[SI-LIST] Re: Ferrite Beads on Differential Signal Lines?
- From: Dave Cuthbert <telegrapher9@xxxxxxxxx>
- To: weirsi@xxxxxxxxxx
- Date: Fri, 12 Sep 2014 06:37:39 -0600
One must be careful applying DM ferrites to signal lines. As mentioned, if
the DM chokes are no identical there will be mode conversion and CM noise
can increase. A fix for this is to add a CM ferrite after the DM ferrites.
Another gotcha is line matching. The DM chokes will increase the driver
impedance above some frequency and load reflections will not be properly
terminated. If the load is resistively terminated this is not as much of an
issue.
All of this can be simulated in SPICE. LTSpice has many ferrite bead models
and one can experiment in simulation.
Dave Cuthbert
EMC, Analog, Power Electronics, SI consulting (presently doing power
design and EMC on an 8 kW power supply)
On Thu, Sep 11, 2014 at 6:34 PM, steve weir <weirsi@xxxxxxxxxx> wrote:
> Mode conversion is not a happy thing.
>
> Steve
> On 9/11/2014 4:11 PM, Craig Francis 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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>
> --
> Steve Weir
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