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

  • From: Craig Francis <craigleefrancis@xxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Fri, 12 Sep 2014 14:11:37 -0700

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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