Piezoelectric Crystals in Oscillator Circuits

STUDY or an explanation of the performance of a piezoelectric crystal in an oscillator circuit involves a study or explanation of oscillator circuits in general and a study of the crystal as a circuit element. Nicolson1 appears to have been the first to discover that a piezoelectric crystal had sufficient coupling between electrical electrodes and mechanical vibratory movement so that when the electrodes were suitably connected to a vacuum tube circuit, sustained oscillations were produced. In such an oscillator the mechanical oscillator}' movement of the crystal functions as does the electrical oscillatory circuit of the usual vacuum tube oscillator. His circuit is shown in Fig. 12.1. Cady2 independently though later made the same discovery, but he utilized it somewhat differently and expressed it differently. He found that when the electrodes of a quartz crystal are connected in certain ways to an electric oscillator circuit, the frequency is held very constant at a value which coincides with the period of the vibrating crystal. He made the further discovery that due to the very sharp resonance properties of the quartz crystal, the constancy in frequency to be secured was far greater than could be obtained by any purely electric oscillator. The development of analytical explanations of the crystal controlled oscillator came along rather slowly. Cady explained the control in terms of operation upon the electrical oscillator to which the crystal was attached. Pie said that the "capacity" of the crystal changes rapidly with frequency in the neighborhood of mechanical resonance, even becoming negative.