Research And Design Of Microstrip Tunable Filter

At present,with the rapid development of modern wireless communications, the occupation of the spectrum is more and more intensive and spectrum congestion is the last thing to be ignored. To solve the spectrum congestion and make better use of the limited spectrum resources, spectrum technologies such as frequency hopping, spread spectrum and dynamic frequency allocation have been widely used in communication systems. As a key component of the reconfigurable systems, electronically tunable filters have attracted more and more attention. In the transceivers and receivers, filters can eliminate and prevent interference of out-band noise, reserve useful signals, and also support multiple information channels.Based on the classical theory of filters, this paper briefly introduces the theory of designing tunable filters and recommends the ways to achieve them. After considering the advantages and disadvantages of the prevalent implementations, microstrip tunable filters with the advantages of high tuning speed and low price turn out to obtain the mojority concern in this work. The major work is concluded as follows:Firstly the basic design theory of traditional filters is introduced. After that, a summarization and comparison of different implementations of electronically tunable filters is made. The use of varactors is choosed as the method to realize the tunable filters.The fully tunable filters occupy a forefront position and have turned to perdominate in the researches of tunable filters.By using tunable dual-mode resonators and external quality factor circuits, a fully tunable filter circuit is designed in this papper. First,the resonance characteristics of the U-shaped tunable dual-mode resonators are proposed and analysed.Meanwhile, a simple circuit is used to implement the tunable transformer and J/K inverter. This fully tunable filter has the advantages of widely controllable bandwidth range and stable performance.At last, a band-pass filter with tunable center frequency is designed based on a two-port rat-race-ring. Firstly, the structure and principle of the rat-race-ring, the characteristics of the bandpass/bandstop are briefly introduced. Then the structure of the bandpass filters and methods to adjust them are given. Two tunable LC resonators located at the orthogonal position of the I/O ports are added to the circuit, converting the stop band to passband. By adjusting the value of the varactors, a filter with a tunable center frequency is genarated. Finally, to solve the problem that the center frequency of the passband shifts caused by the phase difference of the two resonance paths resulting from the use of loaded stubs, a zero-phase difference structure is designed. During the study, simulation and optimization of the filters have been launched out and the proposed filter has also been fabricated and measured. Measurement results turn out to be an excellent agreement with the simulated results.