132 BELL SYSTEM TECHNICAL JOURNAL 



angle between current and voltage of each input frequency. This 

 conclusion is independent of the type of modulation employed. If the 

 phase of the product is then calculated to be identical on the two grids 

 or two plates, it appears in the midbranch; if it turns out to be 

 opposite in phase on the two grids referred to the filament, it appears in 

 the series arm. 



Conjugate Input Grid Modulator 



With the signal introduced in the series arm of Fig. 9a, the sideband 

 potential is built up across the same arm, so that a high sideband 

 impedance must be provided by the input transformer terminated in 

 its filter, — a low pass filter for the modulator, and a band pass filter for 

 the demodulator. The carrier frequency is introduced in the conjugate 

 arm so that the carrier circuit does not directly affect the signal and 

 sideband impedances. There is a second order effect, however, due to 

 the reaction of those modulation products which flow in the common 

 branch. The input impedance may be expected to change also when 

 the coupling between the two high impedance windings of the input 

 transformer is varied, since this effectively changes the impedance to 

 the above mentioned modulation products. 



Modulation is largely eliminated in the plate circuit and the load 

 capacity of the tubes is increased by inserting a choke coil in the 

 common plate branch to suppress the carrier current. This, inci- 

 dentally, tends to reduce carrier leak. The impedance of the choke 

 coil at the carrier frequency is modified by the capacity to ground of 

 the output transformer, and must be designed with this point in mind 

 since the shunt arm impedance may otherwise be materially reduced. 

 Some further increase in load capacity is obtained by having the output 

 transformer and the terminating filter offer a high impedance to 

 frequencies outside the transmitted band. In this way all important 

 components except the sideband are suppressed and the plate circuit of 

 the tube operates as an amplifier so that the plate power dissipation is 

 reduced and the load capacity increased. The same considerations 

 regarding the second order impedance effects of the shunt branch on 

 the series branch exist for the plate circuit as for the grid circuit 

 considered above. The main effect when there is loose coupling 

 between the high impedance windings of the output transformer is to 

 introduce an inductive reactance into the series arm. This tends to 

 increase the reflection coefficient so that it becomes preferable to 

 couple the two windings closely, a comparatively easy thing to do in 

 low impedance circuits. The modulation products accompanying the 

 desired product are indicated in Fig. 9, and it is seen that there will be 

 no introduced distortion up to the third order when the carrier fre- 

 quency is sufficient!}' high. 



