Common mode voltage measurements comparison for CISPR 22 conducted emissions measurements

CISPR 22 (1997) requires the measurement of common mode voltages on Information Technology Equipment (ITE) cabling to determine conducted emission levels over the 150 kHz to 30 MHz range. This paper presents and compares the results of measuring the common mode voltage on a simulated cable bundle using (1) a capacitive (noncontact) voltage probe, and (2) using a capacitive injection clamp in conjunction with a low capacitance FET voltage probe. Results are presented for the common mode voltage measurements made with both methods at different physical locations on the simulated cable bundle. The results show that it is important to have a well controlled common mode impedance, and to make the voltage measurement at that well controlled point. Failure to have a well controlled impedance can result in voltage standing waves, leading to significant voltage measurement errors. Results are also provided that show the capacitive clamp/FET voltage probe combination exceed the 5 pF capacitive loading limit to the cable under test as specified by CISPR 22