A random number generator includes a first oscillator configured to output a first oscillating signal having a first frequency. A second oscillator is configured to output a second oscillating signal having a second frequency different from the first frequency. A sampling unit is configured to receive the first and second oscillating signals. The sampling unit is configured to generate at least one entropy source by combining the received first and second oscillating signals. The sampling unit is configured to generate a random bit corresponding to the generated entropy source using a third oscillating signal. A third oscillator & control unit is configured to control the first and second oscillators and to generate the third oscillating signal. A frequency of the third oscillating signal is lower than the first and second frequencies.