Analysis And Design Of Wideband Bandstop Filters With Transmission Lines And Parallel Open-circuited Stub

With the continuous development of communication technology,spectrum resources are increasingly limited and the interference between various signals is becoming more and more significant.Band-stop filters have ability to suppress the interference of out-of-band signal and improve the dynamic range of receiver,which plays an increasingly important role in all kinds of communication systems.Combined with the design requirements of current 5G communication systems,band-stop filters with large bandwidth and sharp rejection have become a hot research topic at home and abroad.There are usually three methods to implement wideband band-stop filters,but all of them have some unavoidable problems.Filters using coupling are difficult to process.The size of filter using signal interference technology is too large.And filters using open-circuited stubs at quarter wavelength have a finite number of transmission zeros.In order to overcome the above problems,this paper proposes a filter with multiple open-circuited stubs at quarter wavelength.Through synthesis analysis,the number of transmission zeros and transmission poles can be designed.Equal ripples and small optimization of structures can be realized.The specific work contents are as follows:1.The design method of filters with increased transmission zeros and controlled stopband ripples.Firstly,a series of band-stop filters with open-circuited stubs at quarter wavelength are proposed.The ABCD matrix of each part can be multiplied to obtain the overall ABCD matrix.Then,the corresponding relation and variation laws between circuit structure and transfer function are summarized,that is,the number of transmission poles is determined by the number of transmission lines,and the number of transmission zeros is determined by the number of open-circuited stubs.On this basis,the synthesis design method of wideband band-stop filter is proposed.The constraint equations of performance index and circuit parameters,detailed design steps and flow charts are given.Finally,the method of designing transmission zeros and transmission poles,ripple height and band-stop bandwidth can be supported by several examples.2.The design method of miniaturized filters with equal ripples of passband and stopband.To ensure equal ripples in both passband and stopband,more design degrees of freedom can be constructed by adding an additional quarter wavelength opencircuited stub at the center of the filter.At the same time,the circuit size can be further reduced.Finally,a band-stop filter with five transmission zeros and six transmission poles is designed and implemented.The measured results of the filter are in good agreement with the simulation results.The equal ripples and high attenuation rates can be achieved in both passband and stopband.The stopband bandwidth is 129.5% and the stopband rejection is more than 18 d B.Besides,the circuit size is significantly smaller than the first work.