Determining the Maximum Allowable Power Bus Voltage to Ensure Compliance With a Given Radiated Emissions Specification

It is generally understood that excessive power bus noise voltage on a printed circuit board will result in unacceptable radiated emissions levels, but how much noise is too much? In this paper, an analysis based on a lossy cavity model is performed to determine the maximum possible radiated field corresponding to a given power bus noise voltage. A closed-form expression relating the maximum power bus noise voltage to the radiation peaks is then derived. This expression is solved in reverse to determine the minimum power bus voltage that is necessary to generate a radiated field above a specified limit. When troubleshooting a radiated emissions problem, this expression can be applied to measured values of power bus noise voltage to determine whether radiation directly from the power bus is potentially the emissions source.