Patent ID: 7822392

Claim:
A frequency modulation circuit for frequency-modulating an input signal and outputting a resultant signal, the frequency modulation circuit comprising: an input terminal to which the input signal is inputted; a VCO for frequency-modulating the input signal by controlling an oscillatory frequency of the VCO in accordance with the input signal, and outputting a resultant signal as a frequency-modulated signal; a feedback loop for feeding, back to the VCO, the frequency-modulated signal outputted from the VCO; and an open loop for connecting the input terminal and the VCO, wherein the VCO includes at least a first variable capacitor having a predetermined capacitance change rate, and a second variable capacitor having a greater capacitance change rate than that of the first variable capacitor, the open loop includes a switch for switching a connection path between the input terminal and the VCO, and when the frequency modulation circuit is applied in a narrowband modulation method, the switch switches the connection path of the open loop such that the input terminal and the first variable capacitor are connected, and when the frequency modulation circuit is applied in a wideband modulation method, the switch switches the connection path of the open loop such that the input terminal and the second variable capacitor are connected.