[Ground-station] The other kind of ground

Bruce Perens bruce at perens.com
Wed Jul 4 02:59:42 EDT 2018


On Tue, Jul 3, 2018 at 12:35 PM, Zach Leffke via Ground-Station <
ground-station at lists.openresearch.institute> wrote:

> I've looked into Ferrite solutions, but not really sure the right place to
> put them, and not sure if that would kill the actual ethernet performance.
> Are there any particular ferrite type filters I could build for this?
>
Chokes on the outside of a shielded twisted pair should suppress
common-mode currents on the shield and should not effect differential-mode
currents on the internal twisted pair. I am over my head regarding how the
choke changes shield effectiveness and what happens to the internal
magnetic field rather than the external one. It's worth trying.

We've all seen the cheap chokes at swap meets. The big question about them
is what formulation they are. If you want to measure them, this paper:
http://www.reeve.com/Documents/Articles%20Papers/Ferrite%20Beads/Reeve-Hagen_FerriteBeads_P1.pdf
gives ways to do so using a scalar network analyzer (spectrum analyzer with
a tracking generator) or a VNA.

I use Palomar (http://palomar-engineers.com/ferrite-products/ferrite-beads/)
when I want to know what the formulation really is.

    Thanks

    Bruce
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