Study On Characteristics Of SIR And Its Application In Microstrip Filters

Microstrip bandpass filters have found wide application in wireless communication and received wide interests and intense research efforts due to their advantages such as small size, low cost,easy fabrication and so on. Stepped impendece resonator(SIR) used as new structure composing filters can satisfy the demand of small size, good out-band suppression. The characteristic of SIR and design method of micostrip filters were studied in this paper. The main contents are on the following:Firstly, the filter history and basic theory of filter and design method was introduced. Then the common structure of SIR was given. The resonance condition and electronic length , spurious resonance frequency related to the stepped length ratio and impendence ratio of half-wavelength SIR were analysed in detail. Secondly, four-pole microsrip SIR bandpass filter was designed to have good out-band suppression, and the skew-symmetric (0-degree) feed structure was used to get another pair of transmission zeros so the frequency selectivity was greatly improved. Thirdly, the reason of coupled transmission lines with different load producing transmission zeros was studied. Two filters with different zero positions were designed and the relationship between the length of coupled lines and out-band suppression was analysed. Lastly, the method using group delay to design micosrip filters was introduced, and was validated by experiment. Both simulated results and measured results show good performance.