MANUFACTURING DEVIATIONS IN CRYSTAL UNITS 275 



closely by the solid curve. This solid curve actually was obtained from 

 measurements made on crystal units using plates supported with headed 

 wires and using a very small amount of solder. The other curves indicate 

 that the temperature coefficient may be increased appreciably due to the 

 presence of a larger amount of solder. Further, when the larger amounts 

 of solder are used, the characteristics depend on the exact amount of the 

 solder, so that the characteristics represented by the dashed curves are 

 hard to reproduce. 



The amount of solder used in this operation also affects the Q of the crys- 

 tal unit and its resonant frequency. Measurements using several crystal 

 plates of relatively small sizes have shown improvements in Q of as much as 

 25 per cent when headed wires are used over that obtained with other wires 

 using larger amounts of solder. 



Variations in the consistency of the solder joint will, of course, afifect the 

 adherence of the supporting wire to the plate. A poor joint will result in a 

 high effective resistance for the crystal unit and will generally cause in- 

 stability both in resistance and in resonance frequency. 



In soldering the supporting wire to the crystal plate two methods have 

 been used for melting the solder; namely, the soldering iron, and the hot- 

 air blast. With either method, lack of sufficient control can seriously 

 change the electrical characteristics of the plate due to twinning. It has 

 been observed that this twinning occurs when there is a large temperature 

 gradient in the quartz, even at temperatures well below the inversion point. 

 Experimental work by G. W. Willard has indicated that it may occur even 

 when the temperature of the soldering iron is as low as 300°C. To avoid 

 such twinning during the soldering operation, it has been found desirable 

 to raise the temperature of the entire plate to just below the melting point 

 of the solder. 



Twinning, when it occurs, will afifect the crystal plate by causing an in- 

 crease in inductance, a change in the resonant frequency, increased effective 

 resistance, and a change in the temperature coefficient, as stated previously. 

 Also, in crystals with divided plating there will be an inductance unbalance 

 between the two halves of the crystal plate set up due to unequal amounts of 

 twinning. Several measurements made, using GT plates at 160 kc, showed 

 that twinning during the soldering operation decreased the resonant fre- 

 quency in a range from 200 to 100 cps and the temperature coefficient of the 

 units ranged from 2 to 6 times that of units using untwinned crystal plates. 



14.34 Effects due to Wire Resonance 



As described in Chapter VIII^^, the characteristics of crystal units may 

 be changed due to vibrations set up in the supporting wires. When any 



12 "Methods of Mounting and Holding Crystals", R. A. Sykes, B. S. T. J., April 1944. 



