10.1.1 Traceability to National Standards

Before discussing focused calibrations, we should review the purpose and process of instrument calibration in general. Calibration is the process of transferring accuracy standards from one source or measurement instrument to another. In many countries, a central standards agency has been established so that all measurements can be referenced through calibration processes to a common set of standards. In the United States, that agency is the National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards (NBS). From the NIST, the accuracy standards are transferred through a number of calibration processes to achieve a high accuracy ATE test measurement. Figure 10.1 shows a typical chain of standards transferal from the NIST standards to an ATE measurement, including a final focused calibration stage.

Figure 10.1: Transferal of accuracy standards from the NIST to ATE measurements.

In the United States, the NIST is in charge of maintaining standards defining the volt, ampere, second, meter, inch, pound, etc. It is also chartered with the task of licensing calibration laboratories, which in turn maintain "copies" of the calibration standards to be used to maintain the accuracy of bench instruments such as ATE calibration reference sources or high-accuracy voltmeters. ATE calibration reference sources are used by the tester as the "golden" standard for the volt, ohm, ampere, second, etc. These highly accurate instruments are removed from the tester on a periodic basis (typically once every six months) and sent to one of the NIST-licensed laboratories...