Dealing with non-stationary signals: Definitions, considerations and practical implications

The paper addresses the question of how to deal with non-stationary power signals. The first part of the solution is at a fundamental level: the recognition that the thing being measured is known by some kind of label in a model. The label is attached to a some parameter in an equation, and is often identifiable by its position in the equation. The paper presents measurement as the act of solving the equation to find the value of the parameter. In other words, the equation is what metrologists term the measurand, and the measurement equipment must be designed around it. To measure a time-varying signal, in a world of digital measurements, one of the first questions that must be addressed is the relationship between the sampling window of the measurement system and the rate at which the signal is varying. A goodness of fit metric is identified. Several changing-frequency cases are examined.