60 BELL SYSTEM TECHNICAL JOURNAL 



Considerable use for the .4 T plate has been found for filters on pilot channels 

 for the coaxial telephone system. 



The CT and DT are analogous to the AT and BT but are termed low 

 frequency shear plates. The motion associated with these cuts is that of a 

 face shear as illustrated in Fig. 6.4. The CT and DT cuts are useful for 

 both filter and oscillator applications in the frequency range from 60 kilo- 

 cycles to 1000 kilocycles. Here again the DT would be most useful in the 

 lower range and the CT the upper range due to the higher frequency constant 

 for the CT cut. 



The GT is similar to the CT except that it is rotated by 45° about the nor- 

 mal to the plate so that instead of a face shear type of motion there are two 

 extensional modes similar to that shown in Fig. 6.3. These two modes are 

 coupled to each other resulting in one of them having a zero temperature 

 coefficient over a wide range of temperature. This crystal is most useful 

 in the range from 100 kilocycles to 500 kilocycles for a primary standard of 

 frequency and in filter networks having extreme phase requirements. 



The filter plates commonly called the — 18° cut and 5° cut are shown with 

 their relation to the crystallographic axes in the central part of Fig. 6.6. 

 The — 18° cut commonl}' used in filters employs a simple extensional motion 

 along its length with small coupling to an extensional motion along its width 

 and practically zero coupling to a face shear type of motion. Since the width 

 is usually the order of half the length these modes are not troublesome. The 

 + 5° cut is useful in filter work because it has a low temperature coefficient 

 and in spite of its strong coupling to the plate shear, it has been found quite 

 useful in both its extensional mode and its flexure mode. The —18° cut 

 is used over the frequenc}' range from 60 kilocycles to 300 kilocycles and 

 forms the basic crystal used in the channel filters of the coaxial telephone 

 system. When driven in flexure the 5° cut may be made to operate as low 

 as 5 kilocycles and is used in oscillator and filter circuits. 



The NT cut is shown at the bottom of Fig. 6.6 with its relation to the 

 crystallographic axis. This is obtained by a rotation of +8.5° about the X 

 axis with a second rotation of ± 60° about the resulting Y' axis. The pur- 

 pose of the second rotation is to give the shear modulus a positive coefficient. 

 This modulus enters into the equation for the flexure frequency and there- 

 fore the effect of the second rotation is to change the temperature coefficient 

 of the flexure mode from a negative value to zero. This crystal has been 

 used to some extent as a low frequency oscillator. Its main purpose so far 

 has been for the control of frequency modulation broadcast transmitters and 

 for low frequency pilot channel filters. 



Another crystal called the MT which is cut in a manner similar to the NT 

 but with angles of 8.5° and 36° respectively has been used for filter work 

 where an extensionally vibrating crystal of zero temperature coefficient is 



