MANUFACTURING DEVIATIONS IN CRYSTAL UNITS 



279 



tides of foreign matter will introduce appreciable changes in crystal per- 

 formance. 



Usually, the presence of foreign matter will act to load the crystal and 

 will reduce the resonant frequency but there are also instances where the 

 added matter tends to stififen the plate and increase its frequency. The 

 latter has been observed to occur as the result of the deposit of a thin film 

 of rosin on the surface of the plate. In the presence of foreign matter on 

 the surface of the plates, the performance will be unstable with time and 

 temperature even after the plate is sealed into a container. Also, erratic 

 variations are observed as the plate is shifted from a normal atmosphere to 

 a container which is evacuated or filled with dry air. Experience has shown 



-4 -3-2-111 2 



FREQUENCY KC TROM CARRIER 



Fig. 14.11. — Efifect of humidity on the discrimination of channel crystal filters. To 

 prevent decrease of discrimination with increase of relative humidity, the crystal units 

 must be hermetically sealed. 



that elaborate precautions for insuring cleanliness are justified by the time 

 saved in the adjusting processes. 



The need for cleanliness is closely related to the effect of humidity on the 

 insulation resistance of crystal units. As used in filters, crystal units must 

 provide extremely high impedances at their anti-resonant frequencies. 

 These impedances may be as high as 100 megohms. With clean crystal 

 plates in relatively dry atmospheres, such insulation resistance can be main- 

 tained up to 1000 kc. However, even a trace of salts or other t>'pes of con- 

 tamination will make the insulation resistance highly sensitive to moisture 

 in the adjacent air. While it is relatively difficult to measure insulation 

 resistance at high carrier frequencies, the effect of the reduced insulation 

 due to moisture is evident on inspection of the discrimination characteristics 

 of the filters. For example, Fig. 14.11 illustrates the result of high relative 



