Research On Miniaturized And Tunable Microwave Bandpass Filter

With rapid development of communication technology,microwave bandpass filters play important roles in RF front-ends.In order to reduce the cost of RF systems,the miniaturization of RF devices becomes more and more important.In addition,in order to meet the requirement of multi-band communication systems,multi-band microwave bandpass filters and tunable microwave bandpass filters are widely used in practice.In the aspect of miniaturization of microwave bandpass filters,this paper first realizes the filter miniaturization by physical bending of resonators and using of multi-mode resonators,and then by using of multi-layer substrate stacking technology.In the aspect of tunable microwave bandpass filters,this paper first uses varactor diodes to achieve a tunable filter,and then in order to reduce the insertion loss of the filter,uses distilled water to achieve the tuning of the filter.The researches in this paper are divided into the following three parts.1.A planar triple-band second-order bandpass filter based on spiral quarter-wavelength short-circuited resonators and multi-mode resonators,as well as a single-band second-order bandpass filter based on multi-layer substrate stacking technology,are designed.Due to the existence of electromagnetic hybrid coupling and source-load coupling,the triple-band bandpass filter has multiple transmission zeros.The single-band bandpass filter consists of two spiral quarter wavelength short-circuited resonators located in different metal layers.Because the input and output ports of the single-band bandpass filter are in the same layer,it can be well compatible with surface mount technology.Simulation and measurement results of the two bandpass filters are in good agreement.2.A tunable dual-band second-order bandpass filter using varactor diode loaded resonator technology is designed.The bandpass filter consists of two pairs of quarter wavelength short-circuited resonators loaded with varactor diodes.A pair of varactor diodes are connected between adjacent resonators to control the electric coupling strength.By changing the electric coupling strength,the two transmission zeros generated by the hybrid electromagnetic coupling structure can be located on the left side of the first passband and the right side of the second passband respectively.Then,based on the tunable dual-band bandpass filter,an external quality factor control element is added to simulate a fully tunable single-band second-order bandpass filter.By choosing the appropriate external quality factor and coupling coefficient,the absolute bandwidth of the single-band bandpass filter can be kept unchanged during the process of frequency tuning.3.A tunable single-band third-order bandpass filter based on a strip-line triple-mode resonator is designed.There are six polytetrafluoroethylene tubes above the resonator.By adjusting the amount of the distilled water in the tubes,the equivalent electric length of the transmission line changes,thus changing the frequency of different modes of the resonator.Simulation and measurement results show that the filter can achieve frequency and sideband tuning,and has small insertion loss.