Opportunities for improved 80%/80% statistical methods with CISPR 32

Abstract

CISPR 22 and CISPR 32 both require statistical compliance, specifically that products demonstrate with at least 80% confidence that at least 80% of the product population passes the radiated and conducted emissions limits. There have been problems applying the statistical test specified in CISPR 22 to real products because the method assumes a normal distribution, and products often exhibit characteristics which do not fit that model. For example, if the shielding or grounding in electronic equipment is not consistent unit to unit, it can result in bimodal or multi-modal distributions of some emissions. The bimodal distribution may be a common cause behind signal frequencies which appear as significant emissions in some units but are unmeasurable in others. CISPR 32 introduces more options for tools which may be used to demonstrate 80/80 compliance, notably the binomial distribution test, which is robust enough to accommodate a wide variety of population distributions. When CISPR 32 replaces CISPR 22 as the emissions standard, this binomial test method should be available for product qualifications.