S-parameter measurements are influenced by various measurement errors, which can be broken down into two categories:
•systematic errors
•random errors
Random errors comprise errors such as noise fluctuations and thermal drift in electronic components, changes in the mechanical dimensions of cables and connectors subject to temperature drift, repeatability of connections, and cable bends. Random errors are unpredictable and hence cannot be estimated and eliminated in calibration. Random errors can be reduced by having the stimulus power at the correct setting, IF bandwidth narrowing, sweep averaging, maintaining a constant environment temperature, observance of the Analyzer warm-up time, careful connector handling, and avoiding cable bending after calibration.
Random errors and related methods of correction are not mentioned further in this section.
Systematic errors are errors caused by imperfections in the components of the measurement system (see Systematic Errors). Such errors occur repeatedly, and their characteristics do not change with time. Systematic errors can be determined and then reduced by performing a mathematical correction of the measurement results.
Calibration is the process of measuring precision devices with predefined parameters to determine systematic errors, and such precision devices are called calibration standards. The most commonly used calibration standards are SHORT, OPEN, and LOAD.
The process of mathematical compensation of the systematic errors is called error correction.
