Compact superconducting dual-band bandpass filter by combining bandpass and bandstop filters

A superconducting dual-band bandpass filter with a mixture of bandpass and bandstop sub-filters is presented. The bandpass sub-filter is designed with λ/2 double spiral resonators mid-tapped by λ/4 connecting lines which act as admittance inverters. The tap position of each resonator is adjusted to meet the required susceptance slope parameter. The bandstop sub-filter is designed with λ/2 folded resonators embedded in and coupled with the centres of the λ/4 connecting lines. The dual-band filter is designed and fabricated on an LaAlO₃ substrate with a compact size of 30.4 × 10.3 mm. The measurements show good dual-band response and match well with the simulations.