518 BELL SYSTEM TECHNICAL JOUNRAL 



10,000 c.p.s., are distributed to all test positions. Oscillators supplying 

 the individual test sets are provided with cathode ray oscilloscopes, by 

 means of which they can be synchronized with any multiple of the 

 three standard frequencies. To set up an odd frequency not coinciding 

 with any multiple it is necessary to interpolate dial readings between 

 two synchronized points. 



Control of atmospheric conditions also plays an important part in 

 the manufacture of crystal filters. The temperature coefficient of 

 frequency of the crystals most commonly used is about 15 parts per 

 million per degree Fahrenheit. For some filters, in order to secure 

 uniform performance throughout the temperature and frequency 

 ranges encountered in service, these crystals must be adjusted within 

 tolerances as small as 40 parts per million. Fluctuations of as little 

 as 2° F., in such cases, must be taken into account during the adjust- 

 ment of the crystals. In addition, crystals, coils and condensers are 

 all sensitive to the effects of excessive humidity. To minimize such 

 difficulties, the assembly and testing of these components and of the 

 filters in which they are used are carried out in air conditioned rooms 

 controlled at 75° ± 2° F. and approximately 40 per cent relative 

 humidity. 



Crystals 



Of the several component parts used in crystal filters, the first to be 

 considered in detail are logically the quartz crystals themselves. Their 

 properties of low loss and high stability are primarily responsible for 

 the unusual performance of filters in which they are employed. 



Natural deposits in the earth constitute the sole source of supply 

 of quartz crystals, since no practical method of producing them syn- 

 thetically has been developed. "Raw" crystals suitable for use in 

 filter manufacture must be unusually large and free from flaws. The 

 principal source is Brazil, the bulk of the quartz being brought in by 

 native prospectors to trading posts and shipped to this country via 

 Rio de Janerio and other coastal cities. The crystals usually range 

 between 3 and 10 pounds in weight, with occasional pieces reaching 

 100 pounds. 



The raw quartz passes through successive stages of inspection and 

 selection, commencing at the trading post and culminating in careful 

 examinations before and during the cutting operations. A concen- 

 trated beam of light from an arc lamp (see Fig. 2) is used in locating 

 internal flaws, which generally appear as small bubbles and inclusions 

 of foreign matter. Quartz takes two distinct forms, left-hand and 

 right-hand, having opposite piezo-electric polarities. Portions of raw 

 crystals containing both forms are not usable. This condition, called 



