Research And Design Of High Performance Filtering Balun Based On Defected Ground Structure

The successful commercialization of the fifth generation wireless communication technology not only further promotes the development of wireless communication systems,but also puts forward higher and more stringent requirements for them.At the same time,with the continuous upgrading of integrated circuit technology,the cost of introducing balanced circuits into wireless electronic systems has also been greatly reduced.Based on the above two reasons,balanced circuits with higher anti-interference ability,higher linearity and larger signal swing are more and more widely used in modern wireless communication systems.As a bridge between unbalanced signal and balanced signal,balun is also increasingly important.Meanwhile,multi-function circuit integrated with filter function and balun function(i.e.filtering balun)can further realize miniaturization and performance improvement of wireless electronic system Therefore,balun with compact area,high in-band balance and excellent filtering characteristics is becoming a research hotspot at home and abroad.This thesis focuses on the design method of high performance filtering balun based on defected ground structure.The first part mainly introduces the background and significance of balun research,and summarizes the relevant research results on Balun at home and abroad in recent years.Based on this,it points out the challenges faced by the current filter Balun design,namely,the balun design and implementation that takes into account compact area,high in-band balance,and excellent filtering performance.The second part introduces the commonly used transmission line structures in microwave planar circuits,and conducts a theoretical analysis of their related circuit parameters.Then,it briefly introduces the basic theoretical knowledge and related technical indicators of balun,laying a theoretical foundation for the design and implementation of filtering balun.The third part provides a detailed analysis of the characteristics of the rectangular dumbbell-shaped defect ground structure and proposes its equivalent circuit model.Subsequently,based on this model,a miniaturized wideband filtering balun is designed.The fourth part analyzes the slow-wave characteristics and low-pass response of step impedance microstrip-line,and then realizes further miniaturization of defect ground structure balun with wide stopband based on these two characteristics.The fifth part analyzes the reasons for the imbalance within the proposed balun,and determines that the phase-and amplitude-imbalance is caused by the equivalent capacitance inequality caused by the asymmetry on the top feedline structure.In response to this,compensation metal strips are introduced into the defect ground structure,which can reduce the equivalent capacitance difference of the output balance port,and effectively improving the in band balance of the balun without affecting the filtering characteristics.Finally,the measurement results show that the filtering balun with compensation structure has achieved 151%of 3-d B relative bandwidth at the center frequency of 1.88 GHz,and its area is only 36.10 mm×28.54 mm(i.e.0.38λ_g×0.30λ_g,whereλ_g is the microstrip guided wavelength at the center operated frequency of the proposed balun).The stopband range of the balun can reach 5.6 times the center frequency,and the rejection level can reach more than 23 d B.The in-band amplitude imbalance and phase imbalance of the filtering balun are lower than 0.4 d B and 1.1°,respectively.By comparing with the relevant research results on balun in recent years,it has been proven that the proposed balun has the characteristics of compact area,high in-band balance,and excellent filtering performance.