A voltage controlled oscillator includes a resonant circuit, a trimming capacitor and a full adder. The resonant circuit includes a negative resistor, an inductor and an oscillation frequency setting capacitor having a variable capacitance according to a controlled voltage based on a oscillation frequency data. The trimming capacitor is connected in parallel to the oscillation frequency setting capacitor. The trimming capacitor has capacitors each of which has a variable capacitance according to an input signal thereof. The full adder is connected to the trimming capacitor. The full adder generates the input signal based on frequency information and an automatic proofreading value received thereto.
