Statistical Correlation Between Conducted Voltages on the Powerline and Those Measured with a Line-Impedance Stabilization Network

In measurements of physical parameters, it is not always possible to obtain consistent results because of the dependence on and interaction of a significantly uncontrolled variable element in the measurement. For consistent results to be obtained, the variation in the element may be eliminated by replacing the element with a simulated representative constant. Measurement of conducted powerline noise from electric/ electronic equipment is subject to such variation. Conducted-noise measurement procedures used in the United States, and also internationally, typically employ line-impedance stabilization networks (LISN's) which, in theory, eliminate the effects of variations in the ac power-distribution-network impedance on the measurement. The LISN is designed to be representative of the actual power-distribution- network impedance presented to equipment connected to the network. The LISN and power-network impedances are both frequency variant. Power-network impedances can assume a wide range of values at a fixed frequency, whereas the LISN impedance, assuming that any line-impedance effects are small, is a single value. This paper explores the correlation between conducted noise voltages measured in a LISN configuration and the actual noise voltages expected to be impressed on the power distribution system. The analysis is conducted at 1 MHz. The paper analytically relates the impedances of the LISN, power distribution system, and equipment. A LISN of the type used in the United States is used in the development; however, the concept and procedure presented is applicable to any LISN, frequency, and equipment.