Researches On Multi-band Filters And Electrically Tunable Filters

With the rapid development of wireless communication system, 3G and 4G network tend to be used widely, making communication standards is more and more diverse. To allow different standards work simultaneously and have no interference between each other, the mobile terminal must be multi-frequency, which promotes the development of multi-band components. As the basic and important parts of communication system, multi-band filters and electronically tunable filters with high-performance will also be an urgent need for development. And they are hot topics now. Under the background, researches on multi-band filters and electronically tunable filters are studied in this paper. It contains the following sections:1. By describing the transformation from low pass prototype to band pass filter, classical theory of microwave filters is given. To provide a theoretical basis for research work, the basic principle of impedance matching network is also introduced. As the examples, several tunably electrical elements are cited.2. On the foundation of the second chapter, dual-mode resonators are proposed to design a dual-band bandpass filter and dual-mode notch elements are proposed to design a quad-band bandpass filter by embedding it into dual-band bandpass filter. The frequency and bandwide of quad-band filter is independent and controllable. Good agreement between simulations and measurements are achieved.3. The reserch on a bandpass filter with controllable bandwides is studied. The basic theory of knowledge and the equivalent circuit of PIN diode is introduced. The basic theory of comb line filters is introduced. By loading PIN diode between resonators, the bandwide can be controlled thorough the offset voltage.4. A electrically tunable bandstop filter is studied in this chapter. Several design methods of bandstop filters are introduced and the theory, equivement circuit and important parameters of varactor are given. A bandstop resonator with controllable frequency and bandwide is proposed. Some formulas are derived. The structure is improved to reduce the varactor range and increase the quality factor.