HP 8753E manual Determining System Measurement Uncertainties, Sources of Measurement Errors

Determining System Measurement Uncertainties

In any measurement, certain measurement errors associated with the system add uncertainty to the measured results This uncertainty deilnes how accurately a device under test (DUT) can be measured.

Network analysis measurement errors can be separated into two types: raw and residual. The raw error terms are the errors associated with the uncorrected system that are called systematic (repeatable), random (non-repeatable), and drift errors. The residual error terms are the errors that remain after a measurement calibration.

The error correction procedure, also called measurement calibration, measures a set of calibration devices with known characteristics. It uses the measurement results to effectively remove systematic errors, using the vector math capabilities of the analyzer. The residual systematic errors remain after error correction, primarily due to the limitations of how accurately the electrical characteristics of the calibration devices can be defined and determined. Also, the random (non-repeatable) and drift errors, cannot be corrected because they cannot be quantified and measured during the measurement calibration and device measurement. However, the effects of random errors can be reduced through averaging. The averaging may then be reduced for device measurement. The residual systematic errors along with the random and drift errors continue to affect measurements after error correction, adding an uncertainty to the measurement results Therefore, measurement uncertainty is defined as the combination of the residual systematic (repeatable), random (non-repeatable), and drift errors in the measurement system after error correction.

The following measurement uncertainty equations and system error models (flowgraphs) show the relationship of the systematic, random, and drift errors. These are useful for predicting overall measurement performance.

Sources of Measurement Errors

Sources of Systematic Errors

The residual (after measurement calibration) systematic errors result from imperfections in the calibration standards, the connector interface, the interconnecting cables, and the instrumentation. All measurements are affected by dynamic accuracy and frequency error effects For reflection measurements, the associated residual errors are effective directivity, effective source match, and effective reflection tracking. For transmission measurements, the additional residual errors are effective crosstalk, effective load match, and effective transmission tracking.

The listing below shows the abbreviations used for residual systematic errors that are in the error models and uncertainty equations.

nEfd, Erd = effective directivity

nEfs, Ers = effective source match

nEfr, Err = effective reflection tracking

Determining System Measurement Uncertainties B-1