Superconducting Planar Filters Using Dual-Mode Cross-Slotted Square Resonators

Passive microwave filters based on high temperature superconducting (HTS) films are very promising for telecommunication applications. Filters based on a dual-mode configuration present the appealing combination of high power handling capability and excellent miniaturization. Here a new kind of dual-mode filter based on cross-slotted square resonators is presented. The novelty of this structure is related to the use of a pair of unequal crossed slots formed on a square patch resonator, so that the size of the device can be significantly reduced. Single stage dual-mode filters are designed with a commercial 2.5D software at frequencies of interest for satellite communications. Prototypes based on a square resonator with 6 mm side operating at 3.7 GHz are fabricated using Nb double-sided films grown on 10 × 10 × 0.5 mm³ LaAlO₃ substrates. The response is measured at 4.2 K and the nonlinearity is investigated observing the power degradation. The effect of additional cuts, opportunely realized on the conductor surface, is discussed in respect to the size reduction and the maximum circulating power. It is found that the resonant frequency can be reduced from 3.7 to 3.3 GHz without affecting the power handling capability.