A spread spectrum clock generator comprises a conventional closed-loop VCO and an open-loop VCO where both are coupled to the same point of the PLL. Both VCOs comprise a V-to-I converter followed by a current-controlled oscillator and are identical in design but only the open-loop VCO receives the modulation current to generate the spread spectrum clock signal. The open-loop ICO is part of the spread spectrum generator and in one embodiment of the invention receives feedback current signals representing the modulation method and modulation ratio. This ensures that the modulated clock output tracks the PLL output frequency. In a second embodiment the closed-loop VCO receives from the spread spectrum generator the feedback current signal representing the modulation method (center/down spread) while the open-loop VCO receives the feedback signal representing the modulation ratio.