Research On Tunable Filters

Tunable cavity filters are studied in this thesis, including calculation of resonant frequencies for a given cavity, design of cavity size fQr desired resonant frequencies, and analysis of dimension condensing effect of cavity filters. A type of disk-loaded cavity filter is studied into detail: some cavities of different structures, which may be used as cavity filters individually, are proposed; different methods are put forward in each chapter to analyze these cavities. In addition, a kind of electronically tunable filter is proposed by loading a variable capacitor module with a fixed cavity. A practical variable capacitor module is realized by electronic switches with lumped capacitors. With enough switches and capacitors, an interesting band can be densely covered. The author's major contributions are as follows: 1. The quasi-static characteristics of non-planar cylindrical microstrip lines are investigated with conformal mapping method by which the planar microstrip line is used as an intermediary. The formulas for analysis are derived, also the working section and objective function are determined for synthesizing the dimensions of lines by means of dichotomy. 2. A basic theory for constructing cavity resonator with a kind of slowwave transmission lines is presented, the procedure of determining sizes of cavity for a designed resonant frequency is proposed. The properties of disk-loaded coaxial resonator with slow wave mode is analyzed by the Borgnis's potential function method. The characteristic equation for calculating the eigenvalue of dominant TM mode is obtained, from which it is shown that a high-order mode can not exist independently as slow wave mode in such cavity. 3. Based on Floquet's theorem, a general method for analysis of cavity resonators containing periodic structures is given, to design the sizes of cavity for a desired resonant frequency. As a typical example, the coaxial cavity with the outer conductor disk-loaded is analyzed to illustrate the application of this method, the results obtained are compared with those of Collin's network method. 4. A coaxial cavity with interdigitally disk-loaded inner and outer conductors is proposed, and a kind of analysis technique for this type of cavity is obtained by applying the network method to the radial transmission line. This type of cavity has compact volume due to the interlace of the disks. 5. A network method for analysis of coaxial cavity resonator with both conductors disk- loaded is proposed. Some characteristics of this kind of resonator are discussed as the angle of disks and the interval of disks are considered as the independent variable respectively. The idea and procedure for analysis and synthesis of this kind of cavity are also given. 6. A variational method for analysis of the disk-load coaxial cavity is suggested. The boundary conditions between subzones are obtained by applying the technique of field matching to the field expressions in subzones. Taking the radius of disks as the intermediate variable, the process for analysis and synthesis of this kind of cavity is also given. 7. A prototype of electronic tunable filter whose cavity is loaded outside with a variable capacitor module is proposed. A passive capacitor module is realized with mechanical switches and lumped capacitors, and the adjustable range of capacitors is studied; with electronic switches a electronic tunable filter is realized, and the frequency shift and insert loss by active circuits are studi...