25-GHz Operation of ERSFQ Time-to-Digital Converter

Abstract

Fast time-to-digital converters (TDCs), used to convert a continuous time interval into discrete number for further processing, serve diverse applications ranging from photon/particle detectors to communication systems employing delay-encoded pulses. Being a multi-rate digital circuit, the TDC is also a good candidate to explore operation of Energy-Efficient Rapid Single Flux Quantum (ERSFQ) circuits at high (>10 GHz) clock frequency. We designed a TDC using ERSFQ cells, targeting the 10-kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> SFQ5ee fabrication process at MIT Lincoln Laboratory. The main elements of the circuit comprise a 9-bit binary ripple counter and a parallel-to-serial converter. A frequency divider and a pulse distribution network with a decision-making element are designed to control the operation. The ERSFQ TDC was operated up to 25 GHz clock frequency (40 ps time resolution) and with a power consumption of around 14 μW for all ERSFQ components. The design specifications such as number of junctions and area will be discussed together with the power delivery technique involving an over-pumped feeding Josephson transmission line (JTL).