Beyond DDMTD—Sub-Picosecond Timestamps for Asynchronous Clocks

Many navigation, sensing, and time-transfer applications call for the high-precision timestamping of events that occur in different digital clock domains. However, state-of-the-art techniques such as digital dual mixer time difference (DDMTD) typically require that the measured clocks be syntonized, i.e., derived from a common reference oscillator and identical in frequency. This article introduces a novel, all-digital technique that can provide accurate collinear timestamps for asynchronous clock domains of arbitrary frequencies. The vernier-referenced digital asynchronous collinear timestamp (VERDACT) system allows timestamps to be generated and compared across any number of arbitrary clock domains with sub-picosecond precision. The novel circuit can be implemented on most field programmable gate arrays (FPGAs) without the need for external components. Under laboratory conditions, an open-source prototype has demonstrated end-to-end precision better than 0.8 ps-rms on a Zynq UltraScale+ FPGA. Potential applications include sub-nanosecond two-way time-transfer using precision time protocol (PTP / IEEE-1588) over an asynchronous multiport Ethernet switch.