Research On Electric Tunable Radio Frequency Filtering Components

Recently, wireless communications have been developing rapidly and a variety of communication services emerge in different frequency bands, especially in frequency modulation communication systems, like Radar and electronic countermeasure, where multi-mode and multi-band reconfigurable radio frequency (RF) front-ends are required to satisfy the data transmissions for each communication systems. RF tunable filters are one of the key devices for reconfigurable radio front-end to accomplish frequency selection or frequency suppression. Therefore, it is very important to study the RF tunable filters with high unloaded quality factor, wide tuning range, high tuning speed, high linearity, high selectivity and miniaturization.In this thesis, tunable filters and frequency selective structure (FSS) based on transmission line resonators and varactor diodes are at first studied, such as varactor-loaded multi-mode resonator, varactor-loaded 1/2 wavelength resonator and varactor-loaded 1/4 wavelength resonator, etc. Secondly, their tunablity mechanism and coupling coefficient are theoretically analyzed by using the transmission line theory. Then, the tunable filters and FSS are designed based on full-wave simulation and circuit simulation. Finally, seveal high-performance planar tunable RF bandpass filters and a tunable three-dimensional FSS structure are fabraicted, measured and discussed.For planar tunable bandpass filters, each resonant mode of quadruple-mode resonator and dual-mode resonator, the regulation are at first studied by using odd-and even-mode equivalent circuit. Then, a filter with tunable bandwidth and a dual-band bandpass filter with independently controllable dual passbands are respectively proposed by using the varactor-loaded quadruple-mode resonator and varactor-loaded dual-mode resonator. Finally, second-order, third-order and fourth-order tunable filters with constant absolute bandwidth are synthesized by using varactor-loaded anti-parallel open coupled line. The main contribution of the work on tunable bandpass filters are listed as follow:(1) Since microstrip multi-mode resonator has several advantages, like "multiple resonant modes existing in one cavity" and the control among each resonant mode is easy to implement, a novel quadruple-mode resonator structure is proposed. Through the study on the mechanism of the resonant modes, it can be observed that the up and down resonant modes can be independently controlled through three (two groups) open-stubs while keeping the two middle ones unchanged. Then, a tunable filter with the fixed center frequency and independently tunable high- and low-side edge of the passband is designed by making use of three (two groups) varactor-loaded open-stubs. This method can provide a novel way to realize the fully tunable filters, and this structure would be advantageous because of its high selectivty and miniaturization.(2) The proposed tunable dual-mode resonator is at first theoretically analyzed to demonstrate its advanced capacity in independent control of the first and second resonant frequencies with resorting to the two distinct even- and odd-mode capacitances. Secondly, design parameters of the coupled-line sections and input matching capacitances are properly selected in order to realize controllable coupling coefficients and external quality factors. A prototype tunable dual-band filter is in final fabricated and measured. This filter is proposed by exploiting the characteristics of dual-mode resonator that the dual-mode can be independently control, other than the traditional two independent signal paths. The proposed resonator can be easily used in designing other independently tunable dual-band circuits, such as tunable duplexer, tunable dual-band bandstop filter, etc.(3) The resonant frequencies and reactance slopes of proposed varactor-loaded microstrip resonator are at first theoretically analyzed. Secondly, according to the synthesis theory of traditional filter, the desired values of J and K inverters of constant 3-dB bandwidth tunable filter are given. Then, by properly selecting the design parameters of coupling-line and semi-lump metallic via, the extracted values of the J and K inverters used de-embedding method can be tuned to meet the desired values of the J and K inverters, respectively, over the desired frequency range. Final, a second- and third-order constant aboulte bandwidth (CABW) tunable filters based on J inverters and a fourth- and sixed-order CABW tunable filters based on alternative J/K inverters are synthezied. This method is quite simple and does not need multiple voltages to control the coupling coefficient and input/output structures, resulting in good prospects in applications.For tunable three-dimensional FSS, a new second-order 3-D tunable bandpass FSS wide tuning range is presented. This structure is based on 2-D periodic array of vertical varactor-loaded microstrip line resonators and a number of horizontal metallic plates. The proposed varactor-loaded λ/4 stepped-impedance resonator (SIR) is first theoretically analyzed to demonstrate its superior performance in implementation of a wide tuning range. Design parameters of the via hole and metallic plate are then properly selected in order to meet the requirement of bandwidth and location of transmission zeros. Finally, a prototype 3-D tunable bandpass FSS with 2:1 tuning range is fabricated and measured to verify our design concept and its tuning performance.The various kinds of novel tunable filter structures and the design methods proposed in this thesis have a certain instruction function to design the tunable filters with wide tunable bandwidth, high tuning speed, high linearity and high frequency selectivity. Meanwhile, since various tunable bandpass filters in this thesis are designed by making using the inherent electromagnetic coupling characteristic of the resonator to realize the controllability, they are easy to control and produce and with broad application prospects.