Crystal Channel Filters for the Cable Carrier System * f 



By C. E. LANE 



SINCE the channel selecting filters used at the terminals of the 

 twelve-channel cable carrier system are the principal filters in the 

 system this paper is concerned primarily with these. Their importance 

 is evident from the fact that they represent over one-third of the cost 

 of the system terminals. 



Many new features appear in these channel filters. The most 

 outstanding is the use as filter elements, along with inductance coils 

 and condensers, of plates cut from crystalline quartz. It is for this 

 reason they are called "crystal filters." In addition, however, the 

 inductance coils, some of the condensers, and also the filter assemblies 

 have in them new features. Only after a number of years of laboratory 

 experimentation with filters using crystal elements, studying their 

 advantages and limitations, was the cable carrier system planned to 

 use such filters. 



There are twelve channel filters which transmit the lower side bands 

 derived from the modulation of the speech signals with carrier fre- 

 quencies spaced 4 kilocycles apart from 64 to 108 kilocycles. An 

 insertion loss frequency characteristic w^hich applies for each of the 

 twelve filters is shown in Fig. 1. Regarding a 10 db loss increase as 

 the cut-ofT as compared with transmission at 1000 cycles, the voice- 

 frequency band for a single-carrier link, largely determined by the 

 characteristics of the channel filters, extends from approximately 150 

 to 3600 cycles. For five links the band extends from about 200 to 

 3300 cycles. This is a 600 or 700 cycles wider frequency band than 

 the present three-channel open-wire carrier system. The maximum 

 delay distortion in the transmission band of each of the filters is about 

 0.4 millisecond. As many as ten of these filters may appear in tandem 

 in the longest talking circuits. The total delay distortion in such 

 cases would then not exceed 4 milliseconds. This is not objectionable 

 since the average listener can not observe the effect of delay distortion 

 unless it exceeds about 10 milliseconds. A representative filter 

 schematic is shown in Fig. 2. The condenser shown by the dotted 

 line at the left is used only in the two lowest frequency filters to obtain 



* This is a companion paper to other papers covering different parts of the twelve- 

 channel cable carrier system. 



t Presented at Winter Convention of A.I.E.E., New York, N. Y., Jan. 24-28, 1938. 



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