116 



BELL SYSTEM TECHNICAL JOURNAL 



In order to test directly the conclusions regarding relative phase of 

 grid and plate modulation products, a circuit was set up which per- 

 mitted two frequencies to be supplied to the grid of a vacuum tube, the 

 resultant currents of sideband frequency being measured in grid and 

 plate circuits by means of a current analyzer.^ As shown in Fig. 2 the 



600* 



/ leoo 



TO CURRENT 

 ANALYSER 



Fig. 2. Test Circuit 



grid circuit contains a high external resistance (for producing grid 

 current modulation when conductive grid current flows as previously 

 explained) in series with a " C " battery to vary the relative amounts of 

 grid and of plate modulation. The relative phase of sideband currents 

 produced in grid and plate circuits was calculated from the currents 

 measured separately in jacks No. 1 and No. 3 and their vector sum in 

 jack No. 2. These measurements verified the conclusions drawn 

 from eq. 13, — that with resistances in grid and plate circuits second 

 order modulation products produced in the grid circuit are exactly out 

 of phase with the same frequencies produced in the plate circuit. 



The effect of the grid circuit resistance when conductive current 

 flows is of course to limit the positive potentials applied to the grid and 

 so, in efifect, to cause the input-voltage — output-current relation of the 

 circuit to be deflected at the upper end more nearly to parallelism with 

 the X-axis than it is for the tube alone. We may therefore consider 

 grid modulation as equivalent to the introduction of a reversed 

 curvature in the operating characteristic. To substantiate this point a 

 tungsten filament tube was used in which the curvature of the lower 

 branch is nearly the same as that of the upper branch, as shown in Fig. 



^ "Analyzer for Complex Electric Waves," by A. G. Landeen, Bell System Technical 

 Journal, April, 1927. 



