Researches On Compact Microstrip Filters In Wireless Communication System

Researches on high performance and compact microstrip filters in wireless communication system have been done in this dissertation. Several design schemes for filter harmonic suppression and miniaturizations are presented. Based on these schemes, some types of new microstrip filters are designed.In Chapter 1, research background and signification of microstrip filter in wireless communication system are introduced and advances for the study of high performance and compact microstrip filters are reviewed. At present, researches on high performance and compact filter are mainly developed from new package technology, miniaturized resonator, slow wave loading, and double negative material transmission line, etc. Although much work has been done in these aspects, there are many problems needed to be resolved.In Chapter 2, resonance properties of stepped-impedance resonator are analyzed with transmission line theory. Resonance conditions and electrical lengthes of two-section and tri-section stepped-impedance resonators are mainly analyzed, which provide the theory foundation for filter design. Compact microstrip bandpass filters with harmonic suppression are designed with tri-section stepped-impedance resonator. Tunable passband property of the stepped-impedance resonator is used for the design of dual-band bandpass filter that can be applied to wireless local area network communication system. Design theory for this type of filter is presented.In Chapter 3, general theory of coupled resonator filter is introduced. Conventional open-loop uniform impedance resonators are used to design a four-pole coupled resonance microstrip bandpass filter. S parameters of the filter are computed by using the even-odd-mode theory and by employing full-wave EM simulator. Based on the conventional coupled resonance microstrip filter, an embedded-resonator topology is proposed which makes the filter have a more compact size. By using the character of incorporating resonators that have same fundamental resonant frequency but with different spurious frequencies in the filter structure, the spurious responses can be suppressed up to 4.5 f₀. In order to further decrease the size of the filter, a two-layer microstrip bandpass filter is also designed. Compared with a single layer structure filter, a significant size reduction of about 50% is obtained. Furthermore, harmonic suppression is also good.In Chapter 4, microstrip bandpass filters based on capacitance loading are designed. Principle of dual-mode filter is introduced. Based on conventional dual-mode filter design, a new capacitance loading structure is proposed. The characters of this type of loading structure are analyzed with transmission line theory. A compact microstrip dual-mode filter is designed by employing the loading structure. When the circuit area is kept the same as presently reported small dual-mode filter, the proposed dual-mode filter has a better performance. A miniaturization microstrip bandpass filter is designed by using gap-capacitance loading. The circuit size is decreased a lot at the same center frequency when compared with unloaded ring resonator.In Chapter 5, the research of this dissertation is summarized and future work is predicted.