Investigation Of Multi-band Filters And Multiplexers

In the modern society, the wireless technology is in rapid development, and people have ever-increasing demands for RF devices, which are not only the low insertion loss in passbands and high attenuation in stopbands, but also sharp skirt selectivity due to the crowded frequency bands. In addition, in the competitive communication industry, filter miniaturization is an important work.Since different wireless standards have been emerged into the communication industry, research on multi-band devices is the demand of the developing society. As an indispensable part in communication system, bandstop filters deserve a total research, which is of vital importance.In this paper, an intensive research on multi-band bandstop filter is based on the main requirements of modern RF filers. Compared with reported multi-band bandstop filter, we aim at proposing a new design method and a significant improvement on performance, to meet the higher practical demands.Main work as follows.1. Study and design of tri-band bandpass filter with well performance. For multi-frequency filter, stub-loaded resonators(SLR) and stepped-impedance resonators(SIR) are commonly used. These two kinds of resonators are mostly used in dual-band filter. In this article, we deeply study these two resonators and use them in tri-band bandpass filter design. We aim at good in-band and out-of-band performance and wide tunable range. We first analyse the theory of the resonators, and we use fullwave simulator to design and optimize the filters. We also give all the design curves. Finally we fabricate and measure the filter. A good agreement between the simulated and measured results proves the correctness of the design and analyse methon. These works have been published in Micorwave and Optical Technology Letters.2. Study of mixed coupling and its application in filter design. Transmission zeros in traditional cavity filters are usually created by cross coupling. However, filters with cross coupling are limited by the inflexible topology, which is inconvenience in industry application. We found that mixed coupling can also introduce an extra transmission zero, which makes it a good substitution. We design a dual-band coaxial cavity filter, using stepped-width coupling windows to introduce controllable mixed couplings. Meanwhile, we use the helical feeding structure, which greatly increases the tunable range of external quality factor. We provide detailed design curves for these two structure. These works have been published in IEEE Microwave and Wireless Component Letters.3. Study on multi-channel diplxers and triplexers. In diplexer and triplexer design, the matching network determines the isolation of each channel. Design of diplexers will be more convenience if indepedant control of each channel is realized. We introduce a novel matching network using open-circuited stubs and short-circuited stubs, which can be used in multi-channel diplexers and triplexers easily. We provide systematic design procedure for the matching network. Moreover, the diplexer is fabricated and measured. The measured results agree well with the simulated results. These works have been published in IEEE Microwave and Wireless Component Letters.