Novel timing method using IEEE 1588 and Synchronous Ethernet for Compton telescope

Abstract

ICx Radiation, Inc. has implemented a novel timing method for use in a Compton telescope that is capable of nanosecond timing resolution. A critical task in Compton telescope design is to minimize the timing variance between detectors in a large array in order to reduce the background. The voxelSPEC has been developed to combine precise timing with pulse processing electronics in a single device, where all timing, communication, and power is transmitted over non-proprietary Ethernet hardware. The IEEE 1588 Precision Timing Protocol (PTP) combined with Synchronous Ethernet (SyncE) performs both phase and frequency locking of the individual detectors clock to a master clock. PTP phase locks each node to the absolute time recorded by the master clock. Hardware assist PTP uses the Ethernet PHY to time stamp the actual receiving and sending times of each PTP packet with sub-nanosecond accuracy, removing the time variance contribution of the CPU and other hardware, which can be on the order of microseconds to milliseconds. PTP-compatible network switches remove the unequal buffering delays in the switch hardware by performing a hardware assisted time correction to the PTP packet arrival and departure times directly (transparent clock mode) or by synchronizing the node to the network switch with hardware assistance (boundary clock mode). In 100MBps SyncE, the transmitters frequency is provided by the master clocks oscillator and propagated through the network switches to each individual node. Each Ethernet PHY connected to this frequency recovers the clock signal for the internal PTP clock. Using the master clock frequency minimizes the frequency drift. Results using both PTP (version 1 without PTP hardware assisted network switch) and SyncE show a node to master drift of about 8ns and a timing resolution of about 25ns between nodes. The implementation of hardware assisted PTP version 2 network switches will further reduce the timing resolution between nodes.