Research On Miniaturized Planar Filters Using Coupled-Line Structures

With the development of wireless communication,radio frequency microwave,people's requirement for the performances of microwave devices has gradually developed toward high performance,miniaturization and low cost.As an indispensable part of radio frequency microwave devices,filters play a vital role in the field of wireless communication.Filters with excellent performance can largely optimize the performance of the communication system.Therefore,Therefore,how to design a miniaturized,excellent and easily processed filter have become a hot research direction for scientists.In this thesis,based on the theoretical analysis and applications of the parallel coupled lines structure,several types of filters have been designed and proved by simulation and fabrication,including high-frequency selective single-ended broadband filters,dual-band bandpass filters,balanced filters,and reconfigurable filter.The main studies are as follows:(1)Two different planar microstrip filters were designed,using the method of stubs loaded and vias.Firstly,a dual-band bandpass filter with a very high frequency ratio was designed.By reducing the electrical length of the structure in the same proportion,a miniaturized bandpass filter with very wide upper stopband was obtained.Then by adjusting the connection and electrical length between the coupling lines,the more resonant modes are yielded.If the first two resonant modes are apart away from the other modes,a dual-band passband can be obtained.(2)A balanced filter has been designed,whose performance is excellent.Firstly,based on the structure of three pairs of coupled lines,a bandpass filter can be achieved,whose bandwidth and selectivity can be adjusted by coupling strength.However,excellent bandwidth and selectivity cannot be satisfied at the same time.Therefore,stepped impedances are added to improve the performance of the single-ended filter and the transmission zeros and poles are calculated by using the ABCD matrix and odd-and even-analysis.On the basis of the single-ended filter,a balanced filter is designed,which guarantees high performance of the differential mode,has strong common mode suppression,and can be protected from environmental noise.(3)Based on the parallel coupled lines and microstrip lines,a broadband filter with high selectivity is proposed.Then,on the basis of the filter,a reconfigurable filter which can switch among the bandpass filter,bandstop filter and dual-band filter is designed.Firstly,after analyzing the resonance modes of the series-cascaded of parallel coupled lines with the method of odd-and even-analysis,a high selective wideband filter with 5 resonant modes and 8 transmission zeros is obtained by connecting two symmetrical microstrip lines and shorted coupled lines.Owing to the symmetry of the structure,the bandwidth can be adjusted by tuning the coupling strength of the cascade parallel coupled lines to bring the resonance modes closer or farther away from each other.Then,on the premise of slightly changing the size of the filter,a reconfigurable filter is designed.By switching on and off,when the quarter-wavelength shorted coupled lines is connected,the bandpass filter is obtained.When two microstrip lines are grounded,a dual-band filter is proposed.When the two microstrip lines are connected directly,a bandstop filter can be achieved.