Abstract: Apparatus is disclosed for measuring the fundamental voltage, fundamental frequency and the fundamental voltage divided by the fundamental frequency of the output of a device, such as an inverter. Through the use of an isolation amplifier connected across the output of the inverter and proper component selection, the output signal produced by the isolation amplifier is representative of the fundamental voltage of the output of the inverter divided by the frequency of the fundamental voltage. The output signal from the isolation amplifier is converted into a pulse having a fixed width w, fixed height h and period 1/f to produce a signal representative of the fundamental frequency of the output of the inverter. The output signal from the isolation amplifier is also rectified and applied to a multiplier along with a signal having a fixed width w, height E.sub.in /f and period 1/f and then averaged to produce an output signal representative of the fundamental voltage of the output of the inverter.