Patent ID: 7978013

Claim:
A phase locked loop circuit comprising: a phase comparator for phase comparison between a reference signal and a feedback signal; a charge pump circuit for outputting a charge current based on a result of the phase comparison; a loop filter for filtering the charge current; a voltage controlled oscillator for oscillating according to an output voltage of the loop filter; and an output signal frequency divider for dividing a frequency of an output signal of the voltage controlled oscillator and generating the feedback signal, wherein the charge pump circuit modifies a magnitude of the charge current in accordance with a first control signal, the loop filter has a time constant modified in accordance with a second control signal, the voltage controlled oscillator has a gain modified in accordance with a third control signal, and said phase locked loop circuit further comprising a frequency counter for measuring a frequency of an input signal, and a constant determination unit for determining constants, including the magnitude of the charge current, the time constant of the loop filter, and the gain of the voltage controlled oscillator, so that a proportionality constant of a natural frequency of the phase locked loop circuit for the frequency of the input signal and a damping factor have respective predetermined values, and outputting the first through third control signals based on the determined constants, said phase locked loop circuit further comprising a first-order oscillator for oscillating according to a bias given to the charge pump circuit, wherein the frequency counter counts, as the frequency of the input signal, a number of pulses of the input signal per cycle of the output signal of the first-order oscillator.