42 



BELL SYSTEM TECHNICAL JOURNAL 



elements doubled in magnitude. The input series arm is decreased 

 (ro — 2r\ instead of 2ro — 2ri) and the output series arm is increased 

 by an element Ir^. Since 2^2 is generally much smaller than ro there 

 is a net gain in efficiency. 







O 



1+ 



-AV- 

 -'"2 



I i y^^ 



^0-^ + ^2 



Fig. 6 — Equivalent modulator circuit showing the connection between signal 

 and two sideband circuits. All other modulation current components are suppressed 

 by means of high circuit impedances. 



IV. Low Impedance Outside Band 



The foregoing systems involved high impedances, suppressing the 

 flow of current at all but two or three frequencies. Circuits are as 

 readily obtained in the case of low impedance to all but two or three 

 of the modulation products. The physical systems this approximates 

 are the same as those previously discussed except that the terminating 

 filters must present a low impedance to frequencies outside the band. 



All the external potential drops across the modulating element are 

 taken as negligible except components at the signal and one or two 

 sideband frequencies. The analysis, corresponding to that of the 

 previous sections, uses Ohm's law in the form i = Gv. Thus this 

 analysis, and the equivalent circuits, involve the expansion of a 

 conductance instead of a resistance. The equations corresponding 

 to (8) and (13) are equations in V q, V\+ and Fi_. In the single side- 

 band case a resulting equivalent circuit is that of Fig. 7. This is a 

 simple symmetric tt network. From its well known characteristics 

 the optimum terminating conductance and maximum efficiency are 

 immediately available : 



Gi+ = -^ = W^^l?, (14) 



R 



1+ 



L go + Vgo^ - gi^ J 



(15) 



