Agilent 5991-1265EN Understanding Measurement Risk - White Paper c20140812 [20] free download

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Keysight Technologies Understanding Measurement Risk White Paper 1 Abstract One key reason for performing a calibration is to assess a device as either in- or out-of- tolerance. Common calibration test scenarios compare a device parameter against that of a measurement standard by way of a measurement process. If the difference between the device parameter and the measurement standard is greater than the specified tolerance, the device is deemed out-of-tolerance. However, errors in the measurement process bring about the possibility of an incorrect assessment. An incorrect assessment may result in devices incorrectly declared as in-tolerance (false-accept) or incorrectly declared as out-of-tolerance (false-reject). The risk of making an incorrect in- or out-of-tolerance assessment can be determined by evaluating probability density functions that incorporate a device's parameter population and the measurement error. This paper provides an intuitive explanation of these probability density functions drawing on Monte Carlo simulation to demonstrate the relationship between a device's true value and the corresponding measured value. 2.0 Introduction In manufacturing facilities throughout the world, test engineers design measurement procedures for manufacturing purposes. It is common for test engineers to rely on the specifications of measuring equipment to assess the accuracy of the measurement procedures. This creates a dependency between the measuring equipment specifications and the quality of the manufacturing process. To maintain manufacturing process quality, the measuring equipment requires periodic calibration. For the above scenario, one of the primary purposes of calibration is to verify that the measuring equipment performs at a level consistent with the equipment's specifications. In other words, is the measuring equipment in- or out-of-tolerance? Frequently, calibration involves comparing a device parameter (that is, a parameter of the measuring equipment) against that of a measurement standard. For example, assume we wish to calibrate an RF power source with a power meter. The purpose of the calibration is to assess the RF power source error (the difference between the indicated power and the true power supplied by the source) and determine if it is less than a specified tolerance. If it were possible to use a perfect power meter and a perfect measuring procedure, determining the RF power source's error is simply a matter of noting the difference between the power meter's reading and the indicated value of the RF power source. However, since a real-world power meter is not perfect, knowing the exact RF power source error is not possible. Our lack of knowledge about the exact error is what gives rise to the possibility of declaring a device