Patent ID: 7339420

Claim:
A PLL circuit comprising at least: a voltage controlled oscillator; a phase comparator for comparing phases between an output signal of said voltage controlled oscillator and a reference signal; and a loop filter for providing a low-pass component of an output signal of said phase comparator to said voltage controlled oscillator as a control voltage, wherein the PLL circuit operates at a predetermined natural angular frequency and a predetermined damping factor, wherein said loop filter includes: a series circuit of a capacitance and a resistance connected to an output terminal of said phase comparator; in order to switch said natural angular frequency, capacitance switching section that switches a value of said capacitance according to a switching signal; and in order to keep said damping factor at a constant value, resistance switching section that switches a value of said resistance according to said switching signal in conjunction with switching the value of said capacitance, wherein said capacitance includes a first and a second capacitive elements being connected in parallel, wherein said capacitance switching section includes: a switching element for switching the capacitance value, one end of which being directly-connected to one end of said first capacitive element and another end of which being directly-connected to one end of said second capacitive element; and a charge circuit, input of which being directly-connected to said one end of said first capacitive element and output of which being directly-connected to said one end of said second capacitive element, wherein said resistance is connected to said one end of said first capacitive element, said switching element for switching the capacitance value switches to an on-state or an off-state according to said switching signal, and said charge circuit charges said second capacitive element at the same potential as said first capacitive element when said switching element for switching the capacitance value is switched off.