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BELL SYSTEM TECHNICAL JOURNAL 



second system of standing waves results from too much coupling between 

 the crystal and the two oppositely disposed lead wires. It may be reduced 

 by first placing the wires closer to the nodal point and second, using a smaller 

 amount of solder in the cone to attach the lead wire to the crystal. Measure- 

 ments on this same type of crystal when the above conditions were fulfilled 

 showed practically no effects of secondary standing waves. It is important 

 to keep the energy transmitted to the lead wires low since a soldered connec- 

 tion near a loop of motion resulting from secondary standing waves on the 

 wire will act as a clamp and will materially decrease the resulting Q of the 

 crystal. This is probably the best reason for the use of the headed wire type 

 of lead wherever practical. 



0.020 0.040 0.060 0.080 100 0.120 



DISTANCE FROM HEAD OF SOLDER CONE IN INCHES 



Fig. 8.9 — Effect on the frequency of lower resonances of clamping one lead of 164 kc. 



GT-cut crystal. 



8.3 Air-Gap Type Supports 



A third form of mounting for quartz cr>'Stals is that of the airgap type 

 shown in Fig. 8.10 where the crystal plate is held between two fiat electrodes. 

 Two forms of the airgap type of mounting are shown. In Fig. 8.10A the 

 crystal is free to vibrate between two flat electrodes held together to produce 

 a definite airgap of thickness /. In Fig. 8.10B small lands are left on the 

 corners of the electrodes to produce a uniform airgap on each side of the 

 crystal as well as to clamp the crystal plate. 



This type of mounting has found its greatest use for oscillator crj^stals of 

 the AT and BT type. The factor that determines the choice of mount is the 

 ratio of length to thickness of the crystal. For example, when the length is 

 less than 20 times the thickness, clamping the corners of AT and BT type 

 crystals will decrease the activity in proportion to the clamping pressure. 

 This is apparent from a study of the type of motion for these crystals de- 

 scribed in Chapter VI. This then indicates that AT andBT t}T3e cr^^stals 

 for broadcast frequencies should employ a mounting with the crystal un- 

 restricted as shown in Fig. 8.10A while the higher radio frequency crystals 

 may be clamped as shown in Fig. 8.10B. The clamping pressure will be 



