[SI-LIST] Re: Floating Cable Shield

Ivor, 

That is quite an interesting configuration, and it seems quite the waste of
copper :>).  At the risk of becoming too loquacious I will direct you to
Henry Ott's book "Noise Reduction Techniques in Electronic Systems".  There
is a whole chapter dedicated to this question, and shows the good, the bad
and the ugly ways of terminating a cable including what happens when you
attach the shield at one end vs. both.

CC

-----Original Message-----
From: Ivor Bowden [mailto:ivorlist@xxxxxxxxxxx]
Sent: Wednesday, October 11, 2006 11:29 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Floating Cable Shield


Hi,

I ran into what I thought was an interesting cable and am looking for 
comments. This is an IDE cable on a new PC I got. The construction is 
twisted pair ribbon cable (flat spots for IDCs between the twisted 
sections), about 20" long, IDC both ends, a 3rd IDC about 7" from one 
end. What makes the cable interesting to me is there are two braided 
shields covering the bundled twisted pair sections, one about 8", the 
other about 3.5". These shields have no connections, not to a drain 
wire, not to each other and to to any circuit or frame ground, they are 
entirely floating. My question is, how effective is this type of 
"floating" shield, in terms of SI and EMI?

This also brings up the general question of cable shield termination, 
circuit/chassis ground at both ends v.s. one end only v.s. floating; 
what characteristics to expect in each case?

And that brings up the specific question of shield return treatment in 
the connector area, single drain wire v.s. complete enclosure?

Thanks,

Ivor Bowden
Hardware Engineer
Curtiss-Wright Controls Embedded Computing
Video and Graphics
San Diego, CA 92121


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