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Ferrite Beads vs Inductors in LC Filters

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dknguyen

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Ferrite beads are lossy at the frequencies they are intended to filter so that the noise energy is dissipated as heat. App notes caution against blindly using them with capacitors lest a resonance peak make things worse than better. The app note says that the resonance peaks occurs if the LC resonant frequency between the bead and capacitor is below the crossover frequency of the bead (the frequency where resistance = reactance).

However, I was wondering then...what happens to this noise energy in an LC filter that uses an inductor? The noise energy in the inductor isn't be dissipated so where does it go? I always always kind of under the impression that the inductor stores the noise energy and releases it over time as a lower frequency than the original noise. Why is would dissipating it as heat, as is done in a ferrite bead, be more desirable?

And given that an inductor doesn't have a crossover frequency in it's operating range by design, LC filters using inductors would always suffer from resonant peaking, no? Why do I never see that issue discussed with LC filters that use inductors and only with LC filters that use ferrite beads?
 

MikeMl

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If using a bead as a highQ inductor, then the frequency will be less than ~1/10 of where it would otherwise be used as a lossy bead.

I have a bunch of toroids here that work as RFI filters at 100+Mhz that also can be used as inductors in filters at 10Mhz or less. A toroid slipped on a wire doesn't have to be lossy to act as a series choke.
 

dknguyen

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Ferrite beads are lossy at the frequencies they are intended to filter so that the noise energy is dissipated as heat. App notes caution against blindly using them with capacitors lest a resonance peak make things worse than better. The app note says that the resonance peaks occurs if the LC resonant frequency between the bead and capacitor is below the crossover frequency of the bead (the frequency where resistance = reactance).

However, I was wondering then...what happens to this noise energy in an LC filter that uses an inductor? The noise energy in the inductor isn't be dissipated so where does it go?

And given that an inductor doesn't have a crossover frequency in it's operating range by design, LC filters using inductors would always suffer from resonant peaking, no? Why do I never see that issue discussed with LC filters that use inductors and only with LC filters that use ferrite beads?
If using a bead as a highQ inductor, then the frequency will be less than ~1/10 of where it would otherwise be used as a lossy bead.

I have a bunch of toroids here that work as RFI filters at 100+Mhz that also can be used as inductors in filters at 10Mhz or less. A toroid slipped on a wire doesn't have to be lossy to act as a series choke.
You're referring to using a ferrite bead below it's crossover frequency where reactance dominates over resistane, right?
 

MikeMl

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You're referring to using a ferrite bead below it's crossover frequency where reactance dominates over resistane, right?
Right. However, ferrite toroids are frequently used as common-mode chokes at frequencies where the ferrite isn't particularly lossy...
 

crutschow

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what happens to this noise energy in an LC filter that uses an inductor?
It doesn't necessarily go anywhere.
It's just prevented (blocked) from going through the filter.
 

dknguyen

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It doesn't necessarily go anywhere.
It's just prevented (blocked) from going through the filter.
So what's the difference then between using a ferrite and using an inductor for LC filters? My understanding is the only difference is one is intended to be lossy and one isn't.
 

crutschow

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The lossy (resistance) part of the inductance reduces the Q and thus any peaking in the filter response due to resonance between the inductance and capacitance in the filter.
 

dknguyen

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I see...so it sounds like the main differene between a ferrite bead and an inductor is that the ferrite beads is intended to be lossy to reduce the Q factor in order to reduce the resonant peaking. In addition, operating the inductor in this lossy range allows a much smaller inductor to be used for the same frequency?
 

crutschow

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In addition, operating the inductor in this lossy range allows a much smaller inductor to be used for the same frequency?
Don't see why that would be true.
 
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