ELECTRIC \VA VE FILTERS 281 



A complete circuit for resonant frequency adjustment is set up at 

 the Kearny, N. J. Works of the Western Electric Company where 

 carrier filters are manufactured. There, after the coils and con- 

 densers in the filter are mounted upon their sub-panel (or panels), 

 but before they are wired, the assemblies are brought over to the 

 resonant frequency adjusting circuit, familiarly known as the L-C 

 circuit. The sub-panel is placed in one end of the adjusting shield 

 and the coils on that end of the sub-panel are adjusted in the manner 

 previously outlined, after which the sub-panel is brought over to the 

 other end of the shield, and the remaining coils are adjusted. After 

 the coil rotors have been set at the positions for correct adjustment, 

 they are locked by tightening the four screws which hold the coil 

 terminal strip in place. The coil top presses down on the wedges 

 (one of which is visible in Fig. 7) on each side of the coil case, and 

 these wedges in turn clamp the rotor bearings firmly. Then the sub- 

 panels are removed from the circuit, wired, and assembled in their 

 shields. 



This method of obtaining increased precision in filter manufacture 

 was first employed in the band filters and equalizers for the Type " C " 

 Carrier Telephone System. Previously, the ± 1 per cent limit on 

 solenoidal air-core coils, the ± 0.3 per cent Hmit on mica condensers, 

 and the additional ± 0.7 per cent variation in the eflect of the filter 

 shield in the coil inductance, caused by variation in the coil diameter, 

 variation in the shield dimensions, and variation in mounting location, 

 which added up to ± 2 per cent L-C variation, imposed ail per cent 

 limit on the precision of frequency location. When the L-C method 

 of adjustment is employed, the resonant frequency of a coil and 

 condenser combination may be adjusted to ± 0.05 per cent, the 

 precision of oscillator caHbration, plus the ± 0.1 per cent limit on 

 the adjustment process, or ± 0.15 per cent. In the laboratory it is 

 possible to adjust the resonant frequency of an element combination 

 to limits closer than ± 0.1 per cent, but the ± 0.1 per cent limits 

 were set in the shop in order that production might proceed without 

 undue delay occasioned by the process of adjustment. The ± 0.7 

 per cent tolerance caused by variation in coil diameter, shield dimen- 

 sions and coil location is reduced to a variation of ±0.1 per cent in 

 L-C product, or ± 0.05 per cent in frequency; the diflerence between 

 the adjusting shield and a filter shield caused by the variation in 

 dimensions of filter shields. The precision of manufacture of filters 

 using the L-C method of adjustment is, therefore, ± 0.15 per cent, 

 the limits for the adjusting process, plus ± 0.05 per cent, the margin 

 for variation in filter shield dimensions, or ± 0.20 per cent in frequency. 



