Research On Compact Tunable Filter

More and more multi-functional transceiver systems are used in modern radio system which use ultra-wideband signal, and support multi-band operation. This trend led to the growing demand of the RF front-end of the tunable preselection filter technology. In the design of tunable microwave filters, some higher requirements are put forward: the tunable filter, which are able to support multiple information channels,requires not only the elimination of band noise and interference spectral components,but also the control of the size of the appropriate information channels. Meanwhile, due to the compactness and portability of RF system requirements, the demand of tunable filter miniaturization is highly increased. Therefore, the miniaturization of electronic tunable filter which also have a variety of tuning modes still need further study and research.From the traditional microstrip filter theory, different realizations of the electrically tunable filter are summarized and compared. Finally, with loading varactors and using microstrip structure of the source-load cross-coupling and transformed radial stub(TRS), several electrically tunable filters are designed,The main contents of this paper are as follows: First, the basic principles of microstrip filters are analyzed and synthesized, to lay the foundation for the tunable filter design. Then the relevant literatures at home and abroad are analyzed with a comparison of several different methods to achieve tunable, which advantages and disadvantages are listed. Secondly, the frequency response of the TRS resonator is propertied, using the even-odd mode analysis and a compact bandpass filter(BPF) with wide stop-band is designed. The board size of the BPF is only 0.251 × 0.211λg witch demonstrates more than 22-d B rejection from 3 to 23 GHz(up to 9.2f0). Thirdly, the loaded variable diode is chosen to design tunable filter. The varactor diode characteristics were analyzed, integrated varactor diode indicators, choose a varactor and designed a cross-coupled source-load tunable filter. The tunable filter passband has a nearly transmission zero, respectively, which provide a good selection frequency characteristics. Finally, using the TRS with loaded varactors, a miniaturized frequency tunable filters is designed. And on this basis, with an additional transmission channel, a compact with frequency, bandwidth both tunable filter is designed.