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

  • From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
  • To: "weirsi@xxxxxxxxxx" <weirsi@xxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Mon, 22 Sep 2014 15:03:41 +0000

I would like to echo Steve's response and add  a recent experience.  Folks who 
have seen recent STB designs no doubt have seen the keyboards on the outside of 
the metal chassis (if one exists).  There was one signal sourced from the CPU 
that went to the keyboard and was a significant source of radiation.  A ferrite 
bead fixed the problem.  So in the right spot a ferrite can "save the day"!

As far back as 2003, Lee Hill (Silent Solutions)  and Steward put on an "EMI 
Suppression using Ferrites" presentation that might be of some interest here . 
It  can be downloaded from:

http://www.ewh.ieee.org/r5/denver/rockymountainemc/archive/2003/may13/steward.pdf

Please let me know if you are having difficulties and I will be happy to send 
the presentation to you directly.


Best Regards
Charles Grasso
Compliance Engineer
Echostar Communications
(w) 303-706-5467
(c) 303-204-2974
(t) 3032042974@xxxxxxxxx
(e) charles.grasso@xxxxxxxxxxxx
(e2) chasgrasso@xxxxxxxxx

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of steve weir
Sent: Friday, September 12, 2014 3:30 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ferrite Beads on Differential Signal Lines?

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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Steve Weir
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