Synthesis and implementation of low-temperature superconducting filters with lowpass/dual-bandpass combined responses

Abstract This paper presents the synthesis framework and on-chip implementation of low-temperature superconducting (LTS) lowpass/dual-bandpass filters using a four-layer niobium integrated process. A set of characteristic polynomials defining the lowpass and dual-bandpass responses, along with several transmission zeros (TZs) for enhanced selectivity, is first derived and then mapped to a resonant-coupling prototype. Without frequency normalization, the obtained prototype is composed by bandpass resonators and frequency-dependent impedance inverters that are serially connected one after another. To facilitate the on-chip implementation, the bandpass resonators are carefully rescaled, and we finally determine a simplified lumped network that comprises only lowpass elements in series and mutual reactances between them. Based on the proposed approach, an 8th-order lowpass/dual-bandpass filter, with 0–0.8 GHz, 1.7–3.2 GHz, 4.7–6 GHz sub-bands, and three TZs at 1.7, 4.13, and 6.7 GHz, is demonstrated and experimentally verified at 4.2 K. The measured results, showcasing good agreement with simulations, reveal desired lowpass and dual-bandpass behaviors and enhanced out-of-band suppression. These findings validate the availability of synthesis method and simplified lumped network to LTS co-designed filters for cryogenic applications.