From: [email protected]
Date: Tue Nov 07 2000 - 12:20:03 PST
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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