QUARTZ CRYSTAL APPLICATIONS 183 



as a narrow band filter. At about the same time, L. Espenschied,^ taking 

 advantage of the knowledge of the equivalent electrical circuit of a crystal 

 given previously by Van Dyke/ showed how to combine other electrical 

 elements with crystals in ladder form to obtain band-pass filters. It was 

 not, however, until the crystals were combined with auxiliary coils and 

 condensers into the form of resistance compensated lattice type networks* 

 that much progress was made in achieving the wide pass-band characteris- 

 tics necessary for telephone and radio communication. Such filters have 

 provided very selective devices which are able to separate one band of 

 speech frequencies from another band different by only a small frequency 

 percentage from the desired band. This property makes it possible to space 

 channels close together with only a small frequency separation up to a high 

 frequency, and such filters have had a wide use in the high-frequency carrier 

 systems, and in the coaxial system which transmits more than 480 conver- 

 sions over one pair of conductors. In radio systems such filters have been 

 used extensively in separating one side band from the other in single side- 

 band systems. 



In conclusion we can say, that the science of piezoelectricity was born 

 about 62 years ago, lay dormant for nearly 40 years, but during the last 25 

 years has advanced at such a rate that it can be regarded as one of the foun- 

 dation stones of the whole communication art. 



1.3. Theory of Piezoelectric Materials 



Piezoelectric crystals are of interest in communication circuits because 

 they possess three properties. These properties are: (1) the piezoelectric 

 effect provides a coupling between the electrical circuit and the mechanical 

 properties of the crystal; (2) the internal dissipation of most crystals and 

 particularly quartz crystals is very low, and the density and elastic constants 

 of the crystals are very uniform, so that a crystal cut at a given orientation 

 always has the same frequency constant; and (3), at specified orientations 

 crystals can be cut which have advantageous mechanical properties such 

 as a small change in frequency with a change in temperature, or a free- 

 dom from secondary modes of motion. It is the purpose of this section to 

 discuss the first property, the coupling between the electrical and me- 

 chanical properties of the crystal. 



The piezoelectricity of quartz and other materials is due to the fact that 



« Patent 1,795,204, filed Jan. 3, 1927, issued August 8, 1933. 



' K. S. Van Dyke; Abstract 52, Phys. Rev. June 1925; Proc. I. R. E. June 1928. 



* See "Electrical Wave Filters Employing Quartz Crystals as Elements," W. P. Mason, 

 B. S. T. J., Vol. XIII, p. 405, July 1934; "Resistance" Compensated Band Pass Crystal 

 Filters for Unbalanced Circuits," B. S. T. J., Vol. XVI, p. 423, Oct. 1937; "The Evolution 

 of the Crystal Wave Filter," O. E. Buckley, Jour. A pp. Phys., Oct. 1936; and Patents 

 1,921,035; 1,967,249; 1,967,250; 1,969,571; 1,974,081; 2,045,991; 2,094,044. 



