First order sigma-delta modulator in HTS bicrystal technology

We have designed, fabricated and successfully tested a first-order sigma-de lta modulator using a High-Temperature Superconducting (HTS) multi-layer te chnology with bicrystal Josephson junctions. The circuit has been fabricate d on an SrTiO3 bicrystal substrate. The trilayer was fabricated by laser de position and high-pressure sputtering. The bottom layer served as a superco nducting groundplane. The Josephson junctions were formed at the bicrystal line in the upper layer. The integrator resistance has been made from a Pd/ Au thin Nm. The circuit consists of a Single Flux Quantum (SFQ) pulse gener ator, a Josephson transmission line (JTL), a comparator, an integrator and an output stage. Thr correct operation of the modulator has been tested usi ng de and low frequency ac measurements. The Linearity of the modulator was studied by measuring the harmonic distortions of a 19.5 kHz sine wave inpu t signal. From the recorded spectrum, a minimum resolution of at least 6 bi ts can be estimated. This accuracy was limited by the noise of the preampli fier. The correct operation of the current feedback loop was demonstrated b y cutting the feedback inductance. (C) 1999 Elsevier Science B.V. All right s reserved.