CARRIER-CURRENT COMMUNICATION 645 



channel is the same in all cases except for the frequency of the band- 

 pass filter. For this reason, a consideration of one channel is suffi- 

 cient. Each channel is composed of a voice-frequency hybrid coil, 

 a modulator with its band-pass filter, an oscillator, and a demodulator, 

 together with Its associated filters. In addition, there is, at each 

 end, a carrier hybrid coil together with transmitting and receiving 

 amplifiers which are common to all channels. The arrangement of this 

 equipment is shown schematically in Fig. 6, as previously indicated. 



The modulator, the input of which is connected to the center taps 

 of the hybrid coil line windings, utilizes of two vacuum tubes ar- 

 ranged for push-pull operation. The carrier current which is sup- 

 plied by the oscillator is applied to the two grids by means of a trans- 

 former. Such a circuit generates the two side bands but suppresses 

 the carrier. In order that this suppression may be as complete as 

 possible, the small condenser associated with the grid of one of the 

 tubes is made variable and is adjusted until the carrier current in the 

 modulator output is reduced to a minimum. The band-pass filter 

 transmits one of the side bands and suppresses the other, as well as 

 all miscellaneous resultant currents of a higher order which are pro- 

 duced by the modulator. It also prevents the output currents of the 

 other channels from entering the modulator circuit as this would 

 cause a reduction in their efficiency and give rise to undesirable 

 frequencies. 



The demodulator is very similar to the modulator. The tube ar- 

 rangement is substantially the same and carrier current is supplied 

 from the one oscillator. In the demodulator a complete suppression 

 of the carrier is unnecessary as this is accomplished by the low pass 

 output filter. For this reason, the small balancing grid condensers are 

 omitted. In order to adjust the over-all gain of the channel, the 

 demodulator is provided with an adjustable potentiometer gradu- 

 ated in two transmission unit steps, and in addition, fixed pads are 

 provided for making further gain adjustments. The output of the 

 demodulator is connected to the series winding of the voice-fre- 

 quency hybrid coil. 



The oscillator which supplies the carrier current to the modulator 

 and demodulator is of the usual type. The tuning condenser in- 

 cludes a small variable unit for making small adjustments in fre- 

 quency. Separate oscillators are used at the two ends for each channel, 

 and as these are in no way connected together, it is occasionally 

 necessary to make slight adjustments in order to keep the frequencies 

 at the two ends substantially equal. The oscillators are very stable, 

 however, and such adjustments are seldom required. 



