A 3.8 ps RMS time synchronization implemented in a 20 nm FPGA

TL;DR

This paper presents a high-precision intrachannel time synchronization method in a 20nm FPGA, achieving 3.8 ps RMS accuracy, significantly improving over existing self-phase alignment techniques for multichannel high-speed transceivers.

Contribution

A novel protocol combining a high-precision TDC and tunable phase interpolator for intrachannel synchronization in FPGAs.

Findings

Achieved 3.8 ps RMS time synchronization precision.

Reduced maximum variation to 20 ps.

Outperformed existing self-phase alignment methods.

Abstract

A 3.8ps root mean square (RMS) time synchronization implemented in a 20nm fabrication process ultrascale kintex Field Programmable Gate Array (FPGA) is presented. The multichannel high-speed serial transceivers (e.g. GTH) play a key role in a wide range of applications, such as the optical source for quantum key distribution systems. However, owing to the independent clock dividers existed in each transceiver, the random skew would appear among the multiple channels every time the system powers up or resets. A self-phase alignment method provided by Xilinx Corporation could reach a precision with 22 ps RMS and 100 ps maximum variation, which is far from meeting the demand of applications with rate up to 2.5 Gbps. To implement a high-precision intrachannel time synchronization, a protocol combined of a high-precision time-to-digital converter (TDC) and a tunable phase interpolator (PI) is presented. The TDC based on the carry8 primitive is applied to measure the intrachannel skew with 40.7ps bin size. The embedded tunable PI in each GTH channel has a theoretical step size of 3.125 ps. By tuning the PI in the minimal step size, the final intrachannel time synchronization reaches a 3.8 ps RMS precision and maximal variation 20 ps, much better than the self-phase alignment method. Besides, a desirable time offset of every channel can be implemented with a closed-loop control.