134 BELL SYSTEM TECHNICAL JOURNAL 



sary in selecting the retard coils for the grid and plate circuits. Since 

 the grid retard should have a high impedance to the desired modulation 

 frequencies it must have an inductance of the order of 50 henries or 

 greater at low frequencies in a demodulator. Resonance in the voice 

 band is not harmful so long as the impedance does not drop too much 

 at high voice frequencies. 



The plate circuit retard coil is well balanced to reduce the unbalanced 

 carrier transmitted to the line. An important requirement is that of 

 close coupling so that the reactance in the output circuit may not be 

 great enough to cause a transmission loss or large reflection coefficient. 

 The required inductance then depends upon the relative separation of 

 voice and sideband frequencies. If the lowest sideband frequency is 

 very close to the highest voice frequency it may be impossible to 

 prevent positive reactance from coming into the voice circuit of a 

 demodulator, but the effect may be considerably reduced by utilizing 

 this positive reactance in the mid-series section of the adjacent low 

 pass filter. 



Double Balanced Circuits 



If two balanced circuits of either of the above types are connected 

 with their input and output terminals respectively in series, all the 

 modulation products up to the fourth order except the second order 

 sidebands may be balanced out. There is no hybrid or filter loss and 

 due to more complete suppression of unwanted frequencies the 

 maximum output power obtainable is more than twice that with a 

 single balanced circuit. The complexity of the resultant circuit is such 

 as to rule it out for all ordinary applications. 



For purposes of comparison we proceed to consider the experimental 

 results obtained on a conjugate input grid modulator designed in 

 accordance with the ideas set forth above. 



Experimental Results 

 Figs. 10 and 11 represent the results of experiment on a conjugate 

 input grid modulator with a carrier frequency of 6,800 cycles and a 

 signal frequency of 1,000 cycles. The input and output networks 

 previously discussed and represented in Figs. 6 and 7 were used here 

 with 101-D tubes operated at 120 volts plate potential and 1.0 ampere 

 filament current. The grids were connected to the negative terminal 

 of the filaments. Fig. 10 represents the sideband output current in a 

 675 ohm circuit, plotted as a function of the signal current measured 

 in the 675 ohm input circuit, with the carrier input maintained at 15 

 mils throughout. The upper four curves represent various experi- 

 mental conditions designed to bring out the effect of different circuit 



