236 BELL SYSTEM TECHNICAL JOURNAL 



employed for the final stage grinding. Following this, the plate is thor- 

 oughly cleaned by acid treatments or by the use of solvents and detergents 

 followed by copious washing. It is then etched in commercial hydrofluoric 

 acid to remove all loose particles of quartz that might have remained on the 

 surfaces or in the crevices after cleaning. Etching also smooths off the 

 roughness of the ground surfaces. The effect of this treatment reduces 

 energy dissipation in the plate itself and increases by many times the effi- 

 ciency of the crystal units. Etching also improves the stabiUty of perform- 

 ance of the crystal unit. Standard designs of crystal units require etching 

 of the plates, uniformly on all surfaces, for a period of thirty to forty min- 

 utes. For units of highest precision and efficiency longer etching periods 

 are employed. 



The electrodes employed with types of crystal units being described con- 

 sist of metallic coatings, generally aluminum, silver, or gold deposited over 

 the major surfaces of the crystal plate. These coatings are applied by the 

 evaporation process which results in an extremely thin and uniform coating 

 of metal having excellent adherence to the quartz. 



With reference to the mechanical supporting members for the plate, it 

 has been brought out that such supports should be confined as closely as 

 possible to the nodal points or nodal lines where the motion for all practical 

 purposes is zero. It is common practice for the supporting members to be 

 made of metal so that they will serve also as a means of making electrical 

 connections to the electrode coatings on the surfaces of the plates. 



13.2 Pressure Type Crystal Units 



This type of crystal unit was initially developed for use in telephone 

 filters. Up to about five years ago it was employed in virtually all com- 

 mercial designs of filters. Depending upon the mode of vibration and size 

 of the crystal plate the design of the mounting varies. However the prin- 

 ciples employed for clamping are essentially the same in all cases. Where 

 small longitudinal or face shear plates are involved one pair of pressure 

 pins is used unless two are required for electrical reasons as explained later. 

 For medium size plates of the same type or for face flexure plates, two pairs 

 of pins are employed. In the case of large low-frequency longitudinal 

 plates double anvils are used instead of pins in order to obtain firmer clamp- 

 ing of the plate to prevent translation or rotational movement which might 

 cause wear in the electrode surface at the pressure point with resultant 

 variations in frequency and resistance. The blocks are usually composed 

 of molded steatite and the springs for exerting the necessary pressure are of 

 ph osph or-br onze . 



Figure 13.1 (B) shows a pressure mounting for holding four crystals which 

 have single coatings on each of their major surfaces. The main require- 



