Band-stop Filter Design Research

Bandstop filters are mainly used in a variety of microwave systems, such as wireless communication, testing etc, which play a key role in filtering out the interference signals mixed in the desired signals. In the background of increasing demand for microwave communication system performance, designers tend to higher performances of bandstop filters, such as higher rejection in stopband, lower insertion loss in passband, narrower transition bandwidth, wider upper passband, smaller size and so on.The main contents of this paper are the basic theory of bandstop filter design, structure and methods. Given that better performance, such as narrower transition bandwidth, wider upper passband, smaller size, are demanded, this paper presents two types of cavity bandstop filters, designed for narrow transition bandwidth and wider upper passband respectively, which both are with compact structures.Firstly, this paper describes the bandstop filter design theory, design methodology and general structures briefly. Coaxial short-circuit structure is applied to implement a bandstop filter, which meets the requirement well.Secondly, to obtain a narrower transition bandwidth, the paper proposes a new bandstop unit, with which a bandstop filter is designed, whose frequency centered at2.6GHz. The transition bandwidth ratio the stop bandwidth is1/4. And the filter is of compact size because of the capacitance loaded transmission line.Thirdly, with respect to spurious stop band of the bandstop filter, this paper mainly analyzes three key factors (spurious mode in resonation cavity, high-order mode in transmission line, the capacitance loaded to the transmission line). And an designing example of wider upper passband bandstop filter is presented, whose upper passband frequency is four times away center frequency. The filter is also of compact size for the reason that the transmission line between resonators is S-shaped arranged.Finally, the two bandstop filters introduced in this paper are fabricated and tested, and the measured curves are in good agreement with the simulation result.