THERMIONIC VACUUM TUBES 



73 



of the low frequency wave; i.e., although the modulation is effected 

 by the curvature of the tube characteristic, the modulated output is 

 free from distortion. Furthermore, the modulated output voltage 



is proportional to y— ■ — rr which is a maximum when r = \ r p ; and the 

 modulated output energy is proportional to 



which is a maxi- 



{r + r P y 

 mum when r = \ r p . 



Another type of circuit in which the modulation is dependent upon 

 the curvature of the E g ,I P characteristic is shown in Fig. 40. The 

 two tubes are supposed to be alike; so long as no low frequency is im- 

 pressed on the grids the high frequency space currents are equal, 



Fig. 40 



each passing through one of the primary coils of the transformer T , 

 the order of winding these coils being such that this condition gives 

 zero current in the secondary. However, the presence of a low fre- 

 quency voltage (L.F., Fig. 40) raises one grid potential at the same 

 time that it lowers the other, with the result that the high frequency 

 currents in the two primary coils are no longer equal, and a high 

 frequency current therefore flows in the secondary of T , the amplitude 

 of which is determined by the degree to which the two tubes are 

 unbalanced by the low frequency input. It is apparent that the out- 

 put of this modulator circuit contains only the two side bands and 

 none (or very little, if the tubes are not exactly identical) of the carrier 

 frequency and therefore corresponds to curve E in Fig. 37. It is par- 

 ticularly useful in communication circuits where several telephone or 

 telegraph channels are desired on the same pair of wires. Since only 

 the side bands are transmitted the total current which must be handled 

 by repeaters and other line apparatus is materially reduced. By the 

 use of the proper wave-filter it is also possible to suppress one side 

 band, thereby approximately cutting to one half, the width of the 

 frequency band to be transmitted. As will be pointed out under 

 homodyne detection, the suppressed carrier frequency must be sup- 

 plied locally before detection can occur. 



