Electromagnetic coupling circuit model of a magnetic near-field probe to a microstrip line

Electromagnetic (EM) injection experiments require an accurate and quantitative knowledge of the voltage effectively coupled to a target line or circuit in order to predict disruptive behavior or sensitivity of digital IC circuits to EM threats. To answer this question we derive here a complete quantitative model of the coupling of our magnetic probe to a microstrip line. The novelty of this model is to consider the coupling by analogy with a transformer and then to deduce the corresponding mutual inductance as a function of probe to target relative positions. Its inputs are S-parameter measurements of the actual probe coupled to a 50Ω microstrip line and its output is an electric equivalent circuit that can be implemented in any circuit simulator. Validity of the model extends up to GHz frequencies.