PRINCIPLES OF MOUNTING QUARTZ PLATES 



179 



dimensions along the length of the crystal are small and width sufficiently- 

 large to insure a rigid clamp. Pressure was applied by a phosphor bronze 

 spring in the center of the two top pressure points. This gave a satisfactory 

 mounting and also allowed the use of a divided plating necessary for the 

 balanced type crystal filters. This type of mounting was used in crystals 

 of relatively low frequency, for example, 60 to 150 kc. of the "—18 degree 

 X-cut" type. 



With the use of higher-frequency crystals of different types of vibration 

 than that described above, it has been found that this method of mounting 

 has not been very satisfactory. In order to reduce the size of the mounting 

 in proportion to the decreased crystal area it would be a delicate mechanical 

 job and quite costly. This type of mounting could not be used for crystals 

 which did not employ this type of vibration, for example the face shear type 



QUARTZ CRYSTAL 

 -£ XTO PAPER 



PHOSPHOR BRONZE 

 SPRING 



Fig. 8.1 — Pressure mounting for extensional crystals. 



such as the CT and DT, since there is only one spot near the center which 

 would permit clamping at all. 



To permit a crystal to vibrate freely the object used to support the crystal 

 and maintain contact to the plated surfaces must have a very low mechanical 

 impedance. At the same time it should possess sufficient rigidity that the 

 complete assembly may be shocked without changing characteristics of the 

 crystal as an oscillator. For example, if a rod or bar is held against a crystal 

 at any point we would expect that the crystal in an oscillating condition 

 would tend to generate motion in the bar and as this bar is placed closer to 

 the nodal point we would expect the motion to be less. It can be seen that 

 there are two objects to be accomplished in mounting a crystal: First, that 

 the support must be placed as close as possible to a nodal point; and second, 

 that the support shall have a very low mechanical impedance. This me- 

 chanical impedance needs to be low only at or near the operating frequency 

 of the crystal. One type of support which would meet this requirement is 

 that of a rod in flexure a discussion of which is given in Chaper VI. In this 

 case, however, we may clamp one end of the bar and allow the other end to 



