PLATED QUARTZ CRYSTAL UNITS 255 



13.47 Housing of Crystal Units 



For the pressure-type units first discussed no provision was made for 

 protecting or sealing them other than the hermetically sealed containers in 

 which all the other associated components of the filter were enclosed. How- 

 ever, the wire-supported designs have been worked out so that each unit 

 is sealed in its own individual container. Fortunately, the sizes of virtually 

 all crystal units are in the range which permits the use of relatively inexpen- 

 sive radio tube parts for these housings. There are man}' obvious advan- 

 tages to the individually sealed unit. After adjustment and sealing it can 

 be handled more readily in subsequent assembly operations. It is not 

 subject to variations due to changes in ambient humidity and consequently 

 does not restrict the assembly of apparatus to conditioned space. It can 

 be made up and stored or shipped as an individual unit. It has a higher 

 degree of stability. There is one small effect, however, in the case of units 

 which are sealed in vacuum. Due to the absence of any gaseous medium 

 around the crystal, a slight change in frequency is encountered when the 

 tube is evacuated. However, this change is always the same for each 

 particular type and size of crystal and can be allowed for in the final adjust- 

 ment before seaUng. 



Most designs of crystals can be sealed in an atmosphere of dry air although 

 better performance results from the use of vacuum. Some crystals must 

 be sealed in vacuum for this reason. A decided advantage in favor of 

 vacuum-sealed crystals is the elimination of acoustic effects from air 

 resonance. 



Both metal and glass tubes are used for housing crystal units. Initially 

 it appeared that metal tube radio parts were ideally adapted to crystal use, 

 and it was felt that, instead of welding the stem to tube, this seaUng opera- 

 tion could be done by soldering. However, it was found that while sound 

 solder joints could be obtained, extreme precautions were necessary to 

 protect the button-type glass seals, through which the leads emerge, during 

 the pre-tinning and soldering operations. Even with such precautions, it 

 would have been essential to include in every vacuum type tube a means of 

 detecting whether or not a leak had developed. For air-filled tubes at 

 atmospheric pressure this would not have been necessary since minute leaks 

 can be tolerated with little likelihood of the crystal being afifected over a 

 long period of time. The possibility of welding as is done in the case of 

 radio tubes was considered but did not appear justified on the basis of 

 equipment cost. Moreover, even with welding there still appeared to be 

 problems from leakage and outgassing of the metal since, after the crystal 

 is enclosed, the assembly cannot be exposed to high temperature to drive 

 off adsorbed gases during the evacuation process. In view of these draw- 



