CHAPTER VIII 



Principles of Mounting Quartz Plates 



By R. A. SYKES 



Introduction 



IT IS the object of this chapter to show some of the fundamental consider- 

 ations involved that govern the design of mountings or holders of quartz 

 crystals. This discussion is restricted to the three common types, namely, 

 rod or clamp type, wire type and airgap type. The development of these 

 three types of mountings for applications in telephone transmission and radio 

 systems has led to many and varied forms. Commercial designs of units 

 for telephone uses employing these principles are described in detail in a 

 later chapter. 



In chapter VI regarding the vibrations of crystals we have assumed in all 

 cases that the crystal is free to vibrate. In order that this condition shall 

 be fulfilled it is necessary that any mounting which supports the crystal 

 shall not restrict its vibration or at most the effect shall be made as negligible 

 as possible. 



8.1 Clamp Type Supports 



Of the known types of vibration it is noticed in all cases that there have 

 been nodal points. These points by definition are points of zero motion and 

 in all cases that we have studied appear to be single isolated points or lines of 

 very small size in comparison with the total crystal area. The obvious tyi^o 

 of mounting is then one which simply clamps the crystal with a very small 

 area at these points or nodes. The early type of mountings for low-fre- 

 quency crystals were all based on this principle and the area of the clamp was 

 determined experimentally by reducing it until, with sufficient pressure to 

 hold the crystal, a good Q was obtained. The first mountings consisted 

 simply of two pressure points located as nearly as possible to the nodal point. 

 It was apparent at first that this type of mounting allowed the crystal to 

 rotate about the mounting axis and very shortly the plating or electrode 

 open-circuited. With the development of the "—18 degree X-cut" crystal 

 it was found that the nodal region of a longitudinally vibrating crystal was 

 a nodal line and permitted the use of a knife-edged type of mounting instead 

 of the single point. This type of pressure mounting was used with this 

 crystal for quite a number of years in the crystal filters for carrier systems 

 and is shown in Fig. 8.1. This consists mainly of four pressure edges whose 



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