18 THE BELL SYSTEM TECHNICAL JOURNAL, JANU.\RY 1951 



output. Because of the large spread between input and output signal band? 

 the balanced series modulator arrangement can use a simple condenser 

 input and inductor output filter. Poor and uncontrolled carrier leak would 

 result in change in amplitude and phase of the transmitted carrier used to 

 regulate each channel in the receiving circuit and to demodulate the two 

 sent sidebands of voice and signal frequencies. Selection of germanium units 

 in manufacture provides good balance of the varistor bridge and results in 

 very low level of carrier leak. A controlled amount of d-c. is introduced at 

 the modulator input terminals from the 130-volt battery and sends carrier 

 at adequate level and correct phase for demodulation of the speech side- 

 bands in the demodulator without over modulation on the speech peaks. 

 The transmitted carrier amplitude is stable for channel regulation purposes, 

 varying in amplitude only with the 130-volt battery which, in most offices, 

 will seldom be outside of one per cent limits. 



In the receiving carrier channel the band filter at the input selects the 

 particular channel from the receiving group unit output. The crystals in 

 the oscillator and the band filters are the only differences among the 12 

 channels for this subassembly. The filter design is such that one or more 

 channel units can be removed from service without affecting the performance 

 of the others. 



At the output of ea( h band filter a potentiometer is used to adjust for 

 the transmission inequ.' lities from one channel to the next of the over-all 

 system. The two-stage double-triode regulator automatically adjusts for 

 any subsequent changes in level of the received channel signals. The regulator 

 is operated by d-c. obtained from carrier rectification in the linear de- 

 modulator. T3^ical delayed AVC action is obtained by biasing out part of 

 the rectified carrier voltage with a part of the 130-volt battery supply. 

 The time constant of the regulator circuit is slowed to about 5 seconds to 

 provide adequate regulating speed without false operation on speech or 

 line hits. The demodulator is operated as a linear detector with highest 

 peaks on speech sidebands just below 100% modulation. The unusually 

 high impedance level of 10,000 ohms for the varistor demodulator provides 

 large d-c. voltage for the gain control circuit without undue instability. 

 This results from inclusion of the demodulator in the mu circuit of the 

 regulator. The output of the demodulator is connected to the signaling band 

 filter and the expandor LPF. 



Terminal Transmitting and Receiving Group Units 



The group units serve essentially as terminal repeaters for transmitting 

 and receiving directions. There are four varieties: low-group transmitting 

 (LGT) and associated high-group receiving units (HGR) or high-group 

 transmitting (HGT) and associated low-group receiving units (LGR). A 



