Study On The Structure And Performance Of Miniature Planar Microwave Filters

With the rapid development of mobile communications and the high integration of microwave circuits, microwave filter which act as the key elements of communication system, are demanded toward to high performances and small sizes. At the same time, the incessant upgrade of electronic computers, the finite-difference time-domain (FDTD) method is becoming a powerful tool for analyzing these electromagnetic problems. This paper does a lot of research work on miniaturization and high performance of microwave filters. By taking use of new technologies, such as substrate integrated waveguide (SIW), dual-mode structure, and fractal structure, etc., some kinds of novel filters, such as low-pass filters, band-pass filters and band-stop filters, are proposed. FDTD analysis of low-pass filters is studied. The dissertation is summarized as follows:Firstly, the principal theory of FDTD (finite-difference time-domain) is briefly discussed. And then the difficult questions that use FDTD to compute microstrip circuits are researched, such as the boundary conditions of PML and transmission characteristics. These theories are the foundations of studying and analyzing microstrip filters. The compute results of microstrip low-pass filters, which are proposed in the next chapter, indicate that the computing theories about microstrip circuits are correct.Secondly, fully analysis the stop-band characteristics of a microstrip structure (DMS) and the differences between DMS and defected grounded structure. The lumped circuit parameters are extracted from EM simulation and equivalent circuit theory. This structure can be easily integrated with other active circuits, and avoids the leakage through the ground plane. Two types of three-pole low-pass filters using DMS are proposed, and the FDTD is used to analysis the related results. Based on two types of three-pole filters, a type of five-pole low-pass filters is proposed. Three types of filters all have good stop-band characteristics in high frequencies and excellent pass-band characteristics in low frequencies.Thirdly, based on analysis of triangular and circular dual-mode patch filters, the fractal structures are applied in designing dual-mode microstrip patch filters. A class of fractal-shaped triangular filters and a class of fractal-shaped circular filters are proposed. The degenerate-modes can be well excited, which improves the pass-band characteristics. Two classes of filters all have attenuation poles at each side of pass-band, which efficiently improved the frequency selectivity. Fractal structure also can reduce the resonant frequency of initial filters, which making for the filter's.Fourthly, the basic structure of substrate integrated waveguide (SIW) is introduced and some fundamental characteristics, such as transmission mode and the equivalent condition as rectangular waveguide, are researched. A single cavity SIW dual-mode band-pass filter is introduced. The transitions of SIW with microstrip are solved by using concave transition and grounded coplanar waveguide transition. This kind of filter has high quality factor and low insertion loss, which expands the applications of narrow-band filters.Finally, the split ring resonator (SRR) and complementary split ring resonator (CSRR) that act as fundamental composite units to make metamaterial are discussed. The resonant performance and the planar band-stop filters that based on SRR and CSRR are researched. The complementary split ring resonator (CSRR) which etched in conductor strip of microstrip line is proposed. The electric field is fully used to excite the CSRR, which can decreases the resonant frequency and widen the stop-band bandwidth. At the same time, the lumped circuit parameters are extracted from EM simulation and equivalent circuit theory. Based on this type CSRR, two 1-D CSRR band-stop filters are proposed, both filters all have wider bandwidth and deeper stop-band.