100 BELL SYSTEM TECHNICAL JOURNAL 



of the same order as that of coils and condensers, and require practic- 

 ally no maintenance as compared to vacuum tubes. 



The translation of the channels from the 60 to 108-kc. range to the 

 position required for cable carrier, 12 to 60 kilocycles, is made by a 

 stage of group modulation. A copper-oxide group modulator is used 

 and a carrier frequency of 120 kilocycles. The reverse of this process 

 in a similar group demodulator at the receiving end steps the frequency 

 back to its original range, 60 to 108 kilocycles. These processes of 

 modulation take place at points of low-energy level in the circuit with 

 a comparatively high level of carrier, so that the inter-channel crosstalk 

 which results from unwanted products of modulation is unobjection- 

 able. Low-pass filters are inserted after the group modulator and 

 demodulator, and amplifiers with flat gain characteristics are supplied 

 to raise the levels of the output currents of the group modulators or 

 demodulators. 



Carrier Supply 



The carrier frequencies which are required at a terminal are obtained 

 from the harmonics of a base frequency. The carrier supply system 

 is common to as many as 10 systems in one ofiice. This simplification 

 was made possible by the selection of the channel frequencies as mul- 

 tiples of a base frequency, 4 kilocycles being chosen for this system. 

 This base frequency is produced by an oscillator in which the control 

 element is a tuning fork, the whole unit being designed to have the 

 necessary output and frequency stabilities. The output of the oscil- 

 lator is amplified and fed to a circuit which produces the desired 

 harmonics. All of the carrier frequencies which are required for the 

 different channels as well as for group modulation and demodulation 

 are obtained from these harmonics. A small coil with a permalloy 

 core is the important agent in this process.* 



Failure of the 4-kc. supply, or failure of the 120-kc. supply used for 

 group modulation, would cause failure in the channels of all systems 

 operating from this supply. Provision is made for such a contingency 

 by an emergency carrier supply which is automatically switched into 

 service when the regular supply fails. This reserve source duplicates 

 all of the parts of the regular supply, 4-kc. fork, amplifier, harmonic 

 producer, and amplifier for the 120-kc. carrier. 



Assembly 



The different panel units which make up the terminal of a type 

 K system are assembled on a functional basis with similar panels of 

 other K systems, the channel modulator-demodulator panels in one 



