Re: [SI-LIST] : Inductor Selection

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From: sweir (weirsp@atdial.net)
Date: Tue Nov 07 2000 - 18:17:06 PST


Just be careful that any ferrite you choose really is sufficiently
resistive everywhere that matters to prevent peaking. Look at the
frequency curves carefully and simulate as necessary. You may be surprised
to find that you still need a damping resistor when using ferrites. It
just depends on the application.

Regards,

Steve.

At 03:20 PM 11/7/00 -0500, you wrote:
>Raymond,
>Or we can be lazy/conservative, and use a ferrite bead instead of an inductor,
>so that the high-frequency noise gets turned into heat instead of getting
>reflected back to the source... I don't like using low-loss/high-Q
>components
>in a circuit whose component values/parasitics/frequencies aren't under my
>complete control.
>
>R. Kenneth Keenan's books suggest bypassing power with series ferrite beads
>that have:
>* < 10 ohms impedance at the circuit's clock frequency.
>* > 50 ohms impedance at 5 * the circuit's clock frequency.
>
>The high-permeability manganese-zinc ferrites are usually good up to about
>40MHz. The medium-permeability nickel- zinc ferrites are usually good up to
>200MHz, and low-permeability nickel-zinc ferrites are usually good above
>200MHz.
>Since each company that makes ferrite beads and cores has their own system for
>naming their compositions, you usually have to look at the
>impedance-versus-frequency curves in their datasheets/catalogs to identify
>good
>candidates.
>
> John Barnes Advisory
>Engineer
> Lexmark International
>
>
>
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