Miniaturized superconducting filter realized by using dual-mode and stepped resonators

In this paper, the design principles of a new miniaturized superconducting planar filter configuration is presented. The configuration is based on the combined use of dual-mode cross-slotted patch resonators and stepped resonators. Four-pole filters exhibiting a quasi-elliptical response and with operating frequencies in K-, L-, and C-bands have been designed as a test vehicle of the new configuration. The analyzed topology exhibits a high degree of miniaturization, requiring less than 50% of the area occupied by filters based on dual-mode or hairpin resonators. The physical origin of the transmission zeros is discussed, deriving equivalent-circuit descriptions that are capable of reproducing the measured frequency response. In order to confirm the validity of the introduced design principles, experimental performances on an exemplary L-band filter, realized using a YBCO film deposited on an MgO substrate and exhibiting a 3% fractional bandwidth, are presented. The measured frequency response obtained at T = 77 K agrees well with the simulations, presenting an insertion loss of 0.1 dB and a power handling of 34 dBm.