[SI-LIST] Re: Floating Cable Shield
- From: Doug Smith <doug@xxxxxxxxxx>
- To: a.ingraham@xxxxxxxx
- Date: Sat, 28 Oct 2006 15:49:18 -0700
Hi All,
A (very) simplified way of thinking how shields work is the following:
Current flowing on a shield produces a voltage across the inductive
reactance of the shield. This voltage is induced in the center
conductor with the same amplitude as well as polarity/phase. If both
ends of the shield are connected, the center and shield voltages
cancel each other out around the loop composed of the source, load,
shield, and center conductor. If the shield is not connected at one
end, capacitively coupled current on the shield will generate
inductive drop which results in center conductor voltage. At RF/logic
frequencies, the shield does not even do much for E field coupling,
just helps with induction into the differential loop between enclosed
wires, not much help.
This is illustrated by measurements on a shielded EMC loop in one of
my papers at:
http://emcesd.com/pdf/emc99-w.pdf
"Signal and Noise Measurement Techniques Using Magnetic Field Probes"
Such loops only offer E field protection for fields that are symmetric
around the center axis, such as in the far field. Near a circuit board
such loops only offer E field protection if the gap is on the part of
the loop closest to the board so the board noise capacitively couples
on both sections of the split shield. Under that condition, the
induced voltages in the center conductor cancel.
There is a set of slides on my website from a previous talk several
years ago than bear on this. The link to the slide set is:
http://emcesd.com/talks/hfovervw.pdf
"Overview of High Frequency Measurement Techniques"
The password to open the file is:
need2see
The slides of interest are pages 47-59. There are lots of spoken words
that accompany the slides, but you should be able to get the meaning.
The first few slides show that there is no field inside the current
carrying shield by the right hand rule and the fact that the magnetic
field of a long wire falls off as 1/R.
Doug
Andrew Ingraham wrote:
>>... 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.
>
>
> It's my understanding as well that the shield ought to be "grounded" (i.e.,
> have a current path) at both ends to be fully effective against
> electromagnetic noise, on the large scale. But on the other hand, even a
> floating shield does affect local inductive coupling from nearby sources, by
> setting up eddy currents which tend to cancel some of the AC (varying)
> magnetic fields.
>
> Regardless, the mere presence of metal affects the EM fields in some way,
> whether or not it is grounded. Could be either for the better or for the
> worse.
>
> Andy
>
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