Unsymmetric Voltage of Conducted Noise Measurement System Without Grounding by Estimation of Ground Capacitance

Conducted noise originating from power supply equipment disrupts telecommunication networks, causing problems such as telecommunication interruptions and reduction in the speed of communication. Measurement of noise in data transmission lines at a repair site may require additional work to secure the electrical ground level for the measurement, because the data lines are differential pairs and the ground level is not always accessible. A method to estimate the capacitance between the measurement equipment and the ground was developed to allow measurement without access to the ground, i.e., in floating conditions. The method uses an oscillator and multiple electrodes to measure the capacitive coupling with the ground under the floor. A ground-capacitance-estimating device was fabricated and tested. Its accuracy was further improved by incorporating a linear function to express the relationship between the output of the device and the known ground capacitance in the calibration setting. This calibration procedure and the measurement procedure at the repair site are described. The noise voltage was calculated from the estimated ground capacitance and the voltage reading of the floating measurement equipment. The estimated ground capacitance was in the order of several tens of pF. The error of the corrected noise voltage was within 1.6 dB.