Research On Tunable Bandpass Filter

Since the first generation of analog mobile communication system was put forward in the1980 s,the information technology has been developing rapidly,and the business needs of different fields and different scenarios have also increased,and the communication system standards have also evolved rapidly towards diversification.As an indispensable part of communication system,microwave filter plays the role of signal screening,and its performance directly affects the performance of communication system.Nowadays,communication system is developing towards miniaturization and portability.Therefore,the study of tunable bandpass filter with compact structure,strong design,high selectivity,flexible tuning frequency and bandwidth has practical application value and theoretical guidance.This paper mainly carries out the research of tunable bandpass filter and the related technology of transmission zero introduction in bandpass filter.The researches in this paper are as follows.Firstly,the working principle and structure of the tubular filter are discussed in detail.Then,the π-type resonator in the tubular filter is realized by using the step impedance line loaded with two varactor diodes,and the coupling between adjacent resonators is realized by using the varactor diode.The frequency tuning and bandwidth tuning of the bandpass filter can be realized by adjusting the bias voltage of the varactor.Finally,the proposed tunable filter is simulated,processed and tested.The simulation results are in good agreement with the test results.Based on the proposed tunable bandpass filter,the transmission zero technology is studied.The source/load is coupled through parallel coupling lines firstly,so the transmission path of the input and output is added.By optimizing the parameters of the coupling line,the source/load coupling is changed,and an adjustable filter with three or four transmission zeros in the high band of the filter passband is designed.Then a new type of two-mode resonator is used to generate two transmission zeros,and one is realized at each end of the filter passband by loading an adjustable capacitor.Simulation and test results show that the two schemes improve the selectivity of the filter to some extent.