GRID CURRENT MODULATION 



137 



A few words on the shape of the signal-sideband curve of plate 

 modulators of the van der Bijl type may not be inappropriate at this 

 point since the curve depends to some extent upon the incidental grid 

 modulation produced. Thus at large signal amplitudes the grid of 

 the modulator tube is driven positive and grid modulation is produced, 

 which tends to oppose plate modulation. By reversing the conditions 

 which we have employed in the grid current modulator to promote grid 

 modulation, the net plate modulation may be increased and the 

 sideband-signal curve may more nearly show an asymptotic maximum 

 which is so desirable from the overloading standpoint. This condition 

 is evidently secured with a flanking input filter having a low impedance 

 to the sideband, or by having an input coil which, while not seriously 

 affecting the transmission of signal frequencies, offers of itself a low 

 impedance to the grid sideband. Thus in plate modulators the input 

 coil would have a high winding capacity, and in plate demodulators it 

 would have comparatively low mutual inductance between primary 

 and secondary windings. 



As an indication of the quality obtained with the grid current 

 modulating process, comparative listening tests between carrier 

 telephone systems employing plate and grid demodulators, respectively, 

 conducted by R. W. Chesnut, indicate roughly a 10 T.U. greater load 

 carrying capacity for the grid type over a wide range of input ampli- 

 tudes at about the same quality in both cases. The carrier leak may be 

 reduced to one half mil by a not very critical tube selection, which is 

 quite satisfactory in general. 



The last point remaining is the plate power efficiency, which we have 

 defined as the ratio of the sideband power developed in the load 

 resistance to the d.c. power supplied to the plate circuit under operating 

 conditions — it is really the efficiency of power conversion. At maxi- 

 mum output it is three per cent for the standard plate modulator and 

 fifteen per cent for the above grid modulator. The efficiencies 

 obtained at maximum output for a number of different low power 

 tubes used in the grid current modulator may be tabulated as follows : 



For design information and construction of the experimental models. 



