Rectifiers and inductive co-ordination

NOISE induced from power-supply circuits into circuits used for transmission of speech or music is caused by the harmonic-frequency voltages and currents present in the power-system wave shape.¹ The interfering effect of a foreign voltage on a telephone circuit depends on its frequency, increasing with frequency up to a certain value and then decreasing. The voltage induced in a telephone circuit by a given voltage or current on a power-supply circuit also depends on the frequency, the mutual relation or coupling being directly proportional to frequency. When the effect of harmonics present on the power circuits is considered, it is necessary to give due weight to both factors. The term “inductive influence” is used in discussing them. The inductive influence of a power-system voltage or current wave shape is measured in terms of “telephone influence factor,”² abbreviated “TIF”, which factor takes into account the variation of both the interfering effect and coupling with frequency. Figure 1 shows two TIF curves, one designated 1935 and the other tentative 1941.² The shape of the curve depends, of necessity, on the characteristics of the telephone instruments in use, and the change from the 1935 to the 1941 curve reflects the improvements that have been made in telephone instruments. The 1941 curve is just coming into use now, and the majority of the investigations made in recent years have been based on the 1935 curve.