[SI-LIST] Re: Floating Cable Shield

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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