An accurate method for the measurement of line frequency and its deviation using a microprocessor

A digital method for the accurate measurement and display of line frequency and its deviation from its nominal value is described. The method is based on counting a number of high-frequency pulses proportional to the line period, and then calculating the line frequency and its deviation in a microprocessor system that also provides the monitoring functions. The method provides a resolution of one millihertz and an accuracy better than a millihertz plus the tolerance and stability of the high-frequency crystal clock used.