Closed-Form Expressions of Electric and Magnetic Near-Fields for the Calibration of Near-Field Probes

Abstract

Near-field scan is a powerful method to diagnose EMC issues. Some of related applications require quantitative measurement of near-field, which relies on calibrated near-field probes. A typical and convenient structure to calibrate them is the microstrip line. Although this structure seems simple, determining near-field distribution is not straightforward and 3D electromagnetic simulation is usually preferred. Because of the complexity of electromagnetic solvers and the dependence of results to their configuration which requires a solid expertise, this approach introduces an additional difficulty in the calibration process and a source of uncertainty. This paper proposes closed-form expressions based on quasi-static approximation to calculate near-field distribution over microstrip line and simple calibration structures. The paper specifies the frequency limits of this method, which can reach several GHz, covering most of the needs in EMC diagnosis at PCB and IC level. Several probes are calibrated with these formulations, which are validated through the comparison of the near-field measurements above the same case study. A special care is also provided during the calibration process to ensure that the probe characteristics remains independent of the probe height.