This guide specifies basic principles, application objectives, function, performance, and test methods for Alternating Current (AC) standard signal sources with digital output. This guide applies to AC standard signal sources which provide standardized digital voltage/current output signals in the form of a formatted digital output stream, which are used to simulate the output of an electronic instrument transformer or merging unit (MU) when testing intelligent electronic devices (IEDs). For AC standard signal sources, IEC 61869-9 is applicable.