Research And Design Of Absorptive Tunable Bandstop Filter

With the rapid development of the wireless communication technology,dynamic alloc-ation of frequency resources and high frequency spectrum utilization are needed to be more effective.Since filter plays an essential role in the wireless communication,the absorptive,selectivity and tunable properties are hot spots for research.The paper start with the basic theory of filter design,and then discussed the design theory of absorption tunable bandstop filter based on microstrip line structure deeply.The main contents are as follows:Firstly,we studied about the theory of cross-coupling structure for absorption bandstop filter.Based on the derivation,two filters with different unload Q were designed to analyze the selective of this type of reflectionless filter.Also,the tunning performance was studied by a simulation results.Varactor diode was chosen to be the tunning element.Secondly,we proposed a new absorptive tunable banstop filter based on a 3dB transdirectional coupler.The phase and magnitude of signals which flows in the coupler were analyzed.By controlling the coupling gap and length between the resonator and the feedlines of the through port and the coupling port,we can obtain a reflectionless tunable band-stop filter with good stopband suppression and better passband bandwidth.Finally,we discussed the tuning ability of a coupled line structure which has been proved to design as a absorption bandstop filter with a resistor add to one port of the coupled line.Three coupled line sections with the same dimensions were cascaded for the study of it's tuning performance.Simulation and measured data were compared for a conclusion.