136 BELL SYSTEM TECHNICAL JOURNAL 



elements, while the lowest curve illustrates the performance of a 

 representative conjugate input plate modulator working under con- 

 ditions prescribed for it into a 600 ohm circuit with the same tubes and 

 plate potential. A direct comparison between the two types of 

 modulator as to sideband current output should include a comparative 

 increase of 0.6 T.U. to the grid current modulator output to take care 

 of the difference in the two load impedances. 



The curve labelled "no filters" applies to the circuit of Fig. 9 in 

 which both input and output circuits were connected to 675 ohm 

 resistances. The presence of the retard coil in the plate circuit is 

 accountable for the increase in output at large signal inputs over that 

 of the plate type. When an output filter is added (resistance input, 

 output filter) the gain at low inputs is scarcely affected but the out- 

 put power for large signals is doubled since the load capacity is in- 

 creased by the suppression of the signal frequency current in the plate 

 circuit. If now an input filter is inserted and the output connected to a 

 675 ohm circuit (input filter, resistance output) the gain at low signal 

 currents is increased by about 5 T.U. over that with no filters in circuit, 

 while the increase at high signal amplitudes is of the order of 1.5 T.U. 

 The topmost curve represents the performance of the modulator 

 circuit terminated in the two filters, which shows a modulating gain of 

 21.5 T.U. at small inputs and a maximum power output of 30 mils into 

 675 ohms (0.6 watt). Fig. 11 represents the effect of varying the 

 carrier input at two signal inputs — 0.5 and 6 mils respectively. This 

 illustrates the lack of dependence of sideband on carrier when the 

 carrier is greater than the signal, which was deduced from eq. 20 as 

 characteristic of this type of modulator. The use of a 15 mil carrier is 

 seen to furnish close to the optimum value for the circuit, at least when 

 the signal amplitude does not greatly exceed 6 mils. 



The common input type is capable of yielding much the same 

 results as the conjugate input type with somewhat less care required 

 for the flanking filter impedances, since the proper circuit impedances 

 are obtained by the use of retard coils as shown in Fig. 9h. On 

 theoretical grounds, however, as we mentioned in discussing the general 

 properties of balanced circuits, it is not capable of furnishing as high 

 quality as the conjugate type at the low sideband frequency used here. 

 At high sideband frequencies this objection disappears, so that the 

 reduced filter requirements make it perhaps more attractive in appli- 

 cation than the conjugate type. It should be noted that the plate 

 modulator may be made to have a greater gain than that shown in Fig. 

 10 by changing the input transformer (with the same maximum output 

 level) but this restricts the signal input current to correspondingly 

 smaller amj)litudes. 



