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I've noticed on schematics at work, when I was at uni, and on datasheets that Ferrite Beads usually (90% of the time) are only given with their designator (FB? or L?), then the reactance (XOhms) and the current rating (1.5A for example).

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However, their reactance is obviously a function of frequency (and ideally, it would always be stated "XOhms @ 100MHz" etc.). However, this isn't really the case.

So I was wondering, from any super experienced Electronic Engineers, what frequency for a Ferrite Bead is the reactance "typically" (rule of thumb) given at?

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  • \$\begingroup\$ Useful application note about ferrite beads: Ferrite Beads Demystified by Analog devices \$\endgroup\$ – Nick Alexeev Jan 15 '18 at 1:43
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    \$\begingroup\$ 100MHz was somewhat of a standard at one time, however with more modern higher frequency components some ferrites are spec'd at 1GHz. The manufacturers spec sheet is the only sure way to determine the parameters at the frequency of interest. \$\endgroup\$ – Nedd Jan 15 '18 at 1:48
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    \$\begingroup\$ In general, generic ferrite beads available from the usual suspects are hideously underspecified.... \$\endgroup\$ – ThreePhaseEel Jan 15 '18 at 1:52
  • \$\begingroup\$ I believe 100 MHz is the most common, but always check the datasheet if it matters. It is worth noting that at lower frequencies, the ferrite bead is really just an inductor. So keep that in mind. It does not provide much damping at low frequencies and can possibly resonate with a capacitor. \$\endgroup\$ – mkeith Jan 15 '18 at 5:42
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Ferrite is mix of magnetic, conductive and insulative (dielectric) materials that becomes baked as a composite ceramic. The recipes are proprietary.

As a result the wide range of magnetic parts may have low or high coercivity, low or high hysteresis losses and low or high Q, (Q=reactive/resistance impedance ratio).

In parts like inductors, L and transformers, X, they are designed for their inductive properties. If used with DC current then rated for a 10% drop in L.

Ferrite Beads, FB are different.

Although the magnetic material is inductive serves to raise the impedance with frequency, the self capacitance is formulated to be more lossy above self resonant frequency so that they become more lossy from resistance in addition with Eddy current losses a major contributor as the impedance begins to drop with rising f.

Here is an example of the impedance for a 3A rated SMT ferrite bead.
1.6mm x 0.8mm.
Impedance @ 100 MHz Z=300Ω
Maximum impedance Z= 450 Ω @ 250 MHz
Rated current 2000 mA @ 40'C rise
DC Resistance 0.15 Ω max
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The frequencies where impedance is defined is always in the datasheet. There is no standard frequency for all ferrite but 10MHz, 100MHz and 1GHz is popular.

Inductors vs Ferrite beads

Inductors as they are intended for energy storage mainly below the self-resonant-frequency (SRF) and the series resistance is called DCR since it is measured at DC. A higher Q is a good quality factor.

misc design info.

Ferrite beads are lossy but most useful for absorbing energy up to and above the SRF resonance due to eddy currents. The difference is in the ceramic mix of magnetic , conductive and insulative materials. High resistance over a wide bandwidth over several decades is a good quality factor but is also a tradeoff with the Q and highest impedance of the ferrite. It is not often graphed with DC Amps since the DC current does not change the impedance is not as significant as in inductor designs used to store energy. However a 10% Z reduction at rated DC current would be expected which is more determined by temperature rise.

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