Design And Implementation Of Microstrip Tunable Filter

Nowadays,with the rapid development of radar and communication technology,microwave filters have been widely used in many fields such as microwave communication,microwave guidance and navigation,satellite communication and electronic warfare.Traditional filters with fixed performance cannot meet requirement in some engineering fields for the lack of flexibility.As comparison,tunable filters have the advantages of flexibly adjusting center frequency,passband bandwidth,and transmission zero,which have become focus now.Especially,tunable microstrip filters one important kind of tunable filters.This thesis focuses on the research on tunable microstrip filters.Some novel ones have been presented.The main contents of this thesis are as follows:1.A microstrip filter with tunable center frequency is presented,which incorporates multiple open-ended microstrip stubs.By adding varactors to change the electrical length,the center frequency of the filter can be changed.At the same time,two open-ended stubs are added at the feed lines to suppress multiple harmonics of the filter.The measured results show that the center frequency of the filter can be adjusted from 0.94 to 1.26 GHz.Meanwhile,the 3rd harmonic suppression is greater than 35 dB.2.A tunable filter composed of a parallel-coupled structure and an open-stub structure is presented.The center frequency can be adjusted by using varactors in the filter.The structure is analyzed by using the odd and even-mode analysis method and transmission line theory.The simulated results show that the filter has two transmission zeros at finite frequencies,and the center frequency of the filter can be shifted by adjusting the varactor voltage.The measured results show that the filter can achieve a frequency tuning range of 73.3% relative to the center frequency.3.A tunable filter consisting of an open-loop resonator with length of ?/2 and a parallel-coupled line structure is presented.The structure is analyzed theoretically by using the odd and even-mode analysis method.The study shows that the filter has two adjustable transmission zeros at finite frequencies,and the bandwidth of the filter is adjusted by changing the diode capacitance.The measured results show that the filter has a bandwidth tuning range of 12.1%-40.87% relative to the center frequency with lower loss and smaller size.