Superconducting NbN band-pass filter and matching circuit for 30 GHz RSFQ data converter

We present the planar microwave filters developed as the front selective and impedance matching circuit for the RSFQ based (Rapid Single Flux Quantum) A/D converters, operating at cryogenic temperature ( T = 9 K ). The structures are to be patterned in thin NbN (niobium nitride) superconducting films with critical temperature Tc ∼ 16 K. The filter design is experimentally verified at lower frequency ( ƒ0 = 5 GHz, microstrip ) on the Ro3006 substrate. During the design, we aim to reach high stopband attenuation and matching of different input - output impedances. The filters use high impedance level in order to obtain suitable dimensions of the lines, important for the final accuracy. A layout of 1 cm² NbN filter designed for ƒ0 = 30 GHz and CPW is presented. This NbN filter is intended for the extraction of parameters such as kinetic inductance or London penetration depth, in order to improve the simulation accuracy of our NbN process.