CHAPTER XIV 



Effects of Manufacturing Deviations on Crystal 

 Units for Filters 



By A. R. D'HEEDENE 



14.1 The Effect of Deviations in the Characteristics of Crystal 

 Units on Filter Performance 



THIS chapter emphasizes primarily the need for close control in the 

 manufacture of crystal units for use in filters. The first telephone 

 use of crystal units in the commercial manufacture of filters was made by 

 the Western Electric Company in about 1936. To make such commercial 

 manufacture practical, it was necessary to establish accurate design informa- 

 tion and allowable manufacturing tolerances. The quantitative data 

 collected for this purpose provided the chief source of material for this 

 chapter. While the data is quite extensive, it will be observed that there 

 are still some factors which must be treated qualitatively. 



While filter crystal units are like oscillator crystal units in that they must 

 have low internal dissipation and a close control of resonant frequency, they 

 are different in tliat many additional characteristics of the filter crystal 

 units must also be controlled accurately. Two typical illustrations will 

 demonstrate how characteristics other than resonant frequency and Q may 

 react on filter performance. 



The first characteristic considered is the slope of the reactance with 

 frequency curve in the vicinity of the series resonant frequency. This slope 

 is sometimes referred to as the impedance level of the crystal unit. A con- 

 venient measure is the inductance of the equivalent electrical circuit. 

 When this inductance departs from its nominal value, the performance of 

 the filter using the crystal unit may undergo appreciable change. This is 

 particularly true of filters in which the schematic contains a lattice or some 

 other type of bridge circuit with crystal units contained in all the bridge 

 arms. For example, in Fig. 14.1 the solid curve illustrates the transmission 

 characteristic obtained from a lattice-tj-pe crystal filter, in which both the 

 series branches and the diagonal branches contain two balanced crystal 

 units. High loss results from a close impedance balance between the 

 branches of the lattice. Wlien the inductance of any of the crystal units 

 departs from its nominal value, the bridge balance is disturbed and the 

 transmission characteristic of the filter is changed. The two dotted curves 

 of Fig. 14.1 illustrates the characteristics that result when the inductance 



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