Loading-coil meaning

A coil placed in an electric circuit, as of a telephone cable, to increase its inductance.
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A toroidal (i.e., ring shaped or donut-shaped) device comprising a powdered iron core, or sometimes a soft iron wire core, around which copper wire is wound. A loading coil is spliced into an unshielded twisted pair (UTP) copper local loop, where it functions as a lumped inductor, which is to say that at a specific point in the circuit the process of inductance takes place, to compensate for the distributed capacitance between the two parallel wires. In effect, the loading coil tunes the copper circuit, optimizing it for mid-voiceband performance. The loading coil also functions as a low-pass filter, increasing loss above the voiceband cutoff frequency of 4 kHz, while reducing mid-voiceband attenuation by as much as 80 percent. Loading coils are passive, i.e., not electrically powered, devices commonly placed on local loops that exceed approximately 18,000 feet (5.5 km) in length. The first loading coil is placed approximately 3,000 feet (.9 km) from the central office (CO) and at intervals of 6,000 feet (1.8 km) or so, thereafter.The presence of loading coils renders local loops unusable for ADSL, ISDN,T-carrier and other loops operating at high data rates, as they filter out the high frequencies associated with those higher data rates. Where such services are to be deployed, the local loops must be properly conditioned, which entails removing the loading coils, bridged taps, and other impediments. The presence of a loading coil also has the effects of increasing the impedance of the circuit, which significantly reduces the velocity of propagation (Vp), i.e., speed of signal propagation, to 10,000
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