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

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Fri, 12 Sep 2014 14:53:53 -0700

Craig, no matter how you ultimately choose to address your issue, the 81 
slide presentation linked here should be instructive:

     http://www.x2y.com/publications/cmcreplace.pdf

Steve
On 9/12/2014 2:11 PM, Craig Francis wrote:
> SI-LIST,
> Thanks for the insight.
>
> My application is like Randy described: low data rates (think 1Mbps) driven
> by microprocessors and cables between PCBs within a chassis.
> Some people put ferrites on single-ended signals before the connector to
> reduce the high frequency content of the signal and prevent unnecessary
> high-frequency radiation.
>
> Murata sells ferrite arrays for this purpose:
> http://www.murata.com/en-us/products/emc/emifil/bl but I understand that
> they are in the business of selling ferrite beads.
>
> I was wondering why I never see the ferrites applied to differential pairs,
> only common-mode chokes. But it sounds like there are better matched
> filters for differential pairs to reduce common mode conversion.
>
> However, there appears to be a large disagreement on using ferrites on
> signal lines at all. Application notes and books offer contradictory
> information and now I am more confused than I started.
>
> Thanks,
> Craig Francis
>
> On Fri, Sep 12, 2014 at 2:04 PM, steve weir <weirsi@xxxxxxxxxx> wrote:
>
>> Some generalizations are more general than others.
>>
>> Steve
>> On 9/12/2014 8:47 AM, Orin Laney wrote:
>>> All generalizations are false.
>>>
>>> -----Original Message-----
>>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>> On
>>> Behalf Of Lee
>>> Sent: Friday, September 12, 2014 8:15 AM
>>> To: clfranci@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
>>> Subject: [SI-LIST] Re: Ferrite Beads on Differential Signal Lines?
>>>
>>> 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
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>> MS 34689
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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

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