The Bell System Technical Journal 



Vol. XXIV April, 1(^45 No. 2 



Piezoelectric Crystals in Oscillator Circuits 



By I. E. FAIR 



12.00 Introduction 



A 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. Nicolson^ 

 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 oscillatory 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. Cady 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 vibrat- 

 ing 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. 

 He said that the "capacity" of the crystal changes rapidly with frequency 

 in the neighborhood of mechanical resonance, even becoming negative. 

 This "capacity" connected across the oscillator tuned circuit or in other 

 places prevented the frequency from changing to any extent, as any fre- 

 quency change caused such a "capacity" change in the crystal as to tend to 

 tune the circuit in the other direction. Cady, however, devised one circuit, 

 Fig. 12.2, in which no tuned electrical circuit was used, but he confined his 

 explanation to "a mechanically tuned feedback path from the plate to the 

 grid of the amplifier". Pierce came along later with a two-electrode crystal 

 connected between plate and grid, and no tuned circuit, and also with a 



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