Repeating coil
In
telecommunication, a
repeating coil is a voice-frequency
transformer characterized by a closed
core, a pair of identical
balanced primary (
line) windings, a pair of identical but not necessarily balanced secondary (drop) windings, and low transmission loss at
voice frequencies. It permits
transfer of voice currents from one winding to another by
magnetic induction, matches line and
drop impedances, and prevents direct
conduction between the line and the drop.
It is a special application of an isolation transformer, and is often used to prevent ground loops or earth loops, which cause humming or buzzing in audio circuits. It also prevents low direct current voltages from passing.
Source: from Federal Standard 1037C and from MIL-STD-188