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BELL SYSTEM TECHNICAL JOURNAL 



requirements and did not exceed the maximum loss requirements in 

 the transmitting bands. The margins, however, were not very great. 

 It is evident, therefore, from Fig. 5, that a very small shift in the 

 frequency location of the loss characteristic of a filter would throw it 

 outside the required limits. The shift which would cause this to 

 happen was. in fact, so small, ±125 cycles, that the manufacture of 

 these filters by methods currently in use resulted in enough rejections 

 at the factory to warrant the development of a more precise method. 



L| C| L4 C4 L5 C5 



REQUIREMENTS 



Fig, 



25 



5— Att 



26 27 28 29 30 31 32 33 



FREQUENCY IN KILOCYCLES 



enuation characteristic of 38-N filter, showing requirements. 



The attenuation characteristic of a manufactured filter seldom 

 conforms exactly with that desired because of manufacturing tolerances 

 which must be allowed in its component elements. The inductances 

 and capacitances of the coils and condensers which make up a filter 

 differ somewhat from their specified values for several reasons. The 

 precision of their adjustment can be no greater than the precision of 

 the circuits in which they are measured. It is usually less than this 

 figure because the coils and condensers used in precision filters are 

 potted in a moisture-proofing compound, after adjustment, and this 

 potting produces a small change in the capacitance of the condensers 

 and the distributed capacitance of coils which is not uniform and can- 



