| With the rapid development of mobile communication and wireless multimedia, people pay more attention to the UWB (Ultra-Wideband) technology. It is different from conventional wireless technology, because it is used in PAN of short distance. In order to be adapted to the broad uses of UWB technology, the UWB filter, regarded as one of the most important RF devices of a UWB communication system, should be applied not only to microwave circuits of military applications such as orientation, exploration, and so on, but also to the microwave circuits of civil applications such as using high speed WLAN. Therefore, research on UWB filters is becoming a hot spot.With development of MMIC, some microwave active circuits, such as amplifiers and modems, are getting smaller and smaller. However, many problems still exist in reducing sizes of microwave filters and oscillators. It is especially difficult to reduce the size of microwave filter of passive circuit. Therefore, researching and developing of miniaturized microwave filters with good performance are being pay much attention.In this paper, current research of miniaturized UWB filters inside and outside the country has been presented, analyzed at first. Different methods to design UWB filters have also been compared with each other. According to micro-strip transmission line resonators with SIR structures, the structures and resonance of multi-mode resonators are studied. It is found that impedance matching is improved in a wide frequency band with coupling structures of parallel and interdigital coupled lines by adding aperture to the ground plane. The UWB filter is designed with numerical simulation. It shows that experimental results have good agreement with theoretic results.According to nonlinear dielectric characteristics of BST film, a BST ferroelectric thin film in the substrate is deposited. The permittivity of the BST film changes along with change of the bias offset voltage, which causes the variety of bandwidth of UWB filter. |
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