Grid Current Modulation 



By EUGENE PETERSON and CLYDE R. KEITH 



Synopsis: The term grid current modulator is used to describe those 

 vacuum tube circuits in which modulation is initially produced in the grid 

 circuit of a three-electrode vacuum tube due to the non-linear grid current- 

 grid voltage relation. Comparison with a representative plate current 

 modulator using the same tubes and the same plate potential shows that by 

 modulating at maximum efficiency in the grid circuit and using the plate 

 circuit solely for amplification, the maximum power output is increased 

 about eight times, the power efficiency is increased about five times and the 

 ratio of sideband output to signal input is increased approximately three 

 times. Under these conditions more carrier input power is needed for the 

 grid than for the plate modulator. 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 their best ad- 

 vantage. 



Normally modulation is also produced in the plate circuit which is 

 shown to be out of phase with that produced in the grid circuit. By in- 

 serting high impedances to the input frequencies in the plate circuit, plate 

 circuit modulation is prevented, and the reduction of grid circuit sideband 

 is likewise avoided. By including in the grid circuit an impedance which is 

 high to the desired sideband frequencies, the maximum grid sideband 

 voltage is obtained. In this way the power and modulating efficiencies of 

 the tube circuit are made maximum. 



Where modulation occurs only in the plate circuit of a tube, the sideband 

 amplitude is proportional to the product of the amplitudes of the input 

 frequencies when these amplitudes are small. In the present type of grid 

 current modulator the sideband amplitude is proportional to the smaller of 

 the two input amplitudes provided the ratio between these is greater 

 than about 3/2. This feature makes the modulator particularly valuable in 

 communication systems. 



The article concludes with an application of the fundamental principles 

 involved to an experimental carrier telephone system in which the operating 

 features of tubes, filters, and transformers are discussed. 



Introduction 



BECAUSE of the extensive application of vacuum tube modulators 

 in systems of carrier communication, they constitute an important 

 tool in the hands of telephone engineers. As such they have justified 

 extensive laboratory investigation. The purpose of this paper is to 

 discuss some of the properties of a type of modulator utilizing the 

 non-linear relation existing between grid voltage and grid current 

 and the advantages which recent laboratory investigations indicate 

 that it may possess. Further studies are in progress to determine 

 the conditions under which it can be employed practically. 



There are two distinct classes of vacuum tube modulators which may 

 be designated for convenience as grid and plate types, according to the 

 circuit in which the modulation is initially produced, although some 

 modulators may involve both circuits. As an example of the plate 



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