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Does inductance vary over frequency in passive components. Consider a Coax cable for example, does the inductance value change as a function of frequency, lets say from 5GHz to 100GHz, does the inductance value ever change?

*My understanding is no, inductor values and capacitor values are not a function of frequency but are a function of their physical character.

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  • \$\begingroup\$ Inductance and capacitance are both parametric once packaged, and their values will vary as the voltage across them and the current through them varies \$\endgroup\$ – EM Fields May 24 '16 at 0:29
  • \$\begingroup\$ @EMFields. 'Eddy' currents and dielectric leakage also change with frequency and current, but I was not sure the OP wanted us to 'split-hairs' over such details, as we would be including unknown and/or out-of-range parameters. \$\endgroup\$ – Sparky256 May 24 '16 at 0:33
  • \$\begingroup\$ @Sparky256: "For almost all electronics work, a soldering iron/pencil is the better choice and one which is temperature-controlled is, arguably, the best choice. A soldering gun is, however, useful in electronics work, and is - these days - best suited for soldering, say, shield braid or heavy ground wires to a chassis or running a bead of solder around/between the body and the lid of a metallic hermetic enclosure. \$\endgroup\$ – EM Fields May 24 '16 at 0:38
  • \$\begingroup\$ @EMFields. UUUmmm...What is your comment in reference to? \$\endgroup\$ – Sparky256 May 24 '16 at 0:40
  • \$\begingroup\$ @Sparky256 :Splitting hairs is the best way to get rid of ambiguities. Do you have a problem with that? \$\endgroup\$ – EM Fields May 24 '16 at 0:46
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Does inductance vary over frequency in passive components. Consider a Coax cable for example, does the inductance value change as a function of frequency, lets say from 5GHz to 100GHz, does the inductance value ever change?

Yes, for just about any practical component, the effective inductance will change with frequency.

A coaxial cable is not an easy example to deal with. You should rather be considering it as a transmission line rather than a lumped inductor or inductor/capacitor network. So I'll leave that aside.

Instead consider an inductive coil. Any real coil has interwinding capacitance, caused by electric field between adjacent coils of the inductor, and modeled (in a simplified way) as a capacitor in parallel with the idealized inducutor. Above some frequency, the interwinding capacitance will tend to dominate the behavior of the device, causing the reactance to be negative and decreasing with frequency rather than positive and increasing.

Similarly, practical capacitors nearly always have lead inductance. This acts as an inductor in series with the idealized capacitor. Above some frequency (often specified as series resonance frequency in capacitor datasheets), the lead inductance tends to dominate the device behavior resulting in positive reactance increasing with frequency rather than negative and decreasing.

The frequencies where these effects become important range from kilohertz to gigahertz depending on the device construction, and you will want to be aware of them when choosing components in your designs.

Furthermore, even if you neglect these geometrical parasitic effects, you'll find that the critcial parameters (permeability of inductor cores and permittivity of capacitor dielectrics) also vary over frequency due to the resonances of the molecules producing the magnetization or polarization that determines those quantities. This will cause the inductance or capacitance of the device to vary (at least slightly) with frequency.

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  • \$\begingroup\$ Regarding your first example of inductive coil, I understand how the capacitance between coil winding might begin to dominate as frequency increases, however, I don't think their values ever increase. Their effect starts dominating after a certain frequency but their value surely can't change, am I correct on this. The only thing that I understand is the changes to permeability and permittivity, which ultimately change capacitance and inductance. \$\endgroup\$ – Sam May 27 '16 at 20:13
  • \$\begingroup\$ The capacitance and inductance of the model with parasitics don't change, but the effective inductance (\$X/(2\pi{}f)\$) of the whole device does change. \$\endgroup\$ – The Photon May 27 '16 at 20:35
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You have answered your own question, which is NO. An exception would be resistors that have a 'cold' value and a increased resistance when hot. Inductors and capacitors do not change value (ideally) unless the device is being driven outside of normal working parameters, or they are stressed to the point of damage. What does change with frequency is impedance. You will find many answers about impedance on this forum and on the web.

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