COPPER OXIDE MODULATORS IN CARRIER SYSTEMS 317 



be used as a demodulator, the modulator becomes complicated by 

 the effects of reflections back and forth into the signal bands of 

 numerous frequency bands of modulation products. 



Circuit Arrangement 



The circuit arrangements used in copper oxide modulators generally 

 are concerned either with carrier suppression, with carrier transmission 

 along with the signal, or with balancing action to suppress certain un- 

 wanted bands of signal frequencies. In most carrier telephone systems 

 economy of frequency space and amplifier load capacity demands the 

 use of single-sideband, carrier-suppressed transmission. 



In Figs. 2(a), 2(b), and 2(c) three types of copper oxide modulators 

 are shown, each arranged to suppress the carrier in both the signal input 

 and the signal output circuits. In Figs. 2(d) and 2(e) the carrier is 

 balanced out in only one signal branch. In the usual arrangements 

 a signal band selective filter must be used in each signal branch to 

 restrict transmission to that of the wanted frequency band. Largely 

 in this way interferences are guarded against, not only into other 

 channels or systems to which the modulator output circuit is connected 

 on the line or at the distant end, but also back into the complex array 

 of facilities to which the input circuit may be connected. 



In any of the circuits shown, modulation results from either the 

 reduction or reversal of the current flow between the input and output 

 signal circuits at periodic intervals as the carrier varies the copper 

 oxide resistance back and forth between high and low values. In 

 Fig. 2(a) where the connections of the input and output signal circuits 

 are periodically short-circuited by the carrier-actuated copper oxide, 

 transmission of the modulated signal into the input circuit or the 

 unmodulated signal into the output circuit is prevented by filters, each 

 of high impedance at the frequency of the other signal. In Fig. 2(b) the 

 connections between the signal and modulated signal circuits are open- 

 circuited periodically by the carrier. In this case each filter should 

 have a low impedance at the other signal frequency. In Figs. 2(c), 2(d) 

 and 2(e) the copper oxide rectifiers are made to become alternately low 

 and high resistance in pairs as the polarity of the carrier is either in 

 the same direction as the arrows or in the opposite direction. As a 

 result, current flow from the input signal circuit into the output is 

 periodically reversed by provision of a periodically reversing low im- 

 pedance path. In effect each signal is balanced from the other's 

 circuit. 



Although an indefinite number of other circuit configurations can 

 be used, no novel transmission feature would be found which was not 



