N-1 CARRIER TELEPHONE SYSTEM 431 



band characteristics of these filters are shaped to provide the equaHzation 

 needed in the individual message channels. 



A narrow band filter centered at 3700 cycles selects the signal frequency 

 at the receiving terminal and provides the suppression against all other 

 frequencies needed to prevent false operation of the receiving signaling 

 circuit. The design of this filter, which is also shown in Fig. 7, makes use of 

 a cage type assembly similar to the other filter units but is somewhat smaller. 



Unit Adjustment and Inspection 



All carrier frequency filter and equalizer units are equipped with mag- 

 netic slugs to facilitate accurate adjustment of critical circuit resonances. 

 In the case of the carrier frequency high- and low-pass filter units and the 

 carrier frequency equalizer units, adjustments are made at attenuation peak 

 frequencies. After adjustment, transmission measurements made at these 

 same frequencies only are suflicient to determine satisfactory performance. 



Adjustment and inspection of the channel filter units are accomplished 

 by the use of a special test set which displays four traces on a cathode ray 

 tube. One trace displays the transmission characteristic of the unit under 

 test, the second trace displays the characteristic of an accurately adjusted 

 reference filter unit and the two remaining traces display two reference 

 discrimination levels. See Fig. 9. Blanking pulses are applied to the intensity 

 grid of the cathode ray tube to blot out the traces at points corresponding 

 to liz 4 kc from the mid-band frequency. The blotted out portion of the 

 traces together with the discrimination level traces provide a coordinate 

 system to establish bandwidth limits for inspection purposes. The magnetic 

 slugs are adjusted so that the displayed characteristic of the filter unit 

 under test is symmetrically located with respect to the displayed character- 

 istic of the reference filter unit. If the adjusted characteristic of a filter unit 

 passes through the coordinate established by the blanking pulses between 

 the discrimination level traces it meets its requirements. 



The electrical performance of the voice frequency low-pass filters and 

 the 3700 cycle signal band filter is determined by transmission measure- 

 ments at critical frequencies using standard test equipment. 



Equipment 



Equipment design of N-1 Carrier terminals and repeaters has been directed 

 particularly towards small size and weight, low manufacturing cost, sim- 

 plicity of engineering and installation, and ease of maintenance. Size and 

 weight have been minimized by arranging the miniaturized components 

 compactly in die cast aluminum frames of a size and shape to fully utilize 

 the rack space available in depth as well as in breadth and height. This 

 may be called "cubic" construction as contrasting with the "planar" con- 



