It is not only for test and measurement of great importance to synchronize clocks of networked devices to timely coordinate data acquisition. In this context the seek for high accuracy in Ethernet-based clock synchronization has been significantly supported by enhancements to the Network Time Protocol (NTP) and the introduction of the Precision Time Protocol (PTP). The latter was even applied to instrumentation and measurement applications through the introduction of LXI. These protocols are usually implemented in software; however, the synchronization accuracy can only substantially be improved by hardware which supports drawing of precise event timestamps. Especially, the quality of the timestamps for ingress and egress synchronization packets has a major influence on the achievable performance of a distributed measurement or control system. This paper analyzes the influence of jitter sources remaining despite hardware support and proposes enhanced methods for up to now unmatched timestamping accuracy in Ethernet-based synchronization protocols. The methods shown in this paper reach sub-nanosecond accuracy, which is proven in theory and practice. 1. Introduction In instrumentation and measurement, the General Purpose Interface Bus (GPIB) was for a long time the system for data collection and networking of equipment. This bus system has a dedicated wiring for triggering devices and to simultaneously start measurements. The reason for the continuous usage of this relatively old technology is the excellent tool and driver support and the simplicity of the system. Despite these arguments, GPIB has several drawbacks in the handling (connectors, cable) and generality of the approach. First of all GPIB is limited in terms of cable length and number of bus devices. The parallel data transfer and strict arbitration scheme also limit the achievable data rate and make handling and configuration quite complicated for the user. Second, GPIB is also limited in terms of its functionality and does not comply to modern networked systems. A solution for the test and measurement industry to tackle the drawbacks of GPIB can be found in the LAN extensions for instrument (LXI) [1] approach. This de facto standard uses the well-established Ethernet technology to network measurement devices. The advantage is clearly that one can embed such a system seamlessly into office and lab networks having all advantages of a full network functionality. The application in test and measurement is however only feasible if it can be ensured that the devices are properly triggered. The
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