GRID CURRENT MODULATION 133 



Common Input Grid Modulator 



Another useful type of grid current modulator is shown in Fig. 9&, in 

 which both signal and carrier are applied across the same input 

 terminals. The modulation currents flow in the plate (and corre- 

 sponding grid) circuits as shown in the above schematic. Where the 

 ratio of carrier to signal frequency is large so that a single input 

 transformer cannot be used efficiently, separate transformers with 

 associated filter networks may be used for each of the two inputs. 

 Since the second order sidebands {p ± q) appear in the midbranches, 

 it is not necessary to have the impedance high to these frequencies in 

 the input coil, but only from the midpoint of the input coil to ground. 

 This is most conveniently accomplished by a high inductance retard 

 coil in the midbranch of the grid circuit, although transformers and 

 high impedance networks may be used in general. The grid circuit 

 sideband across the midbranch is amplified and appears in the plate 

 circuit midbranch. The fundamental currents together with all odd 

 order modulation products are eliminated by a high impedance, high 

 mutual retard coil in the series arm of the plate circuit. 



Since the present practice is to use suppressed carrier, a hybrid ^^ 

 coil must be used to introduce the carrier if this circuit is to be used as a 

 demodulator, although the signal and carrier currents may be intro- 

 duced through filters when used as a modulator. Either frequency 

 discrimination or balance is required in any case to keep carrier current 

 out of the signal circuit. 



The chief advantage of the common over the conjugate input type of 

 circuit is that the high impedance required for the modulated product 

 is provided by a distinct element, and no high impedance requirements 

 are placed on other elements in either input or output circuits. Another 

 advantage of this arrangement is that the amplified fundamentals are 

 balanced out, making the singing gain about 20 T.U. less than that of 

 the conjugate input type. The only modulation products (up to the 

 fourth order) not balanced out of the output are the second harmonics 

 of carrier and signal. This type of circuit may be used as a demodu- 

 lator at any frequency, but as a modulator only when the second 

 harmonic of the highest voice frequency does not come in the sideband 

 range — it is therefore not well adapted to modulate low carrier 

 frequencies where high quality is required. 



Although the output of this modulator is affected but little by the 

 filter impedance in either input or output circuits, some care is neces- 



" By using a hybrid coil having eight times as many turns in the signal circuit as 

 in the carrier circuit, the equivalent current losses to signal and carrier are 0.5 T.U. 

 and 9.5 T.U. respectively instead of 3 T.U. each, as is the case for the usual equality 

 ratio hybrid coil. 



