The Research On Mixed-cavity And Mixed-mode Filter Characteristics

With the rapid development of modern communication technology, higher andhigher demands are proposed on the performance of passive components in RF andmicrowave systems. Filter is one of these indispensable components. The requirementsof filters are becoming increasingly stern. Miniaturization, low-loss, high suppressionand high selectivity are future trends of filters. Therefore, in order to improve theperformance of communication systems, studies of meeting these requirements are ofgreat significance.This paper makes a systematic study of the mixed-cavity and mixed-mode filters.Conventional mixed-cavity and mixed-mode technology can be merely applied todielectric filters. Since dielectric filters exhibit the worst performance of parasiticpassband characteristics among all types of filters, mixed-cavity and mixed modetechnology is just utilized to improve the band rejection of filters. The Q value of asingle cavity, which composites a filter, is studied first in this paper. Then, the effect ofthe Q value of a cavity on the filter characteristics is investigated. The influence ofdifferent combinations of Q value on filter characteristics is then investigated based onequivalent circuits of filters. Finally, conclusion that the combination of low Q value attwo ends and high Q value at middle is the best case for series mixed-cavity andmixed-mode filters is given. This combination form can effectively improve filters’rectangular coefficient, band rejection characteristics, insertion loss and miniaturization.In the case of introducing cross coupling, this combination is still the best.Two types of mixed-cavity and mixed-mode filters are designed based onconclusions summarized from equivalent circuit. One is a mixed-cavity bandpass filtersof coaxial resonator structure, and the other is bandpass filters composed by microstripresonators and dielectric resonators. Simulated and measured results show that theconclusions obtained from these two types of filters with mixed-cavity and mixed-modetechnology is completely consistent with that obtained from the third chapter byequivalent circuits. A high power mixed-cavity bandpass filter is designed, which shows excellentperformance and meets the requirements: the insertion loss is less than0.5dB and thereflection loss is greater than20dB. Two transmission zeros are introduced at the highend of the filter’s passband, which improve the out band rejection efficiently. The outband rejection is greater than40dB at the range of1.3GHz-5.5GHz. Parasitic passbandcharacteristic is significantly better than general filters. Another mixed-cavity andmixed-mode bandpss filter is proposed composed by microstrip resonators anddielectric resonators, which also shows good out band rejection. Passband insertion lossis relatively low and the size is smaller than dielectric filter.