PLATED QUARTZ CRYSTAL UNITS 253 



chosen spacing of 25-30 mils between the crystal plate and the bumper 

 should ensure against any service failure of the unit in this regard. In 

 order to center the crystal laterally and longitudinally in the bumper system, 

 the plate is assembled first in the cage by means of spacers. The fine wires 

 are then soldered to the vertical rods or "straights" as they are called, and 

 the spacers removed leaving the plate suspended in position. In order not 

 to set up any strains in the junctions of the wire to the straight which might 

 tend to displace the plate after this operation, the spring wires are usually 

 pre-formed to come within about 5 to 20 mils of the straight. The junction 

 is made by immersing the intersection of the wire and straight in a ball of 

 molten solder. As the wires are withdrawn the ball of molten solder comes 

 with them, solidifying in the air and thus joining the fine wire to the straight 

 without strain. 



It will be noted from Fig. 13.2 that the wires to the longitudinal crystal 

 are equipped with little weights close to the plate. This practice has been 

 found desirable on virtually all types of crystal units to alleviate problems of 

 wire resonance^ which arise in occasional units thereby causing high resist- 

 ance as well as a shift in the frequency of the plate. Initially, while these 

 effects were noted to some e.xtenl in the course of laboratory developments, 

 it was not thought that they would be prevalent enough to warrant taking 

 precaution to eliminate them by loading the wires, since they can usually 

 be corrected by refloating and resoldering the crystal plate thereby changing 

 the effective length of the wire. However, it has turned out that in manu- 

 facture a large enough percentage of crystals contain resonant wires to 

 warrant the use of weights. For low-frequency crystals (up to 200 kc) solder 

 balls are placed on the wire at the desired location using a method worked 

 out in conjunction with the Western Electric Company. The process is 

 performed in somewhat the same manner as that described above for con- 

 necting the crystal support wires to the straights, except that the weight 

 of the solder deposited and the distance from the plate is more critically 

 controlled. For higher-frequency crystals above 200 kc in which more pre- 

 cise positioning of the weight is essential, small metal discs are employed. 

 They are threaded onto the mounting wire and held in the correct position 

 by a definite amount of solder on the back to obtain the desired loading. 

 Since the free length of wire must be accurately controlled, the manufac- 

 turing aspects of this job have been greatly simplified by the use of headed 

 wires in which the variation in height of the solder cones is very small. The 

 chart shown in Fig. 13.11 shows the weights of solder balls or discs and the 

 position they should take on crystals having frequencies up to about one 

 megacycle. It should be noted that the chart covers .0063" phosphor 



8 "Principles of Mounting Quartz Plates," R. A. Sykes, B.S.T.J., April 1944. 



