Abstract: The nonlinear characteristics of a voltage controlled oscillator (VCO) in a phase-locked loop are approximately matched by a single diode in the audio circuitry to provide an essentially constant frequency deviation over the entire frequency range of the phase-locked loop during frequency modulation of the phase-locked loop. A compensating circuit which incorporates the diode samples the error voltage of the phase lock loop and biases the diode accordingly. The audio signal is converted to a current which in turn is used to modulate the bias current through the diode. The resulting AC voltage across the diode is thereby amplitude compensated by the bias current to approximately match the nonlinear characteristics of the VCO at each frequency setting of the phase-locked loop.