130 BELL SYSTEM TECHNICAL JOURNAL 



ficiently sharp cutoff. In most cases a maximum variation of 2 T.U. in 

 the attenuation over the transmitted band is a reasonable figure for a 

 band pass filter. Each transformer and filter tends to increase the 

 attenuation at the edges of the transmitted band more than in the 

 center so that frequencies from 800 to 2,000 cycles are always trans- 

 mitted with minimum attenuation, which is independent of the fre- 

 quency-output current characteristic of the modulating elements. 



The above consideration of filter attenuation is substantially 

 independent of filter impedance since the latter is determined mostly 

 by the end section. From the previous consideration it is evident that 

 either may have a very pronounced effect, so that in measuring the 

 frequency characteristic of a modulator or demodulator both attenu- 

 ation and impedance effects must be taken into account. The effects 

 of input and output impedances can be partially separated when the 

 carrier is suppressed because of the fact that the output impedance has 

 but little effect at small inputs and the input impedance has but little 

 effect at large input currents. 



Balanced Tube Circuits 

 The present practice in carrier telephone systems is to suppress the 

 carrier current and one sideband in order to conserve frequency space 

 and to reduce the energy levels and the cross-talk in associated 

 equipment. The elimination of undesired components of a wave may 

 be carried out by two distinct processes, — frequency discrimination by 

 filter networks, and phase discrimination or balance by bridge circuits.'" 

 Each method is useful and both find places in carrier systems. When 

 the frequency separation between desired and undesired components 

 becomes relatively small, frequency discrimination becomes impractical 

 and expensive. The balance method is used to separate frequencies 

 according to their respective phase relations in two or more similar 

 modulating circuits, the phases of the output components depending on 

 the relative phases of the input currents. Consequently only certain 

 combinations of modulation products can be separated by balance and 

 these only to an extent determined by the balance attainable in 

 transformers and vacuum tubes, both of which are subject to manu- 

 facturing variations. Due to the proximity of the carrier and second 

 order sideband frequencies the suppression of carrier current by filter 

 circuits alone is impractical. Balanced circuits must be used for this 

 purpose and in spite of unavoidable variations in tubes and circuits it is 

 usually possible to reduce the carrier on the line to less than five per cent 



'" For an illustration of balanced circuits, reference may be made to U. S. Patent 

 1,343,306, issued to J. R. Carson. 



