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

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Sun, 14 Sep 2014 19:54:40 -0700

Your model is probably correct. People who are under time pressure 
frequently give in to copying an idea or implementation in the hope that 
it will work, rather than spend time doing the engineering.  That can 
really foul things up.  I don't have a simple answer.  There is always a 
lot of time pressure.  At the same time, the cost of doing foolish 
things can be really high.

Steve
On 9/14/2014 6:22 PM, Chen, Sherman wrote:
> I also saw extra bead worsened the transient response. It's definitely not 
> something "better than none".
>
> Go back to the original question from Craig. I did a quick sim and here are 
> the results:
> 1. for multiple GHz applications, ferrite bead totally messed up the sparam 
> due to its nonlinearity. Meanwhile inductor can slow down the edge as 
> expected. As mentioned this may have practical problem due to lack of matched 
> inductors. As to the low freq. application such as 1MHz, bead probably can 
> server the purpose. Worst case, CM ferrite can be added to correct the mode 
> conversion.
> 2. if loss margin is sufficient, winding couple of extra inches can 
> effectively slow down the edges.
> 3. connecting a CM choke in a "differential way", aka, reversely connecting 
> the two leads on one side so that the CM choke is acting as a "DM choke", may 
> worth trying. My sim result gives a distorted group delay. This could be due 
> to the model problem.
>
>
> Best Regards,
>
> Sherman Chen
> Signal Integrity
> EMC Global Hardware Engineering
> Tel: +86 21 60951100-3329
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
> Behalf Of Lee
> Sent: Sunday, September 14, 2014 12:53 AM
> To: weirsi@xxxxxxxxxx; Lucci, Christopher; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Ferrite Beads on Differential Signal Lines?
>
> As Steve notes, I saw this mistake many times.  Even had a client or two who 
> went out of business because of this error.  One FPGA vendor surrendered a 
> large part of its market to a competitor by clinging to this misconception 
> for far too long.
>
> -----Original Message-----
> From: steve weir
> Sent: Friday, September 12, 2014 3:17 PM
> To: Lucci, Christopher ; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Ferrite Beads on Differential Signal Lines?
>
> You might be surprised to learn how many times that mistake was made over the 
> years especially in the 80's and 90's.  Lee saw it and had to fix it many 
> times, usually by just removing the inappropriately applied beads.
>
> Steve.
> On 9/12/2014 3:13 PM, Lucci, Christopher wrote:
>> Thanks!  Completely understood and agreed.   I wasn't thinking that anyone
>> would omit decoupling caps and hence the confusion.
>>
>> Thanks Again,
>> C. S. Lucci
>>
>> -----Original Message-----
>> From: steve weir [mailto:weirsi@xxxxxxxxxx]
>> Sent: Friday, September 12, 2014 6:10 PM
>> To: Lucci, Christopher; si-list@xxxxxxxxxxxxx
>> Subject: Re: [SI-LIST] Re: Ferrite Beads on Differential Signal Lines?
>>
>> 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
>>> IPBLOX, LLC
>>> 1580 Grand Point Way
>>> MS 34689
>>> Reno, NV  89523-9998
>>> www.ipblox.com
>>>
>>> (775) 299-4236 Business
>>> (866) 675-4630 Toll-free
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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 and
>> notify the sender.
>>
>>
>
> --
> 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 and notify 
> the sender.
>
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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.
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