Phase-locked loop

Abstract

A phase-locked loop (PLL) includes a voltage controlled oscillator (VCO) and a pair of charge pump circuits (CP) and provides a stable oscillation clock signal. A phase comparator compares a reference clock signal with an oscillation clock signal generated by the VCO and generates two comparison signals. The comparison signals are input to the first CP, which generates a first CP output signal. The first CP output signal is filtered with a first low pass filter (LPF) and the filtered signal (control voltage) is provided to the VCO, which produces the oscillation clock. The second CP receives two clock signals and generates a second CP output signal. The second CP output signal is filtered with a second LPF and the filtered signal is converted to a digital signal with an A/D converter. The digital signal is applied to a bias circuit, which then produces first and second control voltages. These control voltages are then applied to the first and second CPs to control the charging and discharging currents of the CPs.