Research And Design Of Tunable Coaxial Filter

In military communications systems,frequency hopping communication and adaptive frequency selection technology are commonly used to prevent interception and interference,its operating frequency will be randomly hopped during communication.Therefore,a narrow band tunable filter with performances of high suppression and fast tuning speed is the key device in electronic countermeasure.The concept of software defined radio was first introduced in the 1990 s,aimed to realize multi-band,multi-standard and multi-function compatible wireless communication platform by combining hardware reconfigurable and software upgrade.Tunable filter is an important part of reconfigurable hardware platform.Based on the design concept of software defined radio system,broadband digital transceiver has gradually become a research hotspot.The narrow band tunable filter is designed according to the bandwidth of the transmitted signal,which can suppress the outside harmonic and stray interference signals to the maximum extent and improve the performance of the whole communication system.Considering the low Q value of microstrip structure and the narrow tuning range of dielectric structure,this paper focuses on the technology of tunable coaxial filter,which is devoted to the realization of low insertion loss,constant absolute bandwidth,high selectivity and wide tuning range,in order to meet the demands of the above systems.The research works in this paper is summarized as follows:This paper will introduce the design,simulation and measurement of three high performance tunable coaxial filters,that are based on varactor loaded coaxial structure.Firstly,we start with the second order tunable coaxial filter.High speed switching varactor and high Q value coaxial resonator are selected to achieve low insertion loss and fast frequency tuning performances.Then,several varactors in the tuning circuit are set in parallel to widen the frequency tuning range of the tunable coaxial filter.Next,the performance of constant absolute bandwidth during frequency tuning range is realized by combining electromagnetic hybrid coupling analysis and asynchronous tuning techniques.Finally,a transmission zero is generated on both sides of the passband by introducing the source load coupling to improve the selectivity of the filter response.A 698-960 MHz second-order tunable coaxial filter is realized through both simulation design and measurement verification,which has good performances of low insertion loss,constant absolute bandwidth,high selectivity,wide tuning range and fast tuning speed.In order to further improve the out of band suppression level of filter responses and verify its high order scalability,a third order tunable coaxial filter is designed.Based on the advantage of flexible cavity arrangement in coaxial cavity,cross-coupling topology design is added between non-adjacent resonators to produce three transmission zeros altogether.Through simulation design and measurement verification,the center frequency is adjustable in the range of 707-963 MHz,and the out of band suppression level is more than 60 d B.Based on rational structure optimization and reasonable design of dielectric plate and tuning circuit structure,a 1-3 GHz ultrawideband coaxial filter is simulated and processed in sections.And the level of 40 d Bc interference suppression can be achieved with the frequency range of f₀ ± 18.92%,which shows good passband selectivity.