COPPER OXIDE MODULATORS IN CARRIER SYSTEMS 325 



obtained under less practical operating conditions. Experimental loss 

 measurements are shown in Fig. 6 for a double-balanced modulator 

 using single 3/16 inch diameter discs in each bridge arm. This 

 modulator was designed to simultaneously modulate sixty speech 

 channels occupying a 240,000-cycle band width. The modulator loss, 

 like the impedance, depends on the impedance terminations of the 

 modulator at all the modulation product frequencies as well as on 

 the internal losses of the modulator. Short circuit,. open circuit, or 

 reactive terminations at the unwanted frequencies, permit energy 

 losses only through reflections at the signal circuit junctions to the 

 modulator or within the modulator. With proper terminations and 

 loss-free copper oxide, 100 per cent efficiency frequency translations 

 are theoretically possible. In a practical case, a larger carrier ampli- 

 tude results in a smaller percentage of the time in which the rectifier 



740 780 820 860 900 940 980 1020 



FREQUENCY OF LOWER SIDEBAND OUTPUT IN KILOCYCLES PER SECOND 



Fig. 6 — Loss in a double-balanced group modulator for coaxial systems. 



elements have impedances that are comparable to the connected 

 circuits and that are neither blocking nor conducting. Signal energies 

 are lost in this time interval, so a higher efficiency modulator results. 

 The time spent on the intermediate resistance parts of the rectifier 

 characteristic can be further reduced by introducing harmonics into 

 the carrier wave, so that a square type of wave results. The resistance 

 of the rectifier is abruptly switched back and forth between blocking 

 and conducting values in this manner. When the connected circuit 

 impedances at the unwanted frequencies are very high or low, best 

 efficiencies result when transmission between the signal circuits is 

 blocked most of the time. Thus in a circuit like that of Fig, 2.(a) when 

 the filters are high impedance at the unwanted products, highest 

 efficiency results when the copper oxide is a low resistance short circuit 

 for the major portion of the carrier cycle. In Fig. 2(b) an open circuit 

 is desirable most of the time. 



