Research On Novel Planar Tunable Bandpass-to-bandstop Filter

With the rapid development of wireless communication technologies,on the one hand,our lives have become more and more convenient.On the other hand,there are more requirement on wireless communication systems,such as miniaturization.As an important part of the system,filters have faced many challenge.The existence of a variety of communication standards requires that the filter can operate in multiple frequency bands.In general,a common solution to this problem is to combine multiple multi-band filters.However,as more frequency bands are required to be supported,the design complexity and difficulty of multi-band filters are gradually increasing.In order to solve this problem,tunable filters with small size and simple structure have become hot spots in recent years.On the other hand,with the popularity of electronic devices,the electromagnetic environment around us has become more and more complicated,and the interference between electronic devices has become more and more serious.In this context,the study of tunable bandpass-to-bandstop filter have practical significance.Because in the high-interference electromagnetic environment,the bandstop mode can suppress high-power interference whose frequency is near the desired signal.While in a low disturbing electromagnetic environment,its bandpass mode can receive the desired signal.Therefore,this paper focuses on a novel high-performance planar tunable bandpass-to-bandstop filter.The main work contents and innovations are as follows:(I)A wideband bandstop filter was proposed at first.The design process and method of it provided a theoretical basis for the following bandstop filter design.Then two planar tunable bandpass-to-bandstop filters were proposed.The first filter used half-wavelength open-circuit resonators and quarter-wave short-circuit resonator loaded varactors to adjust the frequency.The second filter used a multi-mode resonator loaded with varactor.The transmission of the bandpass-to-bandstop characteristics in the filter was achieved by adjusting the status of the PIN diode loaded in the middle of the feed line.Experimental results have verified that the adjustment of the PIN diode state can change the bandpass-to-bandstop characteristic of the filter,which also provided a theoretical method for the design of other high-performance filters.(II)Research on controlled bandwidth tunable bandpass-to-bandstop filter has been done.Firstly,the mechanism of controllable bandwidth has been analyzed and researched.By controlling the varactors loaded at both ends of half-wavelength resonators,it could be concluded that there were different capacitance ratios of varactor capacitances at the same resonant frequency through principle analysis.What's more,different capacitance ratios can change the Q_e of the filter and the coupling strength between the resonators.Therefore,it could achieve a controllable bandwidth.Finally,it has been fabricated and measured.(III)Research on dual-band tunable bandpass-to-bandstop filter has been done.It proposed a theory that changing the varactor capacitance loaded in the middle of the feeding lines could change the Q_e of the filter.The simulation results helped to analyze its working principle.Because the coupling paths of both band in the filter were not overlap,the dual-band could be independently adjusted.(IV)A wide stopband tunable bandpass-to-bandstop filter was studied.Firstly,the realization mechanism of wide stopband was studied.And then three commonly used wide stopband/harmonic suppression methods have been proposed.An example of a wide stopband low pass filter has been proposed to illustrate the design of a wide stopband filter.The wide stopband was achieved by using a resistor and capacitor connected in series in the middle of the resonator and loaded open-circuited stubs.The loaded open-circuited stubs were used to generate a transmission zero to achieve wide stopband.