Design Of The Step-Impedance Dual-Band Filter

In this thesis, the related theories and technologies of the step-impedance dual-band bandpass filter are studied mainly, with an emphasis on the synthesis and design of this filter.Firstly, the synthesis and design of the conventional filters (Butterworth, Chebyshev, Cauer) and frequency transformation are given as the fundamental theory. Moreover, the basic structures and characteristics of the step-impedance are introduced, with an emphasis on the half-wavelength step-impedance resonator and its application in bandpass filter, based on which the theoretical foundation of designing a dual-band filter is derived. Then the general method to design the dual-band bandpass filter with step-impedance resonator is presented. And the transmission zero is introduced to improve the out-of-band suppression performance of the filter. At last, by an analysis of the transmission zero, a special structure is presented to generate tunable transmission zero, and its equivalent circuit is obtained by the graph transformation. A typical filter is designed based on the synthesis theory of the filter mentioned above, and the simulation result shows a good agreement with the experiment one, which demonstrates the validation and efficiency of the synthesis method.