WIDE-BAND TRANSMISSION OVER COAXIAL LINES 667 



any point in a fairly wide frequency range. The high gain is usually 

 obtained by presenting a high impedance to the input circuits of the 

 various tubes through tuning the input and interstage coupling cir- 

 cuits to approximate anti-resonance. 



In amplifying a broad band of frequencies, it is difficult to maintain 

 a very high impedance facing the grid circuits. The inherent capaci- 

 tances between the tube elements and in the mounting result in a 

 rather low impedance shunt which can not be resonated over the de- 

 sired frequency band. It is, therefore, necessary to use relatively low 

 impedance coupling circuits and to obtain as high gain as possible 

 from the tubes themselves. The amount of gain which can be ob- 

 tained without regeneration depends, of course, upon the type of tube, 

 the number of amplification stages, the band width, and also upon the 

 ratio of highest to lowest frequency transmitted. 



Repeater Gain 



The total net gain desired in a line amplifier is such as to raise the 

 level of an incoming signal from its minimum permissible value, which 

 is limited by interference, up to the maximum value which the ampli- 

 fier can handle. 



As pointed out above, the noise in a well shielded system is that due 

 to resistance noise in the line conductors and tube noise in the ampli- 

 fiers. In some of the repeaters which have been built, the amplifier 

 noise has been kept down to about 2 db above resistance noise, corre- 

 sponding to about 7 X 10""^'' watt per voice channel. In a long line 

 with many repeaters the noise voltages add at random, or in other 

 words, the noise powers add directly. Assuming, for example, a line 

 with 200 repeaters, the noise power at the far end would be 200 times 

 that for a single repeater section. In general, the line and amplifier 

 noise will not be objectionable in a long telephone channel if the speech 

 sideband level at any amplifier input is not permitted to drop more 

 than about 55 db below the level of the voice frequency band at the 

 transmitting toll switchboard. 



The determination of the volume which a tube can handle in trans- 

 mitting a wide band of frequencies involves a knowledge of the distri- 

 bution in time and frequency of the signaling energy and of the require- 

 ments as to distortion of the various components of the signal. The 

 distribution of the energy in telephone signals has been the subject of 

 much study. This distribution is known to vary over very wide limits, 

 depending upon the voice of the talker and many other factors. It is, 

 therefore, obvious that the problem of summing up the energy of some 

 hundreds of simultaneous telephone conversations is a difficult one. 



