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BELL SYSTEM TECHNICAL JOURNAL 



37. Modulation Effected by Controlling Plate Current with Grid 

 Potential. Numerous circuits have been developed for modulation, 

 making use of the fact that the grid potential affects the resistance 

 of the plate circuit. Two circuits of this type are shown in Figs. 



Siorwl — * 



Fig. 41 



41 and 42. In the first, the plate circuit of the tube forms a shunt 

 across a portion of the antenna inductance. As the grid potential is 

 varied, the antenna is, therefore, thrown more or less out of tune, 

 with the consequent radiation of a variable amount of high frequency 

 energy. 



Fig. 42 shows one of the most efficient modulating schemes thus far 

 developed. As the grid potential of the modulator tube A varies, 

 causing a change in plate current through this tube, the plate voltage 



Low Frequency High Frequency 



Choke Choke 



Fie. 42 



applied to the oscillator tube, B, fluctuates, because of the presence 

 of the low frequency choke coil. Under this condition of variable 

 plate-voltage the oscillator gives a variable amount of high frequency 

 energy to the antenna. When used for speech modulation this 

 circuit is very efficient and gives good quality. The tubes A and B 

 are ordinarily of the same type. 



Many other modulating circuits have been designed, and those 

 given above are to be considered merely as illustrative of the general 

 manner in which the properties of the vacuum tube may be applied 

 to the problem of modulation. 32 



82 For other types of modulator circuits see a paper by R. A. Heising, Procd. Inst, 

 Radio Engrs., Aug., 1921. 



