258 BELL SYSTEM TECHNICAL JOURNAL 



13.48 Stabilization of Crystal Units 



Despite the close dioiensional tolerances applying to the manufacture of 

 the indi\ndual crystal plates, the exact frequencies are rarely realized in the 

 mounted crystals. To bring the crystal to frequency it is therefore neces- 

 sary to grind off minute layers from the lengthwise or widthwise edges of 

 the plate depending on the mode of vibration. This adjustment, which 

 causes superficial disruption of the quartz areas affected, results in unstable 

 operation of the unit with respect to frequency and resistance. Unless the 

 crystal plate is properly treated after these operations, considerable drift 

 in these characteristics will take place, particularly during the initial service 

 life of the unit To alleviate this condition, the crystal units are first rough- 

 adjusted to the approximate frequency and then heat-aged in an oven which 

 subjects the units to several heating and cooUng cycles between 240°F and 

 75°F. The units are then mounted in their cages as previously described 

 and fine-adjusted after which they are again aged. This operation also 

 tends to drive off any moisture which might be troublesome. With this 

 type of accelerated aging the crystals are stabilized to the point where 

 changes in performance can be detected only by the use of the most precise 

 measuring equipment over long periods of time. Crystals so stabilized may 

 generally be depended upon, at any one temperature, maintaining their 

 frequency indefinitely within two or three parts per million provided the>- 

 are not subjected to excess voltage. 



13.49 Cleaning of Crystal Units 



Throughout the manufacturing process it is essential that every precau- 

 tion be taken to keep the crystal plate and associated parts absolutely free 

 from contamination and dirt. The rigorous cleaning necessary before the 

 spotting operation has already been discussed. In all the subsequent opera- 

 tions care must be taken to prevent the plates coming in contact with sub- 

 stances that might tend to cause corrosion. Any particles of foreign matter 

 that may have accumulated on the plate or wires before rough and fine 

 adjustments should be carefully washed off. Otherwise the performance 

 and life of the crystal may be adversely affected. A suitable method for 

 cleaning crystal units before seaUng consists of washing and rinsing in 

 chemically pure carbon-tetrachloride or other suitable solvent to remove 

 grease, followed by washing and rinsing in hot distilled water at about 

 150°F. To facilitate removal of unwanted substances, the parts should be 

 scrubbed gently with a soft brush or agitated in the solution during this 

 process. The use of pure alcohol (95%) in addition to carbon-tetrachloride 

 is also good for this process, but is not essential. The cleansed crystal units 

 should be carefully dried out and protected from further contamination 

 prior to the sealing operation. 



