A resonator system is provided in which a combined overtone resonator device is excited with a two-dimensional mode of mechanical vibration in a cross sectional plane of a piezoelectric plate in response to an alternating voltage applied through an interdigitated electrode. The cross sectional plane extends along the width direction and the thickness direction, and the two-dimensional mode of mechanical vibration is a two-dimensional combined overtone mode of second and third order asymmetrical Lamb-wave overtones.