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

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Thu, 11 Sep 2014 19:22:13 -0700

On 9/11/2014 6:53 PM, Chen, Sherman wrote:
> Assuming the two ferrite beads are perfectly matching (anyone has even seen 
> such kind of products?) there will be no mode conversion.
There are no such products.  You can only get close matching with a 
common core, and that's a CM choke.  Beads are just not appropriate for 
diff pairs except in narrow circumstances where one has a low data rate 
and a fast edge rate.  But for such situations an X2Y cap works much better.
> Then the implications will be
> 1. reflections.
That's the whole point:  mismatch the line to the outside world. The 
price is that now memories of energy past can go sailing back towards 
the signal source.
> 2. emissions. Theoretically ferrite bead will convert the energy into heat so 
> no much left to be emitted. But from the typical freq. curve of a bead, it 
> would be likely to emit EM waves at freq. where its impd. is not high.
> So as long as the system can tolerate the attenuation brought by the 
> reflection (the src better has a good matching!)
You bet, or bad things can happen.

> and the EMI will not cause problem, using ferrite beads as the means of 
> lowering tr should be worth trying? The attraction of doing so is some chip 
> vendor only provide fixed tr in their products...
If we are talking about inside a product, the vendors of iron have 
suggested such practices for decades and decades using either beads or 
more complex integrated filters.
>
> 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 Tesla
> Sent: Friday, September 12, 2014 9:21 AM
> To: craigleefrancis@xxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Ferrite Beads on Differential Signal Lines?
>
> The Ferrite beads indeed will attenuate some differential energy, increasing 
> rising time.
> Tesla Lee.
>
>
>
>
>
>
>
>
>
> At 2014-09-12 07:13:27, "Craig Francis" <craigleefrancis@xxxxxxxxx> wrote:
>> 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
>>
>>
>> ------------------------------------------------------------------
>> To unsubscribe from si-list:
>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>
>> or to administer your membership from a web page, go to:
>> http://www.freelists.org/webpage/si-list
>>
>> For help:
>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>
>>
>> List forum  is accessible at:
>>                http://tech.groups.yahoo.com/group/si-list
>>
>> List archives are viewable at:
>>              http://www.freelists.org/archives/si-list
>>
>> Old (prior to June 6, 2001) list archives are viewable at:
>>              http://www.qsl.net/wb6tpu
>>   
>>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List forum  is accessible at:
>                 http://tech.groups.yahoo.com/group/si-list
>
> List archives are viewable at:
>               http://www.freelists.org/archives/si-list
>   
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>    
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List forum  is accessible at:
>                 http://tech.groups.yahoo.com/group/si-list
>
> List archives are viewable at:
>               http://www.freelists.org/archives/si-list
>   
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>    
>
>


-- 
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.

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List forum  is accessible at:
               http://tech.groups.yahoo.com/group/si-list

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

Other related posts: