440 BELL SYSTEM TECHNICAL JOURNAL 



L-2. is a small permalloy core coil which is operated at high magnetiz- 

 ing forces well into the saturated region. The circuit including L2, C2, 

 and the load impedance, which is practically resistive to the desired 

 harmonics, is so proportioned that highly peaked current pulses rich in 

 harmonics flow through it. Two such pulses, oppositely directed, are 

 produced during each cycle of the fundamental wave, the duration of 

 each being a small fraction of the fundamental period. The typical 

 output wave shown in Fig. 2 was obtained by means of a cathode ray 

 oscillograph, the ordinate representing the current in the load re- 

 sistance, and the abscissa representing the fundamental current into the 

 coil. The desired odd harmonics are selected by filters connected 

 across the input terminals of the copper oxide bridge. The even har- 

 monics are obtained by full-wave rectification in the copper-oxide 

 bridge. They appear at the conjugate points of the bridge, and are 

 connected through an isolating transformer to the appropriate filters. 

 Thus the harmonics are produced in two groups, with the even harmon- 

 ics separated from the odds to a degree depending largely upon the 

 balance of the copper-oxide bridge, as well as upon the amount of 

 second harmonic passed on from the amplifier. In this way the re- 

 quired discrimination properties of any filter against adjacent harmonics 

 are reduced to the extent of the balance. 



A particular application of the circuit described above to the genera- 

 tion of carriers for multi-channel carrier telephone systems uses a 

 fundamental frequency of 4 kc, from which a number of harmonics 

 are developed. Of these the 16th to the 27th are used as carriers. 

 A photograph of an experimental model of this carrier supply system * 

 is shown in Fig. 3. The top panel includes an electromagnetically driven 

 tuning fork serving as the highly selective circuit {F), the amplifier {A), 

 the output stage of which consists of a pair of pentodes in push-pull, and 

 the tuned circuit LqCq. The next panel includes the elements L\C\, Li, 

 C2, B, and T, together with a thermocouple and meter terminating in a 

 cord and plug for test and maintenance purposes. The last two panels 

 include the twelve harmonic filters, with test jacks and potentiometers 

 for close adjustment of the output of each harmonic. 



A few of the more interesting performance features are given in 

 Fig. 4. The harmonic power outputs shown in Fig. 4a represent 

 measurements at the input terminals of the filters. The variation 

 observed is produced by the non-uniform impedances of the filters. 

 When these are corrected, the variations due to the harmonic generator 

 proper are less than ± 0.2 db from the 16th to the 27th harmonic. 

 Outside this region the amplitudes gradually decrease to the extent 



* Developed by J. M. West. 



