Designs of Coupled-resonator Filters Using Dual-transmission Lines

Abstract

In microstrip coupled-resonator filter design, stepped-impedance resonator (SIR) can be used for adjusting the resonant frequencies and reducing circuit size. High impedance ratio of SIR which can usually achieve compact resonator size is composed of high- and low-impedance lines. However, low-impedance transmission line is wide in microstrip form, which may be not easy for circuit layout arrangement. To improve this issue, dual-transmission lines are utilized to replace the low impedance lines of SIR and the modified SIR is used to design coupled-resonator filters in this study. In general, there are two types of stepped-impedance resonators (SIRs). One is half-wavelength (1/2λ) SIR which both sides are open. The other is quarter-wavelength (1/4λ) SIR which one side is open and the other side is short. The main differences between 1/2λ and 1/4λ SIRs are the higher order resonant frequencies and resonator sizes. For example, in the same high-low impedance and length ratios, size ratio of 1/4λ SIR to 1/2λ SIR approaches 1/2 and the first spurious frequency of 1/4λ SIR can easily design at higher frequency than that of 1/2λ SIR. However, 1/4λ SIR needs extra via hole to increase the process complexity and to produce unpredicted effect. Therefore, the designer needs to make a trade-off depending on the requirement. This study proposes two coupled-resonator filters. The first proposed filter with fourth-order 0.01-dB equal-ripple response uses modified 1/2λ dual-transmission line stepped-impedance resonators (DTSIRs); the second proposed filter with third-order 0.01-dB equal-ripple response uses modified 1/4λ DTSIRs. Each of the two microstrip filters has been examined carefully and the experiment result has high consistency with simulated result