452 BELL SYSTEM TECHNICAL JOURNAL 



fundamental input and harmonic output may vary widely with small 

 changes in supply potentials and circuit parameters. This troublesome 

 source of variation may be avoided in a number of different ways, of 

 which the simplest is to increase the resistance of the resonant mesh. 

 In the present case this is effectively accomplished without sacrificing 

 efficiency by using pentodes, which have high internal resistances, in 

 the amplifier stage connected to the resonant mesh. 



The efficiency of power conversion from fundamental to harmonics 

 may be found from the fundamental power input to the circuit, as 

 derived from measurements on a cathode ray oscillograph, and from the 

 total harmonic output measured by means of a thermocouple. The 

 maximum efficiency obtainable with the low-power circuits described in 

 the second section is in the neighborhood of 75 per cent, and decreases 

 with increasing fundamental frequency because of the increased dissi- 

 pation due to eddy currents. It should be noted that this figure does 

 not include losses in the primary inductance Li. When only a few 

 harmonics are used, the efficiency of obtaining this useful power 

 naturally drops to a much lower value, which for the particular 

 cases mentioned in the second section, is between 15 and 25 per cent. 



VI. Effect of Extraneous Components 



In any practical case the fundamental input to the harmonic pro- 

 ducer is accompanied by extraneous components introduced by cross- 

 talk, by modulation, or by an impure source. Thus if the fundamental 

 is derived as a harmonic of a base frequency, small amounts of adjacent 

 harmonics will be present. Or if the amplifiers are a.-c. operated, side- 

 frequencies are produced differing from the fundamental by 60 cycles 

 and its multiples. Extraneous components of this sort in the input 

 modulate the fundamental and produce side-frequencies about the 

 harmonics in the output. When the harmonics are used as carriers, 

 the accompanying products must be reduced to a definite level below 

 the fundamental if the quality of the transmitted signal is to be un- 

 impaired. The requirements imposed by this condition can be calcu- 

 lated by simple analysis, the results of which agree rather well with 

 experimental values. 



The method of analysis used is to consider the extraneous component 

 at any instant as introducing a bias ^^ to the non-linear coil. The 

 primary effect of a small bias (h) is to shift the phase of the discharge 

 pulse by =F bIHi radians, //] being the amplitude of the fundamental 

 magnetizing force. The sign of the shift alternates so that intervals 

 between pulses are alternately narrowed and widened. 



