Patent ID: 6593783
Filing Date: 2003-07-15
Classification: H03L

Abstract:
A frequency synthesizer comprising: an oscillator that controls a frequency of an oscillation signal with an appropriate control signal,a frequency divider of a fractional frequency division type that frequency-divides the oscillation signal and generates a comparison signal, a reference clock signal generator that generates a reference clock signal, a phase comparator that compares the phase of the comparison signal with the phase of the reference clock signal and outputs a phase difference signal, a charge pump circuit that outputs current corresponding to the phase difference signal, a low-pass filter that removes a high-frequency component of the current output from the charge pump circuit and supplies its output as the conventional control signal to the oscillator, and a compensation circuit that supplies a compensation current used for compensating a ripple current included in the control signal to the output terminal of the charge pump circuit; wherein the frequency divider includes a prescaler that frequency divides the oscillation signal and a counter that is operated corresponding to the output signal of the prescaler to output the comparison signal; and a correction circuit that includes a detection capacitor connected to the charge pump circuit and is able to generate a reference voltage from the voltage of the detection capacitor; the compensation circuit including a voltage generator that outputs a voltage corresponding to the reference voltage, and a compensation capacitor connected between the voltage generator and the output terminal of the charge pump circuit; the capacitance ratio of the detection capacitor to the compensation capacitor is determined on the basis of one of the frequency division values of the prescaler that varies periodically, one period of the frequency division value which varies periodically, and the time period of the frequency division value; the correction circuit charges/discharges the detection capacitor with the output current of the charge pump circuit only during the time period of one of the frequency division values of the prescaler, and the reference voltage is generated from the voltage of the detection capacitor obtained by means of the charging/discharging.