GRID CURRENT MODULATION 109 



approximately three times. From these figures it is evident that the 

 space current or plate power supplied the grid modulator is sixty per 

 cent greater than it is for the plate modulator, and that this greater 

 power is utilized five times more efficiently. Under these conditions 

 more carrier input power is needed for the grid than for the plate 

 modulator. Where the carrier oscillator employs a tube of the same 

 type as that used in the modulators, sufficient carrier power is, how- 

 ever, available. This improved performance has been made possible 

 by a detailed study of the fundamental processes involved, and by a 

 design of the tubes and associated equipment, such as transformers 

 and filters, to permit these fundamental processes to operate to best 

 advantage. 



In view then of the close interdependence of the circuit elements, 

 we shall start with a discussion of the theory as developed for the 

 simplest circuits, and accompany it by approximate mathematical 

 analyses wherever it appears profitable. No rigorous mathematical 

 treatment appears to be possible or even desirable because of its 

 complexity in the general case, and the sole purpose of our approximate 

 analyses is to help in building up a physical picture of the operation of 

 modulators. With the theoretical conclusions in mind, the character- 

 istics of tubes, transformers, retard coils, balanced circuits, and filter 

 networks which are important in this connection are examined, and the 

 performance of the complete carrier telephone modulator circuit is 

 covered in some detail. The theoretical conclusions are not limited to 

 the carrier telephone modulator which has been used simply for 

 illustrative purposes ; as a matter of fact the same principles have been 

 found operative in difi'erent types of circuits over a wide frequency 

 range. It should be noted that we have not attempted to combine the 

 oscillating and modulating functions in a single circuit as is sometimes 

 done, but have maintained these circuits distinct from one another, so 

 that the best performance of each may be realized. 



Theory of Vacuum Tube Modulation 



In a broad sense the same general phenomena are involved in both 

 grid and plate modulation, since modulation is produced when an 

 impedance is varied in accordance with the amplitudes of the modu- 

 lating potentials, a condition true of both circuits under appropriate 

 conditions. Thus conductive grid current is suppressed at negative 

 potentials in the high vacuum tubes which we employ, and it flows 

 when the grid potential is positive, the grid impedance depending upon 

 the amplitude of the grid potential. 



We can obtain a qualitative idea of the situation when we consider 



