Crystal Channel Filters for the Cable Carrier System

I N C E 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. M a n y 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 frequencies spaced 4 kilocycles apart from 64 to 108 kilocycles. An insertion loss frequency characteristic which applies for each of the twelve filters is shown in Fig. 1. Regarding a 10 db loss increase as the cut-off as compared with transmission at 1000 cycles, the voicefrequency 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.