One particular bead that was specified to have 2Kohms Z at
30 MHz was reasonably modeled by the following ckt up to some freq.
(which I don't remember at the moment) where the frequency dependent
losses of the ferrite material really started to kick in. A more complex
model is needed to accurately model the effects of the freq. dependent
The bead models as:
| | L=18.47uH
| | R=1.6K
This was derived with the HP4194's built-in model firmware.
.subckt bead2k 1 2
Rpar 1 2 1.6k
Lpar 1 2 18.47e-6
Cpar 1 2 1.4e-12
The model is representative of the beads performance up to a
certain point and then runs out of gas.
A much better way to model a bead is to measure the s-parameters
of the device you are interested in and then create a "black-box" model
realized as a ratio of polynomial controlled voltage and current sources
(a Pade' approximation). A program such as Apsimtech's SPAR or HP's
ADS (MDS) with the Picosecond modeling option can assist in making such
You really need more than just a generic bead model if you
want your simulation to mean anything.
I believe this topic was discussed on si-list around April 98.
Also, the people at Georgia Tech's High Performance Packaging Lab have
done some interesting modeling of discrete components utilizing s-parameter
based macro models.
Sun Microsystems Inc.
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