Thesis presented July 26, 2007

Abstract:
RSFQ (Rapid Single Flux Quantum) superconducting logic is suitable to process very high speed digital data with very low power dissipation and with performances well beyond what should be possible with CMOS technology in the next decades. The RSFQ circuit technology, based on superconducting niobium nitride (NbN), presently developed at CEA-G, involves NbN/TaXN/NbN internally shunted Josephson junctions with high critical current density and high maximum switching frequency close to 1 THz at 10 K, as required by ultra-fast RSFQ electronics. The purpose of this work is to apply the NbN technology to an ADC (Analog-to-Digital Converter) for space telecommunications. An ADC with sigma-delta architecture was studied oversampling at 200 GHz clock frequency a signal with a bandwidth of 500 MHz modulated over a carrier frequency of 30 GHz. In particular a clock, a comparator and some other logic gates were studied at 200 GHz as well as a third order sigma-delta band-pass modulator whose the SNR and SFDR performances, after optimization, should satisfy the specifications. Decimation filter architecture complexity was analyzed. Some basic components of the filter, such frequency dividers and shift registers, were studied and designed, and in the end some possible methods to test the modulator were also proposed. The implementation of NbN circuits in a multi-levels technology was treated including the complete fabrication of two wafers. For some technology problems clarified afterwards, these two lots couldn’t reach the ADC logic gate test. However the RSFQ gate margins were determined thanks to junctions, SQUIDs and microwaves filters (resonators) characterization. Finally, we compare some circuits based on self-shunted NbN junctions and other similar circuits in Nb foundry technology using Nb/AlOx/Nb externally shunted junctions operating at 4 K. This comparative study shows all the advantages given by the NbN technology.

Keywords:
Analog-to-Digital Converter, sigma-delta, Josephson junction, RSFQ logic, decimation, registers, microwaves filters, comparator

On-line thesis.