Manufacture of Quartz Crystal Filters 



By G. K. BURNS 



Quartz crystal filters used in modern carrier systems present new 

 problems in manufacturing technique. In the assembly and test- 

 ing of the filters and in the production of component crystals, 

 coils and condensers, special factory facilities are required for ac- 

 curate measurement of frequency and control of atmospheric condi- 

 tions. The manufacture of quartz crystal plates in particular 

 combines several fields of applied science, including crystallography, 

 precision grinding, vacuum technique and high frequency electrical 

 measurement. Inductance coils and fixed and variable condensers 

 for use in crystal filters must consistently meet advanced require- 

 ments, especially in regard to stability. The assembly of these 

 components into filters resembles the manufacture of radio re- 

 ceivers, differing mainly because of smaller quantity requirements. 

 Testing equipment must permit rapid shop adjustment and test 

 of the completed filters with laboratory precision. 



Introduction 



ELECTRICAL wave filters employing quartz crystals ^ are used 

 extensively in broad band carrier systems 2. 3 recently introduced 

 into commercial service. Such crystals exhibit the property of piezo- 

 electricity; that is, an electrical voltage applied to the terminals of a 

 crystal causes a mechanical distortion of the quartz, and vice versa. 

 Because of this interrelation a plate of quartz, at frequencies near its 

 mechanical resonance, behaves electrically like the coil and condenser 

 combination shown in Fig. L The series inductance and capacitance 



Fig. 1 — Equivalent circuit of a quartz crystal plate. Elements L, C\ and R are 

 associated with the piezo-electric property and mechanical resonance of the crystal, 

 while Co represents capacitance between the electrodes. 



represent the mass and elasticity of the plate, respectively, while the 

 shunt condenser represents the capacitance between faces of the 

 crystal. The damping of such a plate may be made very low, giving 

 a ratio of reactance to resistance (commonly termed Q) of 15,000 or 



^ Numbered references are listed at end of paper. 



516 



