Investigation And Design Of Tunable Filter With Constant Bandwidth

With the development of wireless communication technology,there exists the coexistence of multi-mode,multi-band,multi-standard,two-way and one-way wireless broadcasting applications in the space environment,which leads to more congestion of electromagnetic spectrum in space.In order to alleviate spectrum congestion by using frequency hopping,spread spectrum,dynamic frequency allocation,the technology of reconfigurability is introduced into the filter which is the core device of RF transceiver front-end,to realize the adjustable frequency and bandwidth,thereby realizing the adjustable frequency and bandwidth.Compared with the method of filter bank switching mode,the system using tunable filter has smaller size and lower cost.It can also meet the requirement of communication system,such as fast tuning,miniaturization and mass production.Moreover,different channels of wireless communication systems usually require the same working bandwidth of the tunable RF front-end.Thus,the study of tunable filters with constant bandwidth has important theoretical significance and practical application prospects.On the other hand,balanced(differential)circuit structure has strong anti-interference to environmental noise and electromagnetic interference.Thus the tunable balanced filter with constant bandwidth and high common-mode suppression has received increasingly great attention.In this thesis,the filter synthesis of mixed coupling and frequency-dependent coupling is discussed.The frequency-dependent coupling and mixed coupling microstrip bandpass filters(BPFs)are designed.On this basis,the tunable microstrip BPF,balanced microstrip BPF and balanced BPF based on double-sided parallel-strip line(DSPSL)structure with constant bandwidth are designed.The main works and innovations in this thesis are summarized as follows:1.By introducing frequency-dependent coupling and high-order frequency-dependent coupling between source and load,dual-mode microstrip BPF with frequency-dependent source-load coupling and dual-mode microstrip BPF with high-order frequency-dependent coupling are synthetically designed.Because of the generation of transmission zeros(TZs)near the lower and upper passband edges and upper stopband,the compact BPFs have high selectivity and deep stopband characteristics.Furthermore,the tunable dual-mode microstrip BPF with constant absolute bandwidth is designed based on the constant bandwidth synthesis theory.Due to three adaptive TZs,the high selectivity and deep stopband characteristics of the filtering response are achieved.The high selectivity dual-mode microstrip BPF and tunable dual-mode microstrip BPF are implemented and fabricated.The measured results agree well with simulated ones.2.The compact high selectivity tunable balanced microstrip BPF with constant bandwidth is designed by adopting the magnetic coupling between the step-impedance resonaters(SIRs).The desired coupling coefficient can be obtained by the mixed coupling between two resonators.Two differential-mode(DM)TZs near the lower and upper passband edges are created by the mixed coupling and source-load coupling,respectively.Furthermore,the measured CM suppression is better than 45 d B across the DM operating passband because of the magnetically coupled resonator structure.The tunable balanced BPF prototype with constant fractional bandwidth is implemented and fabricated.The measured results agree well with simulated ones.3.DSPSL has good differential transmission characteristics on account of its symmetry.Based on the constant bandwidth synthesis theory and the CM suppression characteristic of DSPSL,the tunable balanced BPF with constant bandwidth based on DSPSL is designed.At the same time,two adaptive TZs are achieved near the lower and upper passband edges by the inherent characteristics of the dual-mode resonator and the source-load coupling to achieve high selectivity.The compact tunable balanced BPF prototype with constant absolute bandwidth is implemented and fabricated.The measured results agree well with simulated ones.