Research Of Microwave Balanced Bandpass Filter

In recent years,the continuous development and progress of various wireless communication technologies,especially the continuous maturation of fifth-generation communication systems and large-scale commercialization,has led to an increasingly complex communication environment,prompting higher requirements for the antiinterference capability of microwave devices.Compared with conventional filters,balanced filters are better than conventional filters in suppressing electromagnetic interference,environmental noise,and crosstalk between different components.Therefore,it is an important research topic to study and design compact and miniaturized balanced filters that achieve good differential mode passband performance,high common mode rejection,and low loss in performance.In this thesis,balanced bandpass filters and balanced dual-band bandpass filters are proposed using microstrip lines,and the main research is as follows:(i)To analyze the differential and common mode signals transmitted in balanced bandpass filters,as well as to analyze the conversion relationship between single-ended networks and balanced networks.Firstly,the S-parameters of the conventional singleended RF network are introduced;secondly,the S-parameters of the mixed mode of the balanced RF circuit are introduced,which include the two transmission modes in the balanced circuit,i.e.,differential mode and common mode,as well as the mixed mode Sparameters are introduced;finally,two methods are given in detail to derive the conversion between the S-parameters of the single-ended network and the S-parameters of the balanced network,which provide the theoretical guidance for analyzing the characteristics of the balanced band-pass filter.Finally,two methods are given to derive the conversion between single-ended network S-parameters and balanced network S-parameters,providing theoretical guidance for the analysis of balanced bandpass filter characteristics.(ii)A simple structured balanced bandpass filter is proposed,which mainly consists of a ring resonator with loaded opened/shorted stubs,and the resonator is fed by a balanced coupling line.In the ring resonator,three methods can be utilized simultaneously to achieve high common mode rejection.First,the ring resonator has different differential/commonmode resonant frequencies,which results in good in-band common-mode rejection.Second,loading different lengths of studs causes the input/output ports to have different coupling strengths,which can deteriorate the common-mode bandpass response.Third,loading ground resistors can effectively absorb the common-mode signal.Meanwhile,loading ground resistors in the balanced coupled line structure can effectively absorb the reflected common-mode signals.Its structure,working principle and design method are described in detail.For demonstration,a prototype balanced bandpass filter operating at2.4 GHz is designed and measured to achieve a high in-band common-mode rejection of49 d B.The simulated and measured results verify the effectiveness of the proposed balanced bandpass filter and its design.(iii)A simple multi transmission pole and zero balance dual frequency bandpass filter is proposed based on a simple planar dual frequency bandpass filter,and the filter achieves high selectivity and high isolation.According to the frequency response transformation,two passbands are realized on both sides of the operating frequency.In addition,based on the rigorous scattering parameter theory and even-and odd-mode analysis method,an analytical(closed-form)extraction procedure for the eight transmission poles and seven zeros of the balanced dual-band bandpass filter is given.Common mode rejection is achieved by loading open stubs at symmetrical positions as well as resistors.The balanced dual-band pass filter operating at 1.615 GHz and 3.09 GHz with 3d B fractional bandwidth of 40.9% and 19.4% is designed and simulated.The results of circuit simulation and fullwave simulation agree basically,which verifies the feasibility of the design.