[SI-LIST] Re: Floating Cable Shield
- From: Yu Wang <wangy_km@xxxxxxxxx>
- To: a.ingraham@xxxxxxxx
- Date: Fri, 27 Oct 2006 10:27:20 -0700 (PDT)
I'm not sure if my model is correct or not. anyway,
it's just my 2 cents. I think the Floating Cable
Shield can be modeled as a wave guide and we all know
that wave guide can somehow confine and conduct the
wave energy, although it dosen't eliminate the wave
(or dissipate the energy) to a significant extent.
However, for passing EMC test, it may be helpful
because the attena dosen't cover 4-pi solid angle, a
wave guide can steer the radiation off the beam. To me
it's like the trick poeple played with smeared clock,
energywise it does not help, spectrumwise, it does.
--- Ivor Bowden <ivorlist@xxxxxxxxxxx> wrote:
> on 10/24/2006 3:26 PM PDT Andrew Ingraham wrote:
> >> When I specify cable shielding I always ensure
> that it is contiguous,
> >> preferably enclosing the connectors at both ends.
> I would not specify
> >> floating or single ended shielding. I wonder if
> there are cases, in
> >> which such configurations could be useful?
> >
> > Personally I can't envision the usefulness of a
> fully floating shield ...
> > but there are cases where grounding the shield at
> only one end is useful,
> > especially where desired to avoid multiple ground
> loops. It depends on the
> > frequency ranges and such. And obviously the
> shield must not be part of the
> > signal path (i.e., not coax).
> >
> > Regards,
> > Andy
> >
>
> Hi Andy,
>
> Thanks for the response. I bought Noise Reduction
> Techniques in
> Electronic Systems by Henry Ott (1st edition, 1974,
> ~$12 used) as
> suggested by Craig. I've been muddling through it,
> and am not too far
> along yet, but if I understand correctly, the case
> is made that while a
> shield grounded at one end only can be effective for
> reducing
> capacitively ("electrostatic") coupled noise, it is
> useless for
> inductively ("electromagnetic") coupled noise - that
> requires grounding
> at both ends to be effective.
>
> I'm unclear about the effects of a fully floating
> shield. I'd think that
> it would pick up currents from the signals it is
> "shielding", and those
> could couple as noise into other signals in the
> "shield" (increasing
> crosstalk noise) or the environment (increasing
> EMI?). But as Moustapha
> mentioned, he got a significant reduction in EMI
> with this
> configuration. I wonder how this would model.
>
> -Ivor
>
>
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