Timing Circuits for RSFQ Digital Systems

A low-skew frequency divider and clock controller have been designed for high-frequency timing of superconductor rapid single-flux-quantum (RSFQ) digital systems. The circuits have only about 10-ps skew between input and output signals and are applicable for multirate digital systems (e.g., oversampling analog-to-digital converter and bit-serial digital systems). Several circuits have been fabricated in conventional Nb-trilayer technology with a critical current density of 1 kA/cm2. The most complex clock controller generates trains of 224 single-flux-quantum pulses with a period of less than 70 ps. The long-term relative stability of these intervals has been measured to be better than 6 x 10-5. The basic component of the controller, a frequency divider, operates at input frequencies above 85 GHz.