Investigation of using ferrite cores to control common mode impedance of cables 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. Section C.1.4 indicates that the ITE cable bundle common mode impedance can be fixed at 150 ohms (+/- 20 ohms) by the positioning of ferrites along the cable between the EUT and the AE. Data is presented showing the measured common mode impedance of a simulated cable bundle as ferrite cores are moved along its length. Measurements are shown for core permeabilities of 125, 850, and 10000. The data show that common mode impedance control using ferrites is difficult, and may not be achievable for all frequencies over the intended emission frequency range of 150 kHz to 30 MHz. An example of a common mode impedance stabilization network is also provided which shows controlling the common mode impedance within CISPR 22 requirements for loads ranging from a short circuit to an open circuit.