[SI-LIST] Re: Floating Cable Shield

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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