An inductive probe for the measurement of common mode currents on differential traces

Measuring common mode currents on differential microstrip transmission lines is a complicated task, because the common mode current, being the parasitic mode, is usually much weaker than the intended differential current. Hence, the measurement technique has to provide sufficient rejection of the differential mode. The probe described in this paper uses a shielded loop probe combined with a metallic screen to enhance the differential mode rejection of the current probe. The proposed techniques were tested on a test board at frequencies up to 6 GHz.