A signal generator includes a main ring oscillator and a first ring oscillator. The main ring oscillator is supplied by a power voltage, and is configured to generate an output oscillation signal. The main ring oscillator is coupled through a power mesh to the power voltage. The first ring oscillator is supplied by the power voltage. The first ring oscillator is similar or identical to the main ring oscillator. The first ring oscillator is coupled through the power mesh to the power voltage. The first ring oscillator is used to calibrate a frequency of the output oscillation signal.