COPPER OXIDE MODULATORS IN CARRIER SYSTEMS 323 



voltages the impedance is a resistance in parallel with a condenser. 

 This resistance decreases rapidly with increasing frequency while the 

 shunt capacity decreases only a moderate amount. At moderate 

 positive voltages (about 1/2 volt) the impedance becomes resistive and 

 does not change appreciably with frequency. Experimentally it is 

 found that the signal impedances in resistance terminated modulators 

 can be made largely free of reactance at high frequencies by using 

 either large carrier amplitudes, inductive tuning of the copper oxide 

 capacities, or lower impedance connected signal circuits to accentuate 

 the importance of the low-resistance part of the copper oxide character- 

 istic. Very much lower circuit impedances must be used at the 

 higher frequencies. Where 600 to 1000 ohms is a satisfactory im- 

 pedance at speech frequencies, less than 50 ohms may be the best 

 impedance to use, at three megacycles. 



Impedance measurements on a double-balanced modulator designed 

 to translate a twelve-channel group of frequencies for cable carrier 

 systems from a band at 60 to 108 kilocycles to a 12- to 60-kilocycle 

 band are shown in Fig. 5 for several resistance terminations. Absence 

 of any impedance irregularities with frequency is apparent. Also, the 

 tendency is shown for the modulator impedance to become less 

 reactive with lower resistance terminations. 



Inasmuch as copper oxide discs are available in sizes from 1/16 inch 

 to more than an inch in diameter, a wide range of circuit impedances 

 are possible varying from only a few ohms to thousands of ohms. 

 Large area discs roughly are equivalent to small area discs in parallel. 

 Thus by usiftg a disc of n times the area of a small one or n of the 

 small ones in parallel, the best circuit impedance becomes 1/wth at 

 the same carrier voltage. Either discs in series or ones of smaller 

 diameter enable the impedance to be raised in a corresponding manner. 

 The lower impedance and greater energy dissipations of larger discs, or 

 paralleled smaller ones at the same carrier voltage, obviously allow 

 greater input signal energies before the signal impedance and loss begin 

 to vary with the signal, and overload distortion appears. Similarly 

 series stacks of discs, or series-parallel combinations, offer wide choice 

 in both the signal levels that can be satisfactorily modulated and in the 

 impedance levels. Usually r.m.s. carrier voltages across individual 

 discs in the conducting direction will best be made somewhere between 

 3/10 and 3/4 volts. 



The impedances of the connected circuits at the modulation product 

 frequencies react back on the signal impedances in a way similar to 

 the way that the two terminating impedances of a four-terminal linear 

 network react on each other. In the case of the copper oxide modu- 



