COPPER OXIDE MODULATORS IN CARRIER SYSTEMS 321 



or disc selection, by artificial balancing with some means such as 

 condenser-resistance potentiometers, or by statistical averaging 

 through use of numbers of discs in each bridge arm. 



In single-channel modulators interferences caused by the signal into 

 its own signal band will occur only in the presence of the signal. In 

 such cases they need be only 20 to 30 db below the signal, except in 

 special cases, as for example, modulators for broad-band program 

 channels. In multi-channel systems interferences may be produced in 

 the silent channels by the active channels. This kind of interference 

 or crosstalk is ordinarily made to be 70 db or more below the wanted 

 signals for commercial telephone service. In such cases overlapping 

 bands of frequencies not improved by level adjustment are avoided 

 by judicious choice of the carrier frequencies. 



Circuit Impedance and Loss 



In all modulators the carrier serves merely as a means for obtaining 

 a simple periodic variation of the impedance presented to the signals. 

 It is not only immaterial to the signals how this variation is obtained, 

 but the signals also are totally unaware of whether electrical, mechan- 

 ical or other means are used, just so long as the signals themselves are 

 unable to affect the time variation of this impedance. In a copper 

 oxide modulator, only by making the carrier amplitude large com- 

 pared to the signal amplitudes across the rectifier elements, can the 

 impedance of the rectifiers be made to vary at carrier rather than signal 

 rates. Too large a signal amplitude not only results in the production 

 of undesired frequencies, but also the impedance and loss character- 

 istics of the modulator vary with the signal amplitude. With small 

 signals the carrier energy is used up in maintaining the copper oxide at 

 prescribed impedance values at each instant of time, and none of the 

 modulation products involving the signal receive more than a negligible 

 amount of energy from the carrier. As a result the output signal 

 energy will always be less than that of the input signal, partly because 

 of iV losses within the copper oxide, and partly from the diversion of 

 the input signal energy into the energies of the many modulation 

 product frequencies. 



The signal impedance of a copper oxide modulator is a combination 

 of a characteristic impedance of the rectifier elements and the im- 

 pedance of the connected circuits at all the modulation product fre- 

 quencies. The characteristic impedance of the rectifier can be viewed 

 crudely as an average of the impedance encountered by a small signal 

 over a cycle of the carrier, treating each instantaneous value of the 

 carrier voltage as a d-c. bias. If the impedance for small super- 



