[SI-LIST] Re: Why is shield grounding necessary for Inpair Skew (Intra pair skew)?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: JS <jsven007@xxxxxxxxx>
- Date: Fri, 02 Aug 2013 19:59:56 -0700
JS it is the return path impedance at signal frequencies that affects
the performance. Ground the cable or hook it to an arbitrary DC voltage
that is bypassed and the shield will provide a return path independent
of cable length. Leave the shield open, and how it affects the signals
will be frequency and length dependent. Essentially, an open shield
forms a tuned resonator with the inner conductors.
Steve.
On 8/2/2013 4:26 PM, JS wrote:
> Thanks Steve. Capacitance-to-ground (shield) unbalance is major cause
> of IPS. Is this because common mode currents require a ground
> connection? The pair's shield is present in both cases, just in one
> cases it is grounded. So if we measure the pair with shield
> ungrounded, we just measuring mutual capacitance unbalance (instead of
> it and cap-to-ground unbalance)? Anyone with a thorough explanation
> much appreciated! Many thanks!
>
>
> ------------------------------------------------------------------------
> *From:* steve weir <weirsi@xxxxxxxxxx>
> *To:* si-list@xxxxxxxxxxxxx
> *Sent:* Thursday, August 1, 2013 10:57 PM
> *Subject:* [SI-LIST] Re: Why is shield grounding necessary for Inpair
> Skew (Intra pair skew)?
>
> In two words: unequal capacitance.
>
> Steve.
> On 8/1/2013 6:25 PM, Jus wrote:
> > Hi, I am interested in in-pair skew. Hoping another expert could
> help! :) Does anyone know why grounding a differential pair shield
> (ex. twinaxial or twisted pair foil) is needed to generate high
> within-pair-skew (intra-pair skew)? For instance, with the pair's
> shield grounded the in-pair skew value may be 10 ps/ft, while it is
> only 2 ps/ft with shield ungrounded (shield left floating). Why does
> grounding the shield dramatically increase In-Pair Skew? Are there any
> cases where a differential pair's overall shield or individual shield
> should be left ungrounded when measuring in-pair skew? I think I know
> the answer, hoping for confirmation. Any assistance much appreciated!:)
> >
> > Jus
> >
> >
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> --
> Steve Weir
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Steve Weir
IPBLOX, LLC
1580 Grand Point Way
MS 34689
Reno, NV 89523-9998
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
All contents Copyright (c)2013 IPBLOX, LLC. All Rights Reserved.
This e-mail may contain confidential material.
If you are not the intended recipient, please destroy all records
and notify the sender.
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