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

  • From: "Chen, Sherman" <sherman.chen@xxxxxxx>
  • To: "emcesd@xxxxxxx" <emcesd@xxxxxxx>, "craigleefrancis@xxxxxxxxx" <craigleefrancis@xxxxxxxxx>
  • Date: Fri, 12 Sep 2014 01:53:00 +0000

Assuming the two ferrite beads are perfectly matching (anyone has even seen 
such kind of products?) there will be no mode conversion. 
Then the implications will be 
1. reflections. 
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!) 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... 

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
>
>
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