108 BELL SYSTEM TECHNICAL JOURNAL 



modulating gain corresponding to it is sufficient for a determination of 

 the output amplitude or output level which is of prime importance in 

 matters relating to noise and interference. Other significant factors 

 from the standpoint of noise and interference are the closeness with 

 which line and connecting apparatus impedances are matched since 

 this determines the amount of reflection of an incident wave,' and the 

 extent to which carrier current is transmitted to the line (carrier leak) 

 in carrier suppression systems. 



It is highly desirable of course to have the efficiency of energy 

 conversion from the plate battery to the sideband output power as 

 high as possible, since the amount of power supplied to the plate 

 circuit is thereby minimized, and the necessary power capacity of the 

 plate supply is reduced. Another kind of plate efficiency in which we 

 are sometimes interested is the efficiency of energy transfer from the 

 plate supply to the external plate impedance. This tells us the 

 amount of energy dissipated by the plate of the tube and fixes its 

 structure; it differs from the first efficiency only when other current 

 components than the sideband flow in the plate circuit. Inasmuch as 

 we shall deal in the following with low power tubes which have ample 

 load capacity, only the first-mentioned efficiency is important. 



We are also interested ordinarily in the quality of the system. This 

 is determined in large part by the width of the transmitted frequency 

 band, by the presence of new interfering frequencies introduced by the 

 process of modulation, and by the linearity of the modulated output in 

 terms of the signal input. The first and last conditions are equivalent 

 to the requirements that the modulating gain be maintained both over 

 the ordinary range of signal input amplitudes, and over the frequency 

 band essential for good signal reproduction. Finally the system as a 

 whole is required to have a high degree of stability, so that ordinary 

 variations of battery potentials, or even the replacement of a tube by 

 another of the same type, will not impair the operation of the system. 



The specific forms of grid current modulator with which we shall be 

 concerned as an application of the theory are those adapted to carrier 

 current telephony, in which the carrier current is suppressed and a 

 single sideband transmitted. Comparison with a representative plate 

 current modulator using the same tubes at 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 eight times, the power efficiency is increased 

 five times, and the ratio of sideband output to signal input is mcreased 



1 The reflection is measured by the quotient of the difference by the sum of the two 

 connected impedances; this is known as the reflection coclhcicnl. 



