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

  • From: "Orin Laney" <olaney@xxxxxxxxx>
  • To: <sherman.chen@xxxxxxx>, <emcesd@xxxxxxx>, <craigleefrancis@xxxxxxxxx>
  • Date: Thu, 11 Sep 2014 21:38:20 -0700

If you want to manage risetime, why use a bead?  An EMI bead is not
characterized as an inductor per se, having a characteristic that varies
between inductive and dissipative depending on frequency.  You could use a
formal inductor to create an L/R time constant (1 pole filter), or use an RC
time constant to do the same, or go whole hog and design in a proper 2 or 3
pole Bessel filter matched to the characteristic impedance of the line
(which is what a classic non-ringing rise time filter is).  

The other remarks are on point regarding reflections.  Either put your
filter directly at the driving source or use a constant impedance topology.
Tossing in a piece of ferrite characterized for EMI suppression and hoping
that it will do something on general principles is a use often referred to
as "prayer beads". Suffice to say that EMI beads are not intended or
characterized for this sort of application and are not likely to yield
optimal results.  

The other remarks are also on point regarding differential signals.  Basic
advice: don't go there with individual beads.  You could use a differential
mode choke, which is simply a common mode choke with one winding reversed.
Pay attention to trace layout, which does not flow as smoothly in this
configuration use unless the part is deliberately manufactured in a
particular way for DMC service.

Orin Laney

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Chen, Sherman
Sent: Thursday, September 11, 2014 6:53 PM
To: emcesd@xxxxxxx; craigleefrancis@xxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ferrite Beads on Differential Signal Lines?

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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