SHOHT-IIAITL CAUUIKU TKLKGHAl'lI 669 



nals. The high-frequency system is based on six midband tVcciucncies 

 spaced from 200 to 240 cycles apart. The frequency sliift ranges from 

 ±40 cycles in the lowest channel to ±50 cycles in tlie highest. These 

 wider spacings and shifts were adopted to ease the problem of designing 

 inexpensive filters and oscillators for the higher frecjuencies. 



Channel 1 of the high-fre(iuency system employs adjacent frecjuency 

 assignments for the two directions of transmission. The lower frequency 

 path employs a downward shift for marking signals and the higher- 

 frequency path an upward shift for marking signals. In half-duplex 

 operatit)!! this minimizes interference from the strong signals at the 

 transmitter output to the weak signals at the receiver input. The steady 

 marking frec[uencv, which is being sent against the flow of traffic, is 

 sliifted away from the band o\er which the message is passing. 



CHANNEL TERMINAL CIRCUIT 



Sending Circuit 



The sending portion of the channel terminal circuit is shown in the 

 upper part of Fig. 2. When the teletypewriter sending contacts open 

 the loop to send a spacing signal, the sending triode is cut off. When the 

 contacts close the loop to send a marking signal, the grid is made positive 

 with respect to the cathode, the tube conducts and the potential at the 

 plate is decreased. A varistor bridge modulator is connected between 

 this plate and a source of potential having a value lying midway be- 

 tween the marking and spacing plate potentials. Thus, the potential 

 apphed across the modulator during marking signals is opposite in 

 polarity to that applied dining spacing signals. When the voltage across 

 the varistor bridge is in the conducting direction, the varistors provide a 

 low impedance path to alternating currents. Thus additional capacitance 

 is coupled to the oscillator tank circuit and the oscillator operates at 

 the lower of its two signal frequencies. When the voltage across the 

 bridge is in the non-conducting direction, the varistors are biased to a 

 high-impedance portion of their characteristic, the capacitor is effec- 

 tively disconnected from the tank cii'cuit and the oscillator operates 

 at its higher signal frequency. 



The reversing switch in the dri\-ing circuit of the modulator permits 

 either the higher or the lower frequency to be used for marking signals. 



The oscillator output power is adjusted by the send level poten- 

 tiometer and passed through a buffer amplifier and a band pass filter 

 to the send bus and line. 



The filter is an impedance transfoi'ming structure which contains a 



