MANUFACTURING DEVIATIONS IN CRYSTAL UNITS 273 



I 



In baking the silver spots, care must be taken to prevent "heat" twinning. 

 If the temperature of a quartz plate is raised above the inversion point 

 (573°C) and then is reduced again, the plate will be electrically twinned.^ 

 The firing temperature of the silver paste currently used for the spots is not 

 many degrees below this inversion point. Hence, the firing temperature 

 may easily become so high as to result in twinned plates. In addition, 

 it has been observed that the twinning may occur at a considerably lower 

 temperature if the plate is subjected to large thermal stress. For this 

 reason, care must be taken to heat the plates uniformly during the baking 

 operation. 



14.32 Division of Coating 



The next operation is to evaporate a coating of silver on the surface of 

 the quartz plates. The plates must be thoroughly cleaned before this coat- 

 ing is applied in order to ensure firm adherence of the coating. Poor ad- 

 herence may cause the coating to peel off the plate, changing all of the 

 electrical characteristics of the plate. In many cases the coating must also 

 be divided. ^'^^ Two methods are in general use for dividing the silver 

 coating on crystal plates, namely, an abrasive method and an electrical 

 stylus method. In general, the abrasive method of dividing the coating is 

 superior to the electric stylus for all cases requiring a simple straight line 

 division, but it has not been found practical for complicated divisions such 

 as are desirable for harmonic longitudinal plates and fiexure plates. 



In using the abrasive method for dividing the coating only two factors 

 are likely to change the characteristics of the crystal plate, these being the 

 location and the width of the dividing line. Deviations in the location of 

 the dividing Hne from the lengthwise center line for a longitudinal plate 

 will affect the capacity and inductance balance between the two halves of 

 the plate. Deviations in the width of a properly centered dividing line 

 will cause changes in the inductance of the plate since for a given plate the 

 inductance is a function of the ratio of the plated area to the total area of 

 the plate. So, for a wide crystal plate deviations in the width of the divid- 

 ing hne will be negligible while for narrow plates these deviations can 

 cause an appreciable change in the inductance of the plates. 



When the electric stylus is used for dividing the coating, the location and 



the width of the dividing line again will aflfect the performance of the plates. 



In addition, varying amounts of twinning will occur along the division 



line apparently due to instantaneous high temperature gradients introduced 



by burning oi" the silver at the point of contact of the stylus. In measure- 



^Loc. cit. 

 * Loc. cit. 



*^ "Crystal Channel Filters for the Cable Carrier System", C. E. Lane, B. S. T. /., 

 January 1938, pp. 125-136. 



