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

  • From: Randy Dawson <rdawson16@xxxxxxxxxxx>
  • To: "gnuarm.2007@xxxxxxxxx" <gnuarm.2007@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 12 Sep 2014 12:30:29 -0700

Some stuff on this list is pure nonsense.
There are OFTEN good reasons to put ferrite beads on signal lines, both 
differential and single ended:

If the signal exits the chassis, even a static one that's an input to a 
microprocessor, as in a external switch.  Key up a handie talkie next to that 
wire and see what happens.
Passing emissions tests, signal wires exiting the box may have slew rates not 
warranted by the data bit rate, you will be radiating all over the spectrum.

Randy
KF7CJW

> Date: Fri, 12 Sep 2014 14:51:17 -0400
> To: si-list@xxxxxxxxxxxxx
> From: gnuarm.2007@xxxxxxxxx
> Subject: [SI-LIST] Re: Ferrite Beads on Differential Signal Lines?
> 
> At 11:47 AM 9/12/2014, you wrote:
> >All generalizations are false.
> 
> Including this one.  :)
> 
> Rick
> 
> 
> 
> >-----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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