The single Key Comparison in the TF domain, CCTF-K001.UTC, gives participating laboratories traceability to the SI second by using the published UTC-UTC(k) values, which enable the frequency offsets of the local time scales and clocks to be determined as well as supporting time and frequency measurements. However, CCTF-K001.UTC does not directly confirm the measurement proficiency of the laboratory in the TF domain. One important aspect of this proficiency is the ability to perform time interval measurements, as they are crucial for the correct calibration of time transfer systems, determination of signal delays, and timing control.
This Supplementary Comparison (SC) aims to support CMC claims in time interval (TI) measurement. It is based on experience with cable delay measurements within EUROMET Project 828, which showed that a cable delay is not a well-defined measured quantity and its value is significantly dependent on the shape of the signals used for the delay measurements.
For the purposes of this SC, two types of TI standard were prepared within EURAMET Project 1288:
- TI Generator (TIGen) based on PLL loops and programmable logic and counters, which can generate 127 different time intervals within the range from approximately 20 ns to 12 μs (4 of them are chosen for the SC);
- Set of 3 delay standards (InLambda standards) based on stabilised fibre delays of around 20 ns, 100 ns and 300 ns respectively.
TIs are generated by the TIGen with instability and reproducibility not exceeding 3 ps of assigned expanded uncertainty and by the InLambda standards with instability and reproducibility not exceeding 10 ps of assigned expanded uncertainty – as verified within EURAMET Project 1288.
The pilot laboratory (GUM) will collect all measurement results and evaluate the comparison. However, in order to ensure transparency and impartiality of the pilot laboratory measurement results, it is proposed that the pilot laboratory will perform the measurements on the travelling standards first and will send these and later measurement results to the BIPM as a third party (if the BIPM will agree to act as a third party). After completion of the final report, the BIPM will check and confirm the pilot laboratory measurement results presented in the report.
Each participating laboratory will cover the costs of the measurement, transportation and custom formalities as well as of any damage to the travelling standards that may occur within its country. The participating laboratory must accept that their results will be published in the final report of the comparison, even if they are not considered to be satisfactory by the laboratory. The SC will follow the rules described in EURAMET Guide on Comparisons – EURAMET Guide No. 4, Version 1.1 (12/2016).
The SC aims to present the opportunity for every participating EURAMET NMI or DI to verify or improve its CMC claims for time interval measurements.