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BELL SYSTEM TECHNICAL JOURNAL 



The built-out band pass filter shown in Fig. 7b has a very satisfactory 

 impedance over the voice range and the modifications introduced by 

 the transformer do not seriously affect its efificiency. From the curves 

 of Fig. 7c it is evident that the built-out type of section must be used 

 for channels near the voice frequency range but that the confluent type 

 shown in Fig. 7a may be used for the higher frequency channels. 



The close relation between input filter impedance and modulating 

 gain is illustrated in Fig. 8. Two band pass filters were built having 



24 

 22 



20 

 IS 



- 16 



z 



< 14 



1.0 1.5 2.0 2.5 



FREQUENCY IN KILOCYCLES 



3.0 



Fig. 8 



impedance characteristics approximating the curves shown in Figs. 7a 

 and 7b. It would then be expected that the modulating gain would be 

 proportional to the ratio Z_/(i?o + ZJ). The curves in Fig. 8 show 

 that this is very nearly the case. With no input filter the ratio 

 Z_/(i?o + Z-) would be 0.5 or 6 T.U. less than the maximum possible 

 gain, as is found to be actually the case. This shows that the modu- 

 lating gain may be calculated for any value of input impedance if it is 

 known for any other value. 



Input impedance 

 The impedance looking into the low side of the input transformer 

 when the high side is terminated in the grid circuit of a modulator 

 under operating conditions (carrier at normal value) depends on a 

 number of factors, among which the principal variables are the signal 

 and carrier input currents, the input transformer, and the input 

 generator impedance. As might be expected, the input impedance 

 decreases as either carrier or signal amplitude is increased, and the 

 change of impedance with signal amplitude is small when the signal is 

 small compared to the carrier, as is normally the case. 



