Effects of Manufacturing Deviations on Crystal Units for Filters

Effects of Manufacturing Deviations on Crystal Units for Filters By A. R. D'HEEDENE 14.1 T H E E F F E C T OF DEVIATIONS IN THE CHARACTERISTICS OF CRYSTAL U N I T S ON F I L T E R PERFORMANCE T HIS 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 information 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 that 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 convenient measure is the inductance of the equivalent electrical circuit.