[SI-LIST] Re: Floating Cable Shield
- From: Istvan Novak <istvan.novak@xxxxxxxxxxx>
- To: doug@xxxxxxxxxx
- Date: Sat, 28 Oct 2006 19:40:37 -0400
Hi Doug,
Thanks for sharing your knowledge and experience with the list.
Since I dont recall the original question mentioning what was the
material of the floating 'shield', this brings up some thoughts and
follow-on questions.
In EMC practice, floating clamp-on ferrite absorbers are commonly
used. In PDN measurements I use ferrite tubes and beads around the
cables and probes for two purposes: first to improve the surface
transfer impedance of the thin flexible cables, and second to increase
the common-mode inductance so that the crossover frequency of
ground-loop problems gets pushed to lower values. If the floating
shield is not ferromagnetic, it wont really impact inductance, but its
losses still might help? If the floating shield is ferrormagnetic,
the inductance will also change. Could it be that the intention of
the floating shield was to utilize the loss of the shield and/or its
optional ferromagnetic properties in a certain frequency range?
Regards,
Istvan Novak
SUN Microsystems
Doug Smith wrote:
>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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