GRID CURRENT MODULATION 107 



type we might mention the coupled-plate or Heising modulator which 

 has found extensive application in radio transmitters, in which the plate 

 circuit of an oscillating tube is coupled to the plate of another tube 

 through which the signal is introduced, modulation taking place 

 ordinarily in the plate circuit of the oscillating tube. A carrier 

 frequency amplifier is sometimes used in place of the oscillator. 

 Another type of plate modulator, due in principle to van der Bijl, 

 which has found extensive application in carrier telephone systems, 

 applies the signal and carrier to the grid of the modulator, so that the 

 two components are amplified in common before being modulated. 

 As examples of the grid modulator there are the grid leak and con- 

 denser type used almost universally for radio reception, together with 

 the type which forms the subject of the present paper, employing a 

 generalized impedance in the grid circuit. The three last-meationed 

 modulators may incidentally produce modulation in both plate and 

 grid circuits. This ordinarily acts to reduce the overall efficiency as 

 well as to introduce other undesirable features of operation, so that in 

 the design of the grid current modulator we have been led to minimize 

 modulation in the plate circuit, operating the grid circuit as a modu- 

 lator and the plate circuit purely as an amplifier. 



The criteria of usefulness of modulators include some usually placed 

 upon vacuum tube apparatus in general, together with those peculiar 

 to frequency change; some of most importance are modulating gain 

 and level, plate power efficiency, quality, stability, input and output 

 impedances, and carrier suppression. These will be taken as a basis 

 for discussing the operation of the grid current modulator and com- 

 paring it with that of the other types mentioned above. The modu- 

 lating gain usually expressed in transmission units (T. U.) represents 

 the ratio of the power output of a single sideband to the power input 

 of the signal which produces it. It is a function of both carrier and 

 signal amplitudes, usually decreasing at high amplitudes. This de- 

 crease in gain should not be too rapid or the modulated output power 

 at sufficiently large signal amplitudes may actually decrease as the 

 signal is increased, and lead to prohibitive distortion. Another aspect 

 of the question relates to the maximum modulated power attainable. 

 The signal amplitude fluctuates within wide limits in course of oper- 

 ation and it becomes desirable to limit its effects, so that the resultant 

 modulated potentials may not disturb the operation of associated 

 equipment. This may be accomplished in modulators in which the 

 sideband output approaches an asymptotic maximum as the signal is 

 increased, better than in those which pass through a maximum in the 

 operating range. A knowledge of the signal amplitude and the 



