PLATED QUARTZ CRYSTAL UNITS 239 



conlainers, pressure-type crystal units are not sealed in individual con- 

 tainers. However, the entire filter in which they are employed is dried and 

 sealed off after filling with dry air. 



In pressure-type units of this type, variations in frequency of the order 

 of .01% may be expected if crystals are transferred from one mounting to 

 another or relocated in the same mounting. Consequently, if high-frequency 

 precision is desired, it is necessary to make the final frequency adjustment 

 with the crystal located in its final position. Due to the inherent difficulties 

 of adjustment, coupled with the close manufacturing tolerances on parts, 

 and precision adjustments necessary for the holders during assembly, these 

 designs have been virtually discarded in favor of wire supported designs. 

 However, more recent developments by J. F. Barry on pressure-mounted- 

 type units have brought forth some new ideas which might prove in for 

 wider future application. 



13.3 Wire Supported Crystal Units 



This type of mounting is being used extensively on crystal applications 

 and has superseded the earlier pressure-type units. Various designs of this 

 type of crystal are shown on Fig. 13.2. The wire-mounted crystal possesses 

 the definite advantage in that after the supporting wires are attached to 

 the plate, they remain fixed in position throughout the subsequent manu- 

 facturing process thus facilitating adjustment of the frequency or the 

 frequency-temperature characteristic. Moreover, the supporting wires can 

 be formed so as to provide a spring mounting for the crystal plate which 

 protects it from any shocks or vibration it may encounter in shipment or 

 use. In the wire-supported design the suspension wire is also employed as 

 a means of connecting the electrical circuit to the electrode plating on the 

 crystal. By virtue of the solder bond between the wire and the electrode, 

 this type of unit is free from the possibility of instability in frequency per- 

 formance due to slight changes in position and variations in contact resist- 

 ance prevalent in pressure type designs, and for this reason the stability 

 and frequency precision of wire supported crystals is superior. 



From a manufacturing standpoint the wire-supported unit involves a 

 greater number of processing operations than the pressure-type unit, but 

 the adjustment operations are considerably easier. Moreover, it is possible 

 to realize greater precision of frequency adjustment by a factor of at least 

 two or even three. The crystals are also more uniform in their effective 

 resistance. Since the mount or cage in which the crystal is suspended is 

 comparatively inexpensive, there should in general be little difference in the 

 manufacturing costs of the two types. Consequently, in the wire-supported 

 crystal units a very appreciable improvement should be gained in perform- 

 ance without increasing the cost of the unit. 



