GRID CURRENT MODULATION 



117 



3. The plate circuit sideband was measured as a function of the "C" 

 battery with zero grid resistance so that the plate circuit was re- 

 sponsible for the total sideband production ; the data are plotted on the 



36 34 32 30 2S 26 24 22 20 



16 16 



-Ec 



14 12 10 6 



Fig. 3 



same Figure. It is seen that the sideband drops nearly to zero when 

 the "C" battery is adjusted so that the input voltage swings sym- 

 metrically over the upper and lower branches of the curve. These 

 results could very well be attributed to out-of-phase modulation 

 resulting from reversed curvature. As a matter of fact the algebraic 

 expression for the characteristic involves in general a series of both odd 

 and even powers of applied voltage, but if the axis is taken at the point 

 of symmetry of the characteristic the even powers drop out. Now 

 since the even orders of modulation can be attributed only to the even 

 powers of the static equation, it might be expected that these com- 

 ponents would drop to zero. 



We may conclude from this discussion that best results will be had in 

 the practical design of grid current modulators, when the external grid 

 impedance at the sideband frequency is made as high as possible and 

 when the impedance to the fundamentals is matched. As to the plate 

 impedances, the situation is the reverse of that existing in the grid 

 circuit since we must have the impedances to the two modulating 



