SHORT-HAUL CARRIER TELEGRAPH 



671 



downward transformation (7500:000) from the impcMlance of the })uffor 

 amplifier to that of the Une so that no amphfier output transformer 

 is required. 



Either unity-ratio hue coils or a hy])rid coil may be used to connect 

 the unbalanced sending 'I'ld recei\'ing filters to a balanced line. The 

 hybrid coil is used with a two-wire line when the send and I'cceive 

 frequencies occupy adjaccMit bands. 



Receiving Circuit 



The receiving circuit, shown in the lower part of Fig. 2, is equipped 

 with a filter which selects a narrow band of frequencies centered about 

 the mark and space frequencies of the channel to be received. The 

 receiving band filter has characteristics similar to those of the sending 

 filter, except that it has a greater discrimination against unwanted fre- 

 (juencies and provides an upward transformation (600:140,000) from 

 the line impedance to a value suitable for dri\dng the grid of the first 

 amplifier stage. 



The frequency-loss characteristics of a typical receiving filter used in 

 the voice band and of the corresponding sending filter are given in Fig. 3. 



The carrier signals selected by the receiving filter are passed through 

 a three stage amplifier limiter. Most of the limiting action is provided by 

 the third stage; the first and second stages act as amplifiers only for 

 weak signals but limit strong signals. An adjustment for receiving gain 



ill30 



-200 -175 -150 



-125 -100 -75 -50 -25 25 50 75 100 125 

 FREQUENCY IN CYCLES PER SECOND FROM MID -BAND 



150 175 200 



Fig. 3 — Send and receive filter characteristics, VF allocation. 



