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I am shopping on Digikey for an inductor for use in a radio application. The frequency range I am working in will be 200kHz - 900kHz. I noticed that there is a "frequency test" column where most of the inductors have 1kHz written in (a few have 250kHz written there). The datasheet provides no information about this and I'm not sure how to interpret it.

Do all inductors behave the same over a range of frequencies? If not, can I assume that they will at least behave with consistent inductance up to 1Mhz (a reasonably low frequency)?

What does this mean?

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    \$\begingroup\$ It's "Frequency - Test". The frequency at which the parameters were tested. \$\endgroup\$ – The Photon Dec 1 '15 at 17:25
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    \$\begingroup\$ No they don't. Core losses vary at different frequencies. This is why mains transformers use sheets of iron while RF inductors use ferrite or air cores. I wouldn't trust a 1 kHz measurement at 250kHz or vice versa, but within an order of magnitude I wouldn't worry too much. \$\endgroup\$ – Brian Drummond Dec 1 '15 at 17:32
  • \$\begingroup\$ Not all inductors will behave consistently up to 1 MHz. What are you using the inductor for, or how much current will pass through it at 900 kHz? Work your answer into the original question as an edit. \$\endgroup\$ – mkeith Dec 1 '15 at 17:33
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No. All inductors don't behave the same way over a range of frequencies. That's why the data sheet specifies the test frequency. Rejoice that it does specify that, some don't and leave you guessing.

The frequency range of an inductor is governed by two things, one is the core material, the other is the winding geometry.

The core material tends to have an upper frequency limit, above which it becomes too lossy to use, the effective permeability often changes as well. This is expressed either as Q, for signal uses, or power dissipation, for power uses. Materials designed for high frequency tend to have lower permeability than those for low frequency, which means that low frequency inductors will be 'better' on other specs, like inductance, and residual resistance.

As the frequency goes up, the self capacitance of the windings can start to turn an inductor into a parallel resonant circuit. The cure for this is to reduce the capcitance by reducing the number of turns, and to use fancy winding techniques that pack less wire into the available space. Again, to make a high frequency capable inductor means sacrificing inductance and series resistance. It is because of the windings issue that even 'air-cored' inductors have frequency limitations.

The specified test frequency will be in the range of 'good' use frequencies. Not necessarily at the top end of the range, it depends what equipment the test house has to hand. For high frequency inductors, the low inductance can mean that Q is very poor at 1kHz, and there is little point measuring at such a low frequency.

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It means that the inductance is only guaranteed to be within the specified limits at that frequency, since trying to nail down an inductor is like trying to herd cats.

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