[SI-LIST] Re: Ferrite Beads on Differential Signal Lines?

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Fri, 12 Sep 2014 14:30:16 -0700

While often abused, ferrite beads are a tool that have legitimate 
purposes.  What makes no sense is to either:

1) "Solve" a problem that doesn't exist, or
2) Solve a problem with inappropriate means.

Ferrite beads are very useful in these sorts of situations:

1) As  part of a properly decoupled high insertion loss filters.
2) As components in EMI filters.

Places where they should not be used, or at least not used alone:

1) As power feeds to broadband amplifiers.  (All digital logic devices 
qualify as broadband amplifiers.)
2) In circuits where their low to medium frequency behavior of a high Q 
linear inductor has not been fully considered.

Unfortunately, there have been many times that people have misused 
ferrite beads creating bigger problems than the beads were intended to 
solve.

The National Ferrite Bead Association reminds us:

"Ferrite beads don't kill designs.  Lazy engineers do."

Best Regards,


Steve

On 9/12/2014 8:15 AM, Lee wrote:
> It makes no sense to put ferrite beads in any signal line.  How did this get
> started?
>
> -----Original Message-----
> From: Craig Francis
> Sent: Thursday, September 11, 2014 4:11 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Ferrite Beads on Differential Signal Lines?
>
> SI-List,
> I haven't seen any literature concerning using ferrite beads on each signal
> line of a differential pair.
> Usually only a common-mode choke is used to reduce common-mode EMI.
>
> Can ferrite beads be used on a differential pair to increase the the rise
> time of the signal and reduce unnecessary high-frequency content that may
> radiate?
>
> The two ferrite beads would be sized to allow the bandwidth necessary for
> the differential signal's highest data rate.
>
>
> Thanks,
> Craig Francis
>
>
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