Researches On Synthesis Methods For Multi-band Filters And Multi-band Impedance-matching Networks

With the issuance of3G and4G licenses, a variety of communication standards have beenformulated for the domestic communication system. To allow different standards work simultaneouslyand have no interference between each other, the mobile terminal must be multi-frequency, whichpromotes the development of multi-band components. As the basic and important parts ofcommunication system, multi-band filters and impedance matching networks with high-performanceand small size will also be an urgent need for development. And they are hot topics now. Under thebackground, synthesis methods of multi-band filters and impedance matching networks are studied inthis paper. It contains the following sections:1. By describing the transformation from low pass prototype to band pass filter, classical theory ofmicrowave filters is given. To provide a theoretical basis for research work, the basic principle ofimpedance matching network is also introduced. As the examples, several classic impedance matchingcircuits are cited.2. On the foundation of the second chapter, some equivalent circuits of admittance converter andmulti-mode resonators are proposed to design dual-band and tri-band lumped filter. The measurementof dual-band filter indicates that there are two zeros between passbands resulting in good inhibition instopband. And the circuit has the advantage of minimization by using a small number of lumpedelements.3. The synthesis methods of multiband filters with controllable frequency and bandwidth arestudied. By using the recessive multi-mode coupling element which is a cascading of connectingtransmission line and two multi-mode capacitors, a microstrip tri-band filter is given by adoptinggenetic algorithms to extract parameters of Y-type multi-mode resonators. High isolations areachieved because of the existance of two transmission zeros between adjacent passbands.4. On the basis of capacitive and inductive inverters, the П-and T-type dual-band impedancetransformers are proposed for arbitrary complex impedance and design formulas are deduced. Havinga further study in multi-band impedance matching networks, this paper presents a Г type tri-bandimpedance transformer for real impedance and derives the relevant design formulas. These multi-bandimpedance transformers have advantage of compact dimension by using lumped elements, then mucheasier to be integrated into multi-band system.