like other relaxation oscillators, the neon bulb oscillator has poor frequency stability, but it can be synchronized (entrained) to an external periodic voltage applied in series with the neon bulb. even if the external frequency is different from the natural frequency of the oscillator, the peaks of the applied signal can exceed the breakdown threshold of the bulb, discharging the capacitor prematurely, so that the period of the oscillator becomes locked to the applied signal.