Phase relations in unbalanced two-way telephone repeaters

Owing to the wide use of repeaters in modern high-grade long-distance telephone circuits and the importance of obtaining a high repeater gain without singing, the authors of the paper investigate fundamentally the conditions obtaining in a 22-type repeater in the singing state. The differences between the values for the singing points obtained by estimation by different methods are discussed and an endeavour made to find a means whereby the practical value of the singing point may be derived theoretically from knowledge of the lines, networks and repeater units. In order that a "feed-back" circuit such as a 22-repeater may generate oscillations, it is not only necessary that there should be sufficient gain in the singing circuit, but that the phase rotation in the complete circuit must be multiples of 2pi radians.