Investigation And Design Of Novel Microwave Balanced Tunable Filters

With the rapid development of modern wireless communication,differential/balanced circuits and systems are more and more important due to their high immunity to the environmental noise and electromagnetic interferences(EMI)compared to the single-ended ones.Meanwhile,the radio frequency(RF)front-end circuits are asked to have high requirment under the trend of miniaturized and high-integration wireless terminals and systems.Filter,as a basic and important device,is also very necessary to be designed as balanced topology.In balanced RF systems,a single-ended filter with two baluns(balanced to unbalanced device)can be replaced by the balanced filters,which can reduce the circuit size and the power dissipation and improve the stability etc.Furthermore,with the radio spectral resource getting exiguous nowadays,it is necessary to investigate and design the balanced tunable filters,which can selectively operate at multiple communication frequencies.In this thesis,the research significance,the domestic and overseas research status for the balanced tunable filters are briefly introduced.In addition,a full investigation of the novel balanced tunable filters are carried out.This paper gives the mixed-mode S-parameters,filter synthesis method for the designs of balanced circuits and filters,and several tuning methods based on varactors and microfluidics,respectively.Based on the designs of conventional balanced filters and novel tuning methods,several competitive balanced tunable filters are proposed and implemented.Firstly,a novel balanced tunable bandpass filter(BPF)with constant absolute bandwidth(ABW)is proposed in this thesis.By studying the relationships between the varactor location on the resonator and the frequency tuning capability of the resonator,the frequency tuning range keeps almost the same when the varactor is located at a given area.At the same time,the variation slopes of the required external quality factor and coupling coefficient can be easily controlled and obtained by locating the varactor during this area.Therefore,the ABW of the passband can be kept constant across the frequency tuning range.Meanwhile,the CM suppression is always good.Secondly,the newly developing microfluidic frequency tuning technique is introduced in the balanced tunable BPF designs in this paper.By changing the local effective dielectric constant of the resonator so as to precisely control the passband frequency.The relationships between the microfluidic location and the RF voltage distribution on the resonator are firstly studied in the paper,and the analysis of the microfluidic tuning principle is given and verified by the measurement.The designed single-ended microfluidically tunable BPF shows precisely controled passband frequency with the same hopping step.And then a balanced tunable BPF based on patch resonator is proposed.With the above microfluidic tuning principle,the differential passband frequency can be precisely controlled,and the fractional bandwidth(FBW)of the passband under each tuning state is almost constant.Lastly,in order to meet the requirement of different applications,a wideband tunable CM suppression filter for high-speed differential signals is proposed.In the design,by etching the proposed varactor-loaded slotted-ring resonators on the ground plane underneath the differential microstrip line,differential-mode(DM)signals can be losslessly transmitted through the differential line while the target CM noise can be suppressed in a wide frequency range.The proposed CM suppressed filter shows the features of compact size,low cost and easy integration with other circuits.To conclude,the proposed balanced tunable filters with its design method in this thesis are important in science and applications,and can supply technical references for miniaturization and high performance for the development of the balanced RF front-end circuits.