The different ageing behaviour of fundamental and overtone modes

The general understanding describes the ageing of quartz crystals as a change of the frequency over time for all vibration modes in the same size and direction. In reality, a different ageing behaviour of fundamental mode and overtone modes can be noticed. This effect depends on the blank design and on the electrode design. In the oscillator, only one mode can be measured over time and not the different behaviour of individual modes. The present work shows the influence of varied design parameters on the ageing behaviour. Theoretical calculations are used to determine the mode shape and the direction of the frequency shift for these effects. The calculation correlates with experimental values in ageing and drive level dependence. It can be shown, that electrode design, electrode material and deposition technology has an important influence on the direction of the ageing. Migration effects of the electrode material can explain this behaviour. The measurement of different modes at the same time can give an indication of the direction for the long time ageing in a short time.