112 BELL SYSTEM TECHNICAL JOURNAL 



It seems to have an indefinite life. (Some have been on Hfe tests as 

 modulators for about five years.) The carrier power required is 

 about 1/2 milliwatt to modulate satisfactorily a single telephone 

 circuit level of — 13 db. 



The modulator produces the usual two sidebands and the lower one 

 is selected by the quartz crystal channel filter described in another 

 paper.^ This sideband, joined by eleven others, is stepped down to 

 about the iterative impedance of the shielded office cabling. In the 

 office cabling the twelve channels pass through the high-frequency 

 patching bay to the double balanced group modulator of copper oxide 

 where they are joined by three pilot channel frequencies. 



The group modulator uses the same copper oxide as that in the 

 channel modulator described above, but the carrier power is about 50 

 times greater (about 25 milliwatts) in order to keep down unwanted 

 modulation produced between the twelve sidebands. To that same 

 end the level of each sideband is made low (— 46 db), and the double 

 balanced type of circuit is used to balance out some of the undesired 

 products. It also balances out the twelve incoming bands in the range 

 60 to 108 kc. from the output and so simplifies the following group 

 modulator filter. 



From a level of — 57 db the twelve channels, now in the range from 

 12 to 60 kc, are amplified to + 9 db for delivery to the 19-gauge pair 

 in the lead covered toll cable. The amplifier is a three-tube negative 

 feedback type, using pentodes and operating with 154 volts plate bat- 

 tery which is composed of the usual 24-volt filament battery and 

 130-volt plate battery in series. The last tube is a power tube and 

 does not overload until a single-frequency output of about one watt 

 is reached. 



On the receiving side in Fig. 3, the twelve incoming channels, in 

 the range from 12 to 60 kc, pass from the amplifying and regulating 

 equipment,^ to the group demodulator. This is identical with the 

 group modulator described above and transfers the twelve channels 

 to the range 60 to 108 kc The channels are then amplified to a — 5 

 db level by an amplifier of the negative feedback type using two low- 

 power pentodes with 154-voIt plate battery as described above for 

 the transmitting amplifier. 



From there the twelve channels are separated by the filters which 

 are identical with those on the transmitting side, and are then demodu- 

 lated and amplified to a + 4 db level as shown for one channel in 

 Fig. 3. The demodulator is identical with the modulator but it is 

 poled oppositely on the carrier supply so that the d-c components of 

 modulation in the modulator and demodulator neutralize each other 



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