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By Michael Hiebel, Rohde & Schwarz GmbH & Co. KG

VNA calibration has been the subject of hundreds of papers, and when discussed in terms of its mathematical derivation can be quite complex. The article includes an overview of VNA calibration, a discussion of calibration standards and techniques, along with guidelines that can help designers choose the best calibration technique for specific situations. It also provides tables showing the applicability of the various calibration techniques and the influence of various actions that affect the calibration.

The most common measurement task in RF and microwave engineering involves the analysis of circuits using a network analyzer (VNA). This versatile instrument can evaluate nearly all types of devices, from filters and amplifiers to complex multifunction subsystems. The reason the instrument is so widely used is that it can, through the use of scattering (S)-parameters, evaluate the characteristics of the device under test with a high level of precision. S-parameters have long been the chosen method for this because they are relatively easy to derive at high frequencies and are directly related to the measurement parameters of interest to microwave designers, such as gain, return loss, and reflection coefficient. They are also extremely well suited for use by the electronic design automation tools that designers have come to rely on.

A VNA is only as useful as the accuracy with which it makes measurements, and this requires the instrument to be calibrated. The calibration process employs a technique called vector error correction, in which error terms are characterized using known standards so that errors can be removed from actual measurements. The process of removing these errors requires the errors and measured quantities to be measured vectorially (thus the need for a vector network analyzer). In contrast, scalar network analyzers can only record the magnitude of the measured quantities, precluding them from being used to eliminate systematic measurement errors, which unlike random errors, can be mathematically eliminated from the measurement result.

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•White Paper: Vector Network Analyzer (VNA) Calibration: The Basics