FreeLists / si-list / [SI-LIST] Re: What to do with "isolation" conductors in ribbon cable?

[Xin Wu <xwu_umd@xxxxxxxxx>]

Hi Jeff,=0AIf the isolation line ties to the ground directly, the character=
istic impedance of the signal net changes -- it is simliar to the change fr=
om a microstrip line to a stripline. =0AI think the best approach may proba=
bly be terminating the iso_line with matched loads on both ends.=0ABest, =
=0AXin=0A =0AXin Wu, Ph.D=0AProduct Manager, Ansys Inc.=0AAustin, TX 78749=
=0A=0A=0A----- Original Message ----=0AFrom: "Loyer, Jeff" <jeff.loyer@inte=
l.com>=0ATo: si-list@xxxxxxxxxxxxx=0ASent: Thursday, September 27, 2007 4:1=
3:39 PM=0ASubject: [SI-LIST] What to do with "isolation" conductors in ribb=
on cable?=0A=0A=0AI have a design where I have extra conductors in my ribbo=
n cable=0A("off-the-shelf, same-thing-we've-been-using FOREVER", no shieldi=
ng,=0Aribbon cable), allowing me to put grounds (or whatever I want) betwee=
n=0Asignals.  But, I'm not sure that grounds provide the best isolation.=0A=
I'm wondering if those isolation conductors shouldn't be terminated to=0Agr=
ound using 50 ohm resistors (or whatever the impedance of the cable=0Ais).=
=0A=0A=0AFor example, the pinout may be:=0A=0AGND  S1  ISOLATION_CONDUCTOR =
 S2  GND =0A=0A=0A=0AKeep in mind that all conductors are identical while t=
hey're in the=0Aribbon cable environment.  GND, S1, ISOLATION_CONDUCTOR, et=
c. have no=0Aidea whether they're supposed to a signal, ground, or whatever=
.=0A=0A=0A=0AIs the "ISOLATION_CONDUCTOR" (between S1 and S2) best tied dir=
ectly to=0AGND at both ends of the connector, or better tied to GND through=
 a=0Aresistor matching the characteristic impedance of the cable?=0A=0A=0A=
=0AIf it's tied directly to ground, it seems to represent a resonant=0Astru=
cture. Or, in the time domain, the FEXT/NEXT induced on it would be=0Acoupl=
ed to the other signal, and also bounce back at the interface with=0Aground=
 (rho =3D -1). But, if half the return current is flowing in it=0A(which, b=
y symmetry, it is), I don't want the discontinuity at the=0Aresistor.=0A=0A=
=0A=0ATerminated at both ends, I think it would couple less energy between =
the=0Asignals. =0A=0A=0A=0AThe only empirical data I have remotely tied to =
this is when I had a GND=0Aconductor which was only tied to GND on one end =
(the other was=0Afloating).  Crosstalk was HUGE.  Crosstalk was reduced to =
almost nil=0Awhen I tied both ends to GND, which makes me believe the termi=
nation=0Aresistors on each end aren't necessary. =0A=0A=0A=0AAll sorts of s=
ignals may be going across the ribbon - USB, resets,=0Aclocks, video, etc.,=
 but not super fast (though the ribbon may be=0Asignificantly long compared=
 to the risetimes).=0A=0A=0A=0AAny experience with this sort of thing?   I'=
m wondering if I'm=0Aoverlooking some basic concept that would make a detai=
led analysis=0Aunnecessary to predict the outcome. (or you could call it be=
ing lazy...)=0A=0A=0A=0AThanks  in advance,=0A=0AJeff Loyer =0A=0A=0A------=
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