A new common-mode voltage probe for predicting EMI from unshielded differential-pair cables

A common-mode (CM) voltage probe for prediction of the EMI levels from unshielded differential twisted-pair cables is described. CM currents on I/O cables are primary sources of EMI in many electronic devices. Measuring the CM voltage at the I/O connectors can be an alternative to measuring CM currents. CM voltages at the RJ-45 ports were measured and correlated to the 10m E-field levels of 8 DUTs. A CM-voltage level that correspond to the EN 55022 A limit has been derived. This allows the prediction of radiated emissions based on CM voltage measurements at the connectors. This bench-tool can be instrumental in reducing the time and cost of troubleshooting EMI problems.