Test signal reduction in radiated EMI measurements

Radiated EMI compliance measurements are very time consuming, even when a high degree of automation is used. This is particularly true for measurements made on an open area test site (OATS), due to the presence of ambient signals and weather conditions. The overall test time is largely dependent on the number of signals emanating from the equipment under test (EUT), because their amplitudes must be maximized and measured. The maximization process involves changes in antenna height and polarization, as well as rotation of the EUT. It is mandatory to identify only the relevant signals for maximization and final measurement by using adequate test procedures, selecting proper system parameters, and performing tasks like signal discrimination efficiently. This paper first describes how relevant signals can be discerned in data traces of a swept EMI receiver by using a software algorithm. The significance of a user definable parameter of this algorithm, "peak excursion", is discussed. The overall measurement process also determines the test time. The application of amplitude comparisons to limit lines or margins, comparisons of signal lists and acoustic identification of signals, further reduce the number of emissions throughout the measurement process. An example of this process is outlined. Other tasks like the discrimination between ambient signals and EUT emissions or between EUT emissions and auxiliary equipment also contribute to the reduction of signals. A tool to efficiently accomplish this discrimination task, the polar plot, is also discussed.