Inductors can be separated into two categories: those specifically chosen to be inductors and stray inductances in PCBs and components. For inductors, parasitic characteristics arise from the ohmic losses in the conductor, from capacitance between coils from the core and from mutual inductances with other magnetic components.

The ohmic losses are straightforward as it is understood that there is a resistance associated with the length of conductor used to make the inductor. The capacitance effects are similar to those of the resistors, so those are also fairly clear. As for the inductor cores, the effects are a bit interesting. The ferro-magnetic cores of some inductors can “saturate” with strong fields, and thereby modulate the inductance as a function of applied field. As far as magnetic coupling, this all depends on the amount of magnetic flux that enters the coils of the inductor. Unlike electric fields, which can be shielded by Faraday cages, magnetic fields must be shielded with solid metal enclosures, where the skin-penetration depth of magnetic radiation is much smaller than the metal thickness. For this reason, it is much easier to pick up magnetic noise than electric noise in a shielded circuit. As far as tolerances go, it is hard to manufacture precision inductors, so they have a lower inherent quality (Q) factor are not often used for precise applications beyond power regulation.

Sometimes the inductors do act as precise resonators, but not in the way they were intended. When a series inductor is connected to a parallel capacitance, the pair can act as a tuned-LC circuit and ring at certain frequencies. This can happen intentionally, as in a power system, or unintentionally, where the capacitor is a by-pass capacitor by an IC and the inductor is the trace inductance (~10nH per cm for 10 mil trace), so unwanted ringing can be introduced into the circuit. The amount of ringing depends on how well tuned the circuit is, and thereby how high the Q factors for the various components are. For this reason, a small resistance is sometimes put in series with the inductance to specifically degrade the Q factor of the inductor and thus reduce the ringing.

 
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