New design and implementation of a fast modulator in NbN technology

We have designed a band-pass sigma-delta modulator made in niobium nitride technology. The basic components of such a 10 GHz carrier frequency modulator are a band filtering resonator and an SFQ comparator synchronized with a clock frequency up to 200 GHz sampling rate. We describe the clock design including an internal JTL and its testing circuit based on frequency division. The technology involves NbN/Ta_xN/NbN internally shunted Josephson junctions with Jc∼20kA/cm² and high RnIc product (∼1 mV) at 4.2 K. Junction parameters are shown to be controlled by the reactive sputtering conditions of the Ta_xN barrier. SiO₂/MgO bi-layers are used as dielectrics associated with NbN wiring layers and ground-plane. All layers are prepared in a way compatible with a full processing at about 350°C in order to minimize the London penetration depth of NbN layers and thus to allow circuits operation at about 9 K. High frequency simulations of the modulator´s circuits are presented, including the comparator, the DEMUX, and the resonator response.