Microstrip Dual-band Multi-mode Filter Design,

Wireless communication systems has made great progress in the 21st century, which has spurred a tremendous development in filter that is essential in wireless communication systems and affects the performance of communication systems. Meanwhile, with the development of communication systems, the limited frequency band became crowded, it demands to divide band much finer, and the requirements of noise, interference are increasing. That is the reason why we need to research the filter miniaturization, high performance, low cost, easy to integrate. Microstrip filter become the common filter be used in wireless communication system because of miniaturization, easy to fabricate and precision, low cost, easy to integrate, and wide frequencies performance.Multi-band filters and multi-mode filters be used in wireless communication system are mainly researched in this paper; which used in wireless communication system are proposed based on stepped impedance resonators (SIR) and stub-loaded resonators (SLR). The author's works mainly consist of:Firstly, related theories of filter are described in this paper, such as design parameters of filter, three types of filter, and frequency transformation; the basic structure and electrical character of stepped impedance resonators (SIR) are discussed in detail, such as resonance characteristics, electrical length, high frequencies, and equivalent circuit.Secondly, based on the analysis of SIR, two types of SIR dual-band bandpass microstrip filters used in wireless local area networks (WLANs) are proposed, and they are achieved by two same SIR with anti-symmetric coupling, the performance of two-band filter should be controlled by adjusting the couple-distance between SIRs. With two methods of creating transmission zeros, a new method of creating transmission zeros are introduced in to improve the performance of filter in this paper, which is introducing unequal stubs in input/output ports, and the relationship between dimension of stubs and transmission zeros are discussed in detail; the measurement results show that it is effective to improve the performance of filter with this method to cerate transmission zeros in stop-band.Thirdly, one type of multi-mode filter is designed by changing the structure of stub-loaded resonators (SLR) based on discussing the electrical character of SLR in this article; this change can not only decrease the dimension of resonator, but also create transmission zero to improving the performance of filter.Lastly, with the method of introducing perturbation in resonator to split the degenerate models, a multi-model filter is proposed by introducing slits in half-wavelength resonator, the slits can not only split the degenerate models, but also create transmission zero to improving the performance of filter. Based on the multi-model filter, one dual-band filter with one resonator is designed by changing the dimension of slits to control the transmission zeros in this paper; the affect of dimension of slits for filter performance is analyzed in detail.