NEW SINGLE CHANNEL CARRIER TELEPHONE SYSTEM 177 



each modulation product appears only in one of the four branches of 

 the circuit. For example in the case of the modulator, if a voltage of 

 frequency " F" is applied to the input and a voltage of frequency " C" 

 is applied by the carrier supply circuit, resulting products of modula- 

 tion will appear in the ideal case as shown in Fig. 13. It is obvious 

 that the only unwanted products in the output which cannot be sup- 

 pressed by filters or balance are those which are of the frequency 

 (C ± AV) which for some values of A and V fall in the frequency 

 range of the desired sideband (C -f V) or (C — V). 



These components, however, are normally more than 50 db weaker 

 than the sideband and are not noticeable. Of course, the term A V 

 represents not only odd harmonics of V but odd order intermodulation 



BCtBV 



AC±AV 



A= ANY ODD INTEGER 



B= ANY EVEN INTEGER 



C= FREQUENCY OF APPLIED CARRIER 



V= FREQUENCY OF INPUT 



Fig. 13 — Simplified schematic of copper oxide modulator. 



products such as (2Fi ± V2). The relative amplitudes of the 

 (C ± AV) terms increase with load in a similar manner to that for 

 the distortion products of an amplifier as the overload point is ap- 

 proached, and the effect on articulation is the same. 



For the actual case the modulator balance is not perfect and all 

 products and the original frequencies do appear in all branches of 

 the circuit including the output. However, the balance in most cases 

 is greater than 30 db and the filter requirements are helped to that 

 extent over some portions of the frequency range. This is particularly 

 helpful in connection with suppressing the carrier from the output and 

 input since it lies only about 200 cycles from the pass band and it 

 would be costly to obtain all of the suppression required by means of 

 filters. 



With a single disc in each arm, taking the factory run of discs and 

 making no attempt to select units, the balance for many assemblies 

 would be less than 15 db. By selecting units, this balance could be 

 improved to any desired amount. In the present design, however, to 



