Patent ID: 6121844
Filing Date: 2000-09-19
Classification: H03L

Abstract:
A PLL frequency synthesizer comprising: a reference oscillator for generating a reference frequency signal having a reference frequency; a voltage-controlled oscillator which generates an output signal of a frequency corresponding to a control voltage applied thereto; a frequency divider which divides, based on the frequency number set therein, the frequency of said output signal from said voltage-controlled oscillator to generate a frequency-divided signal; a phase comparator which compares the phase of said frequency-divided signal from said frequency divider and the phase of said reference frequency signal and outputs a phase difference signal indicating their phase difference; voltage detecting means for detecting the present value of said control voltage; storage means which has prestored therein a table indicating the correspondence between a plurality of set values of frequency dividing numbers to be set in said frequency divider and a plurality of set values of control voltages to be applied to said voltage-controlled oscillator and which reads out of said table the set value of the control voltage corresponding to the frequency dividing number set in said frequency divider and outputs said read-out set value; a charge pump which is driven by a signal applied thereto to charge and discharge electric charges; a loop filter which integrates an output from said charge pump to generate said control voltage for application to said voltage-controlled oscillator; voltage value comparison means which compares the present value of said control voltage detected by said voltage detecting means and said set value of the control voltage output from said storage means, outputs the comparison result and generates an output signal corresponding to said set value; switching means which switches said phase difference signal from said phase comparator and said output signal from said voltage value comparison means to output either one of them to said charge pump; and control means which controls said switching means to drive said charge pump by said output signal from said voltage value comparison means when said comparison result indicates that the difference between the detected current value of said control voltage and said set value of the control voltage read out of said storage means is greater than a predetermined value, and at all other times, by said phase difference signal from said phase comparator.