Interaction Features of Stepped-Impedance Stripline Resonators in Comb Filters

Abstract

Mixed coefficients of coupling between the closely spaced stepped-impedance resonators in comb filters of stripline design have been investigated. Transmission zeros at frequencies f_zi correspond to mixed coupling coefficients k_i. These zeros can be moved with respect to the filter passband central frequency f₀ by modifying the shape of resonators. It was proved that the reduction of gap between resonators made it possible to locate frequencies f_z and f₀ closer to one another. The existing restrictions on the minimal value of gap between resonators limit the degree of proximity between f_z and f₀. The N-resonator stripline comb filters with mixed coupling can have N−1 transmission zeros. The absence of cross-coupling links in stripline filters simplifies their construction. It has been established that the thickness of central conductors of stripline resonators affects the positive and negative mixed coupling coefficients. The paper presents measurement data of miniature stripline three-resonator comb filter having an enhanced selectivity at the expense of two transmission zeros. The central frequency of filter is f₀ = 1850 MHz, the bandwidth BW = 100 MHz. The filter having dimensions 5.8×4.2×2 mm was implemented by connecting two ceramic substrates having relative dielectric permittivity ε_r = 92 and the metallized patterns deposited on them.