Comparison of common mode impedance measurements using 2 current probe technique versus V/I technique for CISPR 22 conducted emissions measurements

CISPR 22 (1997) defines the conducted emissions requirements and testing of information technology equipment (ITE). Section C.1.4 of CISPR 22 requires measuring the common mode impedance of an ITE cable bundle with respect to its ground plane, and provides a test methodology for this measurement. This paper presents and compares the results of measuring the common mode impedance using (1) the 2 current probe method cited in Section C.1.4 of CISPR 22 (1997), and (2) using a current probe and a capacitive voltage probe. Results are presented for different wire (cable bundle) lengths and for different load impedances on the wire. The results show that the 2 current probe method can lead to significant impedance measurement errors, and that the current/voltage probe method provides more accurate impedance measurements that are also not adversely influenced by the wire length