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

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: "Lucci, Christopher" <Christopher.Lucci@xxxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <SI-LIST@xxxxxxxxxxxxx>
  • Date: Fri, 12 Sep 2014 15:09:33 -0700

In the first case the decoupling capacitors satisfy the load where the 
bead can't, whereas in the second case they are absent and the load 
starves at medium to high frequency.

Steve.

On 9/12/2014 3:02 PM, Lucci, Christopher wrote:
> Mr. Weir -
>
> Can you further explain what you mean by not using a ferrite in a power feed 
> of a broadband amplifier (e.g. digital IC)?  How is this different from #1 in 
> the first list "As  part of a properly decoupled high insertion loss filters."
>
> Not sure I caught what you meant?
>
> Respectfully,
> C. S. Lucci
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
> Behalf Of steve weir
> Sent: Friday, September 12, 2014 5:30 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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-- 
Steve Weir
IPBLOX, LLC
1580 Grand Point Way
MS 34689
Reno, NV  89523-9998
www.ipblox.com

(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax

All contents Copyright (c)2013 IPBLOX, LLC.  All Rights Reserved.
This e-mail may contain confidential material.
If you are not the intended recipient, please destroy all records
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