High Performance Of Tunable Microwave Filter Theory And Design

As an important component of microwave reconfigurable systems, tunable filters have been a hot topic recent years for their wide applications in multifunction receiver subsystems, electronic countermeasure, dynamic frequency allocation, transmitter stray measurement, etc. Electronic tunable filters with varactors based on printed circuit boards are mostly investigated here due to their frequency agility and low cost.Although the design theory for filters has been matured, there is no special design theory for tunable filters. Firstly, we try to propose a new design theory for tunable filters based on filter synthesis theory and give the stable conditions. Then several tunable filters are designed using the proposed theory. The designed circuits contain two balanced tunable filters with constant bandwidth, two fully tunable filters and a dual-band fully tunable filter.In the design of balanced tunable filters, the techniques of step-impendance resonators (SIRs), balanced circuits, bandwidth control, and common-mode suppression are used. A novel tunable SIR is proposed and analysed. The tunable filters have the advantages of bandwidth control, anti-interference, expanded stopband and so on.Fully tunable filters are the trend and frontier of the study of tunable filters. Based on tunable muti-mode resonators (MMRs) and the theory of external quality factor (Qc) tuning, novel fully tunable filter architecture is proposed. Two design examples are given. Novel cross-shaped tunable MMR is proposed, tunable transformer and tunable J/K inverters are realized. The measured results show the fully tunable filters have wider frequency and bandwidth tuning range and more stable and better performance than others.To meet the needs of muti-band communication systems, a novel dual-band fully tunable filter is designed based on the proposed fully tunable filter architecture. Both the center frequencies and bandwidths of the two passbands are tunable.