Phase locked loop with phase rotation for spreading spectrum

Abstract

A phase locked loop (PLL) with phase rotation spreading includes a phase detector, a charge pump, a filter, a voltage controlled oscillator (VCO) and a selector. The phase detector receives a reference clock signal and a feedback clock signal to thereby produce an error signal. The charge pump converts the error signal into a current signal. The filter converts the current signal into a voltage signal. The VCO produces N clock signals with a same frequency in accordance with the voltage signal, where the N clock signals have phases θto θrespectively, and θindicates a lead of 2π/N over θ, for j=0, 1, . . . , N−2. The selector selects one from the N clock signals in accordance with a predetermined sequence to thereby produce a target clock signal, and finely adjusts a frequency of the target clock signal for a spreading operation.