Self-alignment of automatic test equipment (ATE) when and how often to do it

Automatic Test Equipment (ATE) requires periodic alignment, i.e., making sure the measurement and stimulus instruments are within tolerance. Once this is accomplished, how often should it be repeated? When it is repeated and an instrument is within tolerance, should it be adjusted to be even closer to the true value? These are not academic questions-a set of ATE that we have first hand experience with has a monthly self-alignment performed, Adjustments are made (via software correction factors) even though the instruments are within tolerance. By gathering data before the monthly alignments are made, we have been able to detect drift rates and use trend analysis mathematics to predict when adjustments are required. Two trend models are employed-a linear time dependence and a Brownian motion model. Results of this analysis are presented.