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An analysis of "modulation-rate" distortion originating in frequency modulators is presented. The analysis is based on the solutions of the differential equations representing two fundamental classes of variable frequency oscillators. It is shown that the VFOs of one class produce ideal frequency modulation, while the VFOs of the other class inherently produce distortion. It is shown that this distortion is a function of the ratio of two rates, the rate of change of energy storage capacity and the rate of change of stored energy.