Design And Research Of Electrically Tunable Filters Based On Multimode Resonators

As an important type of filters,electrically tunable filters are playing a more important role in wireless communication and electronic reconnaissance systems.To realize the microstrip filters with tunable working frequency and bandwidth becomes a work with highly scientific research value.As one method of achieving the electrically tunable filter,the electrically tunable filter loaded with varactor is becoming more and more popular which characterized by a fast electromechanical response,a simple bias circuit structure,continuous frequency tuning and low production cost.The focus of this paper is also based on the tunable filter loaded with varactor.In this paper,the latest research progress of electrically tunable filter is investigated.Based on the design theory and analysis method of microstrip filter,the design theory of electrically tunable filter is introduced based on multi-mode resonator.Based on this theory,five microstrip filters are designed,including a dual-band bandpass filter,an electrically tunable filter with tunable working frequency,a fully tunable filter with tunable working frequency and bandwidth,an electrically tunable low-pass filter,and a filter with tunable range of harmonic suppression.In this paper,the electrically tunable filter with tunable working frequency is designed on the passive dual-band bandpass filter.The filter is based on a short stub-loaded resonator and a pair of coupled quarter-wavelength resonator.The insertion loss of the filter is controlled less than 5dB,while the fractional bandwidth is maintained more than 10%,which improves the narrowband characteristic of the filter.The tunable filter is based on the step-impedance resonator structure,compared with existing filters,the translation range of the filter is 350MHz,which have higher engineering application value.In this paper,the low-pass filter with adjustable cut-off frequency and the tunable band-pass filter are cascaded to design a filter which can suppress harmonic components.Compared with conventional filters,the range of harmonic suppression can be adjusted with working frequency.The high harmonic component is suppressed below-20dB,which improve the working stability of the filter.