Patent ID: 7741889

Claim:
A phase-locked loop with phase rotation for spreading spectrum, comprising: a phase detector, which receives a reference clock signal and a feedback clock signal to thereby produce an error signal; a charge pump, which is connected to the phase detector in order to convert the error signal into a current signal; a filter, which is connected to the charge pump in order to convert the current signal into a voltage signal; a voltage-controlled oscillator (VCO), which is a ring oscillator connected to the filter in order to produce N clock signals with a same frequency based on the voltage signal, wherein the N clock signals have phases θ 0 to θ N-1 respectively, and θ j indicates a phase lead of 2π/N over θ j+1 , for j=0, 1, . . . , N−2; and a selector, which is connected to the VCO in order to select one from the N clock signals based on a predetermined sequence to thereby produce a target clock signal; wherein the selector sequentially selects the clock signals with the phases θ 0 , θ 1 , θ 2 , . . . , θ N-1 respectively as the target clock signal when an up spectrum spreading operation is performed, sequentially selects the clock signals with the phases θ 0 , θ N-1 , θ N-2 , . . . , θ 2 , θ 1 respectively as the target clock signal when a down spectrum spreading operation is performed, and sequentially selects the clock signals with the phases θ 0 , θ 1 , θ 2 , . . . , θ N-1 , θ 0 , θ N-1 , . . . , ← N-2 , . . . , θ 2 , θ 1 respectively as the target clock signal when a spectrum spreading is operated by centering on a frequency of the target clock signal, such that frequency of the target clock frequency is spread evenly within a fixed frequency range and energy of the target clock signal is distributed evenly in frequency domain.