to the application and research.This paper mainly focuses on the research and design of the balance microstrip filter.Firstly, the characteristics of 4 ports network, coupling characteristics have been studied. The commonly used methods of producing transmission zeros and inhibiting common-mode noise are proposed which are the basis to design the high performance balanced filter.Then several important components in the field of balanced filter is proposed, including: balanced wide stopband filters, balanced dual-band and balanced diplexer. The main content of the paper is concluded as following:1. To analysis, design and test the balanced filter, the four ports balanced network hybrid S parameters are analyzed firstly to establish the relationship between hybrid S parameters and the traditional four ports S parameters. Also the correctness of using odd-even mode analysis methods to analyze the symmetrical balance device is proved. In order to improve the performance of the balanced filter, the method of producing transmission zeros and inhibiting common-mode noise have been studied.2. The resonance characteristics of the stepped impedance resonators(SIR) are analyzed. A novel compact wide stopband filter with good frequency selectivity, low loss and wide stopband is presented. According to this wide stopband filter, balanced wide stopband filters is designed. The load short circuit dual-mode resonator is presented to increase the common mode rejection. Finally, the predicted and measured performances are in good agreement with each other.3. The perturbation type closed loop dual-mode resonator and the short circuit loading dual-mode resonator are analyzed in detail. A new approach to design the balanced dual-band bandpass is proposed. The bandwidth and the center frequency of the filter are independently controllable. The design process is also given in this paper. The good agreement in the predicted and measured performances demonstrate the validity of the theoretical analysis.4. The method of the DGS(Defected Ground Structure) balanced diplexer design is proposed. The field distribution and the coupling characteristics of the dual-mode resonator are studied. Also, the common step impedance feeding structure is presented and analyzed. Finally, a balanced diplexer with good differential mode and common mode response is designed and fabricated. Simulation and experimental results are in good agreement with each other.