Precise method for digital frequency measurement for Class A instruments

This article describes and compares two methods of frequency measurement used in power monitoring devices. First method uses hardware zero crossing detector and every 10 second computes frequency from real interval duration and count of zero crossing. It uses simple and limited software phase-locked loop. Second method uses fully software based solution. Input data are filtered with IIR filter, zero crossing is detected with advanced algorithm, software PLL is based on FFT and frequency is measured every 10 periods.