620 BELL SYSTEM TECHNICAL JOURNAL 



A particular termination is required at the end of the filter which 

 is connected to the modulator. In order to get the maximum sideband 

 power out of the modulator used, the impedance of the associated 

 band filter, seen from the modulator, must be made low over the range 

 of voice frequencies. 



With the channels placed closely together and with the coordination 

 of different types of systems, depending upon the channel locations, 

 it is important that the band filters remain constant after manu- 

 facturing, and that all filters of the same type be manufactured to 

 meet close requirements. For proper coordination between systems it 

 has been found desirable to keep all the channel bands within ± 125 

 cycles of an assigned location. This means in the higher frequency 

 channels that the filters must be manufactured to a frequency accuracy 

 of the order of 1/2 of 1 per cent. 



The attenuation requirements for the receiving band filter are 

 somewhat different from those of the transmitting band filter. The 

 purpose of the receiving band filter is the suppression of the frequencies 

 of the adjacent channels as they are received over the line. In con- 

 trast to the transmitting filter, which must suppress the unwanted 

 frequencies produced in its own channel, a filter with somewhat 

 different characteristics could, therefore, be used for a receiving filter. 

 While the requirements were determined separately for the receiving 

 and transmitting filters, it was desirable in the interest of manu- 

 facturing economy to build both alike, setting requirements on the 

 basis of a double purpose filter. Thus, this filter had to provide 

 attenuation at each frequency to meet the more severe of the require- 

 ments for either the transmitting or the receiving position. Figure 

 39 shows the transmitting characteristic of a typical filter designed to 

 meet the requirements outlined above. 



As has been explained, the grouping of the channel bands in opposite 

 directions requires the use of so-called directional filters at terminal 

 and repeater points. These filters occur in the circuit in pairs — each 

 pair consisting of one high-pass and one low-pass filter. The "cut- 

 off" point of the filters is determined by the type of system in use — 

 C-S or C-N and its corresponding "grouping point." At repeater 

 points the filters are split for each direction in order to provide 

 selectivity at both the output and input circuits of the amplifiers. 



Considering the closed circuit through the two amplifiers and the 

 four directional filters, the attenuation in this loop must be con- 

 siderably greater than the sum of the gains of the two amplifiers at all 

 frequencies. In the regions outside of the carrier frequencies, the 

 margin between attenuation and gain is made about 10 T. U. For 



