Re: [SI-LIST] : power supply filtering and bypassing

Michael Tsuk (Michael.Tsuk@digital.com)
Tue, 10 Mar 1998 16:54:32 -0500

Talking about ferrites, Ray Anderson wrote:

> Here is a typical ferrite bead model that I've extracted
> utilizing an HP4194 Impedance analyzer.
>
> ----+------L-----+-----
> | | L=18.47uH
> +------C-----+ C=1.4pF
> | | R=1.6K
> +------R-----+
>
> The problem I've found is that this model has a limited
> frequency range over which the model to hardware correlation
> is good.
>
> Does anyone have any comments and/or suggestions on ways and
> means of extending the model to account for frequency and
> current dependency? (or perhaps have run across any good
> articles or papers addressing the issue)

I did some work on this with Rich Mellitz a few years ago. Basically,
coming up with a ferrite model useful over a large frequency range is
very hard. :-) I had a program that tried to create a more complex
circuit
model from measured data, but it wasn't very robust.

It's best if you have both real and imaginary impedances to work with;
matching just the real part can lead to very misleading results.

One idea that might work to extend the range of frequency is to use
more than one of your parallel RLC circuits, each tuned to a different
frequency
of interest.

-- 
Michael Tsuk
Michael.Tsuk@digital.com