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By Anritsu Company

Twenty four years ago Wiltron (now Anritsu) published a Technical Review "Why Tolerate Unnecessary Measurement Errors", long since out of print. Thousands were printed and it was widely used by test engineers. At that time the principal network analyzer used by industry was the scalar analyzer. Today while scalars are still used, the Vector Network Analyzer (VNA) has also become common in both engineering and manufacturing environments. Many measurements are made and recorded with little attention given to the actual accuracy of the measurement.

This Review will include some of the material originally presented in 1975 as it still very important and addresses problems that regularly appear in current applications. It will also treat the VNA which is regarded properly as the most accurate measurement tool; but, when "taken for granted" can provide data that is "out of bounds"!

Uncertainty (Inaccuracy) in Reflection Measurements
Excluding noise, which will be addressed later, the dominant sources of error in a microwave reflectometer measurement system are directivity, source match, and load match. This is true for both scalar and vector systems. The systems differ in that a scalar system's performance is related to the quality of the hardware used, and in general users are well aware of hardware limitations. They will pay more money to obtain a bridge or coupler with higher directivity or a termination with superior return loss. In the case of the VNA these error terms are appropriately termed "effective" directivity, "effective" source match, and "effective" load match. The magnitude of these terms is established by the calibration process and the user is not usually aware of the actual values of these important parameters when measurements are being recorded.

Click here to download the complete application note in pdf format.