MANUFACTURE OF QUARTZ CRYSTAL FILTERS 517 



higher, as compared with a practical limit of 300 for coils. Stability 

 of resonance frequency and compactness of dimensi®ns are two further 

 respects in which quartz crystals surpass the best coils and condensers 

 available. 



Filters designed to utilize these properties generally consist of one 

 or more crystal plates, plus such condensers, inductance coils and 

 resistances as may be required to give the desired overall performance. 

 The principal types used in the Bell System operate at frequencies 

 ranging from 40 to 600 kilocycles and transmit bands varying from 5 

 cycles to 6 kilocycles in width. Physical dimensions range up to 

 3X5X16 inches. 



Unusual manufacturing requirements are imposed by the nature of 

 these filters and of the systems in which they are used. Adjusting 

 tolerances and stability requirements, for example, range from ± 20 

 to ± 200 parts per million on crystals and on coil-and-condenser 

 circuits used in crystal filters. Transmission losses must be measured 

 to accuracies of the order of ± .03 db at 100 KC. To insure stability 

 of adjustment during service life, component apparatus must be 

 protected against dust and excessive humidity. Methods of assembly 

 and testing must be adaptable to a variety of types of filters, one of 

 which, the channel filter,* is manufactured by the Western Electric 

 Company in quantities of 1500 to 5000 per year, while the others 

 range from 10 to 1000 per year. Long service life must be assured by 

 proper choice of materials and technique. 



In order to satisfy such requirements special manufacturing pro- 

 cedures are necessary . In reviewing these features it will be convenient 

 to consider first tho methods or facilities which are used in several 

 or all stages of the manufacture of crystal filters, second the methods 

 employed in producing component apparatus for such filters — particu- 

 larly crystals, coils and condensers — and finally the technique of 

 assembling and testing the complete filters. 



General Facilities 



A primary requisite in the adjusting and testing of both crystals 

 and crystal filters is the precise measurement of frequency. -The 

 equipment used for this purpose includes a standard frequency gener- 

 ator containing a 100 KC crystal oscillator. This generator normally 

 maintains a frequency accuracy of about 1 part in 2,500,000 operating 

 under the control of the Bell System master frequency standard in 

 New York, but will remain accurate within 1 part in 1,000,000 even 

 though the master signal is interrupted for as much as 24 hours. 

 Three sub-harmonics of 100 KC, namely, 100 c.p.s., 1000 c.p.s. and 



