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BELL SYSTEM TECHNICAL JOURNAL 



humidity on the transmission characteristic of a typical crystal filter. It 

 may be observed that the discrimination almost disappears for a relative 

 humidity of 80%. These measurements were made on a filter con- 

 taining well cleaned crystal plates. It will be found frequently that an 

 unsatisfactory discrimination characteristic is produced by considerably 

 lower values of relative humidity when the plates are not so clean. Ex- 

 perience has shown that it is impractical to let the relative humidity sur- 

 rounding the crystal plate exceed 40% for satisfactory filter performance. 

 When a high degree of accuracy is required, the plates are assembled 



-5 



-10 



-15 



-20 



-25 



-30 



10 20 50 100 200 

 MICRO AMPERES PER MM. OF WIDTH 



500 



1000 



Fig. 14.12. — Change of resonant frequency of GT-cut plates due to increase of trans- 

 mitted current. 



in a unit which is either evacuated or filled with air at a relative humidity 

 of less than 5%. 



14.5 Effect of Current Level 



Crystal units will undergo change in effective resistance and in frequency 

 of resonance as the current transmitted is increased. Some change might 

 be expected due to the heating of the plate by the dissipative loss associated 

 with the transmission of current. However, the effects are not identical 

 with those obtained with a change in ambient temperature. Appreciable 



