US Patent 7412019 Spread spectrum clock generator

A spread spectrum clock generator comprising a phase-locked loop circuit and a modulation circuit. The phase-locked loop circuit receives a reference signal at a reference frequency and outputs an output signal at an output frequency periodically varying in a range between upper predetermined boundary and lower predetermined boundary frequencies. The modulation circuit receives the reference and the output signals, controls a period of the output frequency according to the reference frequency, and modulates a variation rate of the output frequency according to the reference frequency, and the output frequency, and the predetermined integers.