GRID CURRENT MODULATION . 123 



sideband current in the plate circuit increases linearly with the signal 

 up to a certain point. At this point, which corresponds to the plate 

 current cut-off, the output departs from the linear relation and 

 increases less rapidly. Further increase of the carrier produces no 

 increase in output but a reduction of the output may result because of a 

 greater swing beyond the cut-off point. 



Inasmuch as the modulating potentials together with undesired 

 modulated products form a wave having a net amplitude considerably 

 greater than that of the useful sideband, it is clear that the maximum 

 output amplitude can be increased by suppressing the undesired 

 current components thus avoiding the loading and heating effects 

 produced in large part by these other components. A method of 

 attaining this desired result will be treated in connection with balanced 

 circuits. The loading effect may be partially ameliorated very simply 

 since one of the products of modulation is a d.c component. The 

 presence of series grid resistance means that we have in effect a 

 negative bias applied to the grid which becomes increasingly negative 

 as the input amplitude increases, — just the sort of thing, in other 

 words, to limit sideband production. If, therefore, we use grid 

 reactances instead of grid resistances we can achieve the same degree of 

 modulating efficiency in the two cases at low inputs, and in addition 

 remove effective grid bias, the maximum output power available being 

 increased to a very considerable extent. Of course the insertion of 

 grid reactance changes the details of the conclusions for the grid 

 resistance, but the main features of performance are retained. 



When grid resistance is used to provide a high impedance to the 

 sideband, the operation of the grid leak and condenser detector is 

 approached, in respect to the undesirable increase of bias with increase 

 of input. As a consequence the output power is limited at large 

 inputs, although the gain is fairly high at small input amplitudes. 



Another point affecting the operation of the grid leak and condenser 

 detector is the plate circuit impedance. According to the conclusions 

 of the above theory for grid current modulation, the output power is 

 increased at large input amplitudes by providing an impedance in the 

 plate circuit which is high to both input frequencies and matches the 

 tube impedance at all desired output frequencies. This conclusion has 

 been verified experimentally at carrier frequencies when operating the 

 tube for maximum output, but is contrary to the usual practice in 

 radio circuits, where the plate circuit impedance to the modulating 

 frequencies is ordinarily made low rather than high compared to the 

 tube impedance. The problem is complicated at radio frequencies by 

 regenerative effects not present to the same degree at the compara- 



