122 BELL SYSTEM TECHNICAL JOURNAL 



detector is used, the voice output is proportional to the product of the 

 received carrier and sideband. Any change in attenuation, expressed 

 in transmission units (T. U.), between the transmitting and receiving 

 station affects the output current by twice that number of T. V. In 

 the above grid current demodulator, however, the output changes to 

 the extent of the attenuation change, and varies no more than in a 

 carrier suppression system with the carrier locally supplied. 



Having determined the grid voltage components, we may now apply 

 the plate circuit coefficients to the grid potential in order to determine 

 the plate current components, just as we did in the previous case. 

 There, it will be recalled, we used a simple representation for the plate 

 current in terms of the grid potential from which amplification and 

 modulation terms were deduced. The same general considerations 

 regarding phase opposition are carried over unchanged. 



Limitation of Sideband Output 



The above method of treatment is quite satisfactory when the space 

 current is never reduced to zero, but when the grid voltage goes 

 sufficiently negative, precisely the same limitations apply to the plate 

 characteristic equation as applied to the grid equation under similar 

 circumstances, and there exists an additional source of distortion in the 

 plate circuit. In this circumstance the method of expansion in Bessel 

 coefficients cannot readily be used because of the large number of 

 components in the wave subjected to additional distortion, which 

 would lead to prohibitive complication. We may nevertheless obtain a 

 qualitative idea of the result in special cases of interest to us in this 

 connection. 



We shall assume that, as we found previously to be advisable, one of 

 the two fundamental currents is substantially suppressed in the plate 

 circuit, so that despite the non-linearity of the plate circuit sideband 

 components are produced only in the grid circuit. We are therefore 

 concerned with the variation of amplification with operating para- 

 meters. Now it is clear to start with, that at sufficiently small grid 

 potentials, the entire variation falls within the region of variation of 

 the plate dynamic characteristic so that the result may be written 

 down as in the previous analysis. As the amplitude is increased, the 

 negative end of the grid swing finally has no effect in varying the space 

 current, and the distortion which results tends to limit the magnitude 

 of the amplified compf)nents. Hence as the sideband potential on the 

 grid is increased by increasing the applied signal potential and keeping 

 the carrier potential large enough (say one and one half times the 

 signal) so as to get the full efficiency of grid current modulation, the 



