A method of developing a signal representative of the instantaneous frequency of an alternating current signal encompasses first determining the apparent frequency of the signal at the ends of sampling periods (e.g., half cycles). From successively derived apparent frequencies the rate of change in frequency is calculated. The actual frequency signal is then generated as a function of the apparent frequency and the rate of change. In a specific implementation, the method is employed to develop a signal representative of the rotational speed of a motor.