Linn Forums

Current time: 2017-10-23, 18:07 Hello There, Guest! (LoginRegister)

Linn Forums / Getting Started / Ask a Question v / Earthing / screening

Post Reply 
Earthing / screening
2017-09-28, 18:53
Post: #11
RE: Earthing / screening
(2017-09-28 11:33)DavidHB Wrote:  When it come s to the screening of Ethernet cables, I don't know offhand whether the screen is normally connected, but, as there is a return (-) wire in each twisted pair used, there are points in the cable (or more likely the connector) which could be connected to the shield.

Ethernet over twisted pair uses differential signalling. TX- carries the inverted signal of TX+, e.g. if TX+ is 500mV above a hypothetical ground, TX- will be 500mV below this ground. Hence you must not connect the shield to any of the cable strands. Otherwise one part of the signal stream will be present on the shield.

(2017-09-28 11:33)DavidHB Wrote:  Even if the shield is not connected, any voltage induced into it will try to find its way separately to an earth point somewhere, and thus dissipate harmlessly, turning the shield into a form of Faraday cage, as previously described.

It won’t dissipate harmlessly, because it induces noise to all strands in the cable. If one strand in the cable has a lower impedance to a suitable earth point somewhere, things get worse, as the noise level induced to the other strands will be higher than to the one mentioned first. This leads to bigger errors in the signalling, especially if it is single-ended.

AEDSM > KEB/1 x 2 > Linn Silvers > A4200/1 x 2 > K200 > MISOs
Find all posts by this user
Quote this message in a reply
2017-09-28, 23:20 (This post was last modified: 2017-09-29 00:02 by BB1.)
Post: #12
RE: Earthing / screening
Shielding is one of the most complex topics I ever came across. There is no universal answer to all use cases.

Take, for instance, a coaxial transmission line. It’s susceptibility to coupled in noise will be minimized by connecting both ends of the shield to ground, earth or any other suitable reference potential. On the other hand, doing so may cause unwanted compensating currents in the shield. These currents are neither good for the signalling nor the radiated emissions of the coaxial transmission line.

If the signalling issues caused by compensating currents are less important than providing the best possible shielding in both directions (coupled in and radiated), the common practice is to use two shields. The inner shield is connected to both sides (providing the best shielding against coupled in noise), whereas the outer shield must be only connected to either the source or the the destination. It’s purpose is to shield the radiation caused by the compensating currents in the inner shield…

I’d like to emphasize that a floating shield is no shield at all. The only thing it does is to alter the transmission line characteristics due to its unpredictable stray capacitance, but it will neither prevent coupling in nor radiation in an effective way as a properly connected shield does. Depending on the actual requirements there are a lot of options when it comes to shielding, but leaving the screen floating doesn’t belong to the suitable ones.

AEDSM > KEB/1 x 2 > Linn Silvers > A4200/1 x 2 > K200 > MISOs
Find all posts by this user
Quote this message in a reply
2017-09-29, 09:42
Post: #13
RE: Earthing / screening
(2017-09-28 23:20)BB1 Wrote:  Shielding is one of the most complex topics I ever came across. There is no universal answer to all use cases.

Just so. Judging by reports on the forum (sunbeamgls' experience with his ExaktLink, for example), there are cases where shielding appears to help, and others where it doesn't. As it happens, the only place I use a shielded cable is my ExaktLink; elsewhere I can separate the Ethernet quite well from other cables, which, I think we would all agree, is the first step one needs to take (if one can).

David

Main system: [Basik/Basik+/K5/Lejonklou Gaio >][Roksan Kandy Mk III >] KEDSM > Akurate Exaktbox 10 > Linn Silvers> A4200 x 2 and A2200 > K600 > Akubariks
Second system: Kiko
Find all posts by this user
Quote this message in a reply
Post Reply 


Forum Jump:


User(s) browsing this thread: 1 Guest(s)