Synchronous circuit capable of properly removing in-phase noise

摘要

The synchronous circuit has: a phase comparator for comparing a feedback signal with an input reference signal to detect a phase difference and outputting a phase difference signal corresponding to the phase difference; a charge pump for outputting charge/discharge signals on the basis of the phase difference signal output from said phase comparator; a low-pass filter for changing a charge amount accumulated in a capacitor on the basis of the charge/discharge signals output from said charge pump and outputting control signals corresponding to the charge amount as differential signals; a voltage-controlled oscillator for changing an oscillation frequency of an output on the basis of the control signals output from said low-pass filter; a frequency divider for multiplying the output from said voltage-controlled oscillator and outputting the feedback signal; and fixing means arranged on an output side of said charge pump and operated, when the phase difference between the reference signal and the feedback signal is near a dead band of said phase comparator, such that levels of the charge/discharge signals output from said charge pump are fixed at predetermined levels. According to the synchronous circuit, troubles causes by the unbalance in charge/discharge operation performed by the change pump can be prevented.