Abstract: Apparatus for comparing an unknown signal with a reference signal is adapted to determine their ratio and, where a variation exists, to compensate the unknown signal to achieve unity ratio. The apparatus includes circuit means connectable to the signals for obtaining sum and difference signals. Means responsive to the sum and difference signals produce a control signal representing the product of these signals and feedback means responsive to the control signal generate a time-averaged feedback signal that is a measure of the ratio of the mean squares of the unknown and reference signals. Control means are disposed in the path of the unknown signal and are responsive to the feedback signal for varying the amplitude of the unknown signal to equilibrate and stabilize the apparatus when the mean squares of the reference signal and the compensated unknown signal are equal.