Telecommunication system loop-back unit.

摘要

A loop-back unit for connection at the subscriber's end of a line (15) to provide a loop-back line testing facility, the unit comprising a transformer (1) with a first winding (3) and second winding (5) of relatively high inductance, connected in series across the line, and a third winding (7). In normal operation the subscriber's unit (17) is connected across the second winding and the transformer is transparent to signals on the line. To test the line a tone is applied to the line which is detected by a detector (25) connected across the third winding and in response to the detector output a control circuit (23) operates a switching arrangement (21) to disconnect the subscriber's equipment temporarily from the second winding and apply a test tone across the second winding. The unit is capable of satisfactory operation despite faults in the subscriber's equipment.