Loading coil
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In electronics, a loading coil is a coil (inductor) that does not provide coupling to any other circuit, but is inserted in a circuit to increase its inductance. Loading coils are archaically known as Pupin coils after Mihajlo Pupin (especially when used for the Heaviside condition), and process of inserting them is sometimes called pupinization.
Loading coils inserted periodically in series with a pair of wires reduce the attenuation at the higher voice frequencies up to the cutoff frequency of the low-pass filter formed by (a) the inductance of the coils and distributed inductance of the wires, and (b) the distributed capacitance between the wires. Above the cutoff frequency, attenuation increases rapidly.
A common application of loading coils is to improve the voice-frequency amplitude response characteristics of twisted cable pairs. When connected in series with a twisted pair at regular intervals, loading coils, in concert with the distributed resistance and capacitance of the pair, convert the line from an RC network into a transmission line (and, coincidentally, form an audio frequency filter) that improves the high-frequency audio response of the pair up to the cutoff frequency of the transmission line.
When loading coils are in place, signal attenuation remains low for signals within the passband of the transmission line but increases rapidly for frequencies above the audio cutoff frequency. Thus, if the pair is subsequently reused to support applications that require higher frequencies (such as carrier systems or DSL), any loading coils that were present on the line must be removed or replaced with a smart coil.
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