Design Of Differential And Balun Filters Based On Dielectric Resonator

With the development of wireless communication technology,the problems of RF/microwave circuits such as signal crosstalk and electromagnetic interference etc.are becoming more and more serious.Compared with traditional single-ended microwave circuits,the differential/balanced circuits have attracted more attention due to its advantages of high immunity to interference,high reliability and high output power etc.In the differential RF system,a differential filter,as a basic device,can replace a single-ended filter and two baluns(unbalanced-to-balanced signal converter),which can significantly reduce circuits’ size,decrease power dissipation and improve the stability.In addition,single-ended and differential devices inevitably coexist in the communication systems,balun still plays a key role.The fusion design of balun filter can provide not only a frequency-band selection as a filter but also an unbalanced-to-balanced signal conversion as a balun.The co-design of multiple-function components cannot cause the enlargement of circuit size.The differential and balun filters in currently reported literatures are mainly designed by using the low quality factor(Q value)resonators which are made of the printed circuit board(PCB),low temperature co-fired ceramic(LTCC)and substrate integrated waveguide(SIW)technologies.They generally suffer from the shortcomings of high insertion loss and poor passband selectivity etc.On the contrary,the dielectric resonator(DR)has the advantages of high Q value,high temperature stability and high power capacity.According to the excellent features,the DR is investigated to design differential and balun filters for satisfying the requirements of high-power communication system such as low loss,high stability and high selectivity.This thesis introduces the importance of differential and balun filters in brief and summarizes the domestic and overseas research status of these filtering devices and DRs,respectivily.The mixed-mode S-parameter and filter synthesis method for the design of differential and balun filters are given.A novel differential feeding structure for the TE01δ dominant mode of ring-shaped DR is proposed in this thesis for the first time.Under this feeding structure,some of the inherent multiple modes of the DR(such as the dominant mode)act as differential mode(DM)responses for constructing DM passband of the filter,some of them(such as the lowest-frequency harmonic mode)act as common mode(CM)responses,while the others can act as both DM and CM responses.Since some high-order harmonic modes only act as CM responses,the DM spurious responses in upper stopband of the designed differential filter can be reduced(compared with traditional single-ended DR filters)so that the performance of stopband rejection can be improved.Based on above research,in order to realize wideband harmonic suppression,the metallic coaxial resonator(CR)is inserted between the two DRs.By utilizing the advantage of the large frequency space between the fundamental mode and lowest-frequency harmonic mode of the CR,the fundamental mode of the CR and the dominate mode of the DR can be designed to have the same frequency to be magnetically coupled for constructing DM passband.While the other harmonic modes of the DR cannot be transmitted through the CR.As a result,the DM upper stopband of the designed differential filter can be extended significantly,and the wideband CM suppression can be realized at the same time.Finally,the three-port balun filters can be obtained by letting one port open on the basis of aforementioned four-port differential filters.The simple and effective fusion-design method is utilized to realize the balun filters based on the DRs,and the balun filters exhibit excellent filtering characteristics and function of unbalanced-to-balanced signal conversion.The designed differential and balun filters in this thesis are all demonstrated by simulation and measurement.