Research On Tunable Low Pass Filter

Wireless spectrum resources are very scarce,and interference between signals of different frequencies is an inevitable problem.Therefore,filters have become key components in radio frequency systems.In order to maximize the use of limited spectrum resources and simplify the complexity of wireless communication systems,tunable filters with parameters such as center frequency and bandwidth that can be adjusted according to actual needs are widely used in a variety of reconfigurable communication systems.In this article,based on the prototype of the quarter-wavelength stub filter,a quarterwavelength stub with tunable input and output impedance is realized by improving the stub structure,and finally a tunable low-pass filter is realized by using it.At the same time,the step impedance line structure and physical folding technology are introduced into the designed tunable low-pass filter to reduce the size of the tunable filter.The designed tunable low-pass filter has good scalability,and can be designed into a tunable low-pass filter that is easier to meet the corresponding requirements according to actual needs.The main contents of this article are as follows:First,the principle of the quarter-wavelength stab line filter is studied in this article.The main study is to change the characteristic impedance of the stub to achieve the characteristics to tune RF filters,and then the law of using the step impedance structure to reduce the size of the filter is studied.Subsequently,the input and output impedance tuning of the main transmission line is realized by introducing a tunable capacitor in parallel with the main transmission line,and then the bandwidth tuning of the low-pass filter is realized;at the same time,the introduction of a tunable capacitor at the open end of the step impedance line can further expand the impedance tuning range achieving a wider bandwidth tuning range.The tunable low-pass filter based on input and output impedance tuning is experimentally investigated in this article.By adjusting the size of the step impedance resonator and the variation range of the tunable capacitor,the bandwidth variation range of the filter is adjusted,and the implemented result of the realized tunable low-pass filter is consistent with the simulated result.