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I am interested which cable/wire type is better to protect against electromagnetic and radio interference/leakage of signal.

  1. Is a twisted pair wire better than an untwisted, to protect against EMI/RFI and to reduce the leakage of signal/information from the cable?

  2. Is a shielded wire better than an unshielded (both in twisted & nontwisted states), to protect against EMI/RFI and to reduce the leakage of signal/information from the cable?

  3. Does the shielding have to be grounded? What happens if the shielding is not grounded? Benefits & Drawbacks?


I have read about these on the internet, but always found contradictions, so I would like if some electrical engineer expert would clarify these for me.

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  • \$\begingroup\$ I supposed you have visited Wikipedia page. It has pretty useful information. en.m.wikipedia.org/wiki/Twisted_pair \$\endgroup\$ – Rubén Sánchez Aug 7 '16 at 16:07
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    \$\begingroup\$ What frequencies are you concerned with rejecting, what is the frequency of your signal, and are you connecting two shielded boxes, a shielded box to an unshielded box, a shielded box to a "stub end" device (i.e. a device with no other connections, such as a computer mouse), or an unshielded box to a "stub end" device? \$\endgroup\$ – ThreePhaseEel Aug 7 '16 at 16:15
  • \$\begingroup\$ I would like to shield against all frequencies. \$\endgroup\$ – Jensen68 Aug 7 '16 at 17:54
  • \$\begingroup\$ Your questions don't work as generic questions absent any useful detail. You can NEVER EVER "shield against all frequencies." At least not here in the Real World. Please revise your question to make sense. \$\endgroup\$ – Richard Crowley Aug 8 '16 at 1:20
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As far as I know a twisted pair wire is ALWAYS better at protecting a signal from outside interference. Belden cables has some webinars that discuss this. SHIELDED cable is the best option and from my experience you generally ground only one side as you can create ground loops if you ground both ends but this also depends slightly on the application as some systems are designed to use and have both ends of a data cable grounded.

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  1. Twisted pair (balanced) is better than unbalanced as long as the source is at least "impedance balanced" and the destination is properly differential. Common mode noise and interference impinges on BOTH of the twisted-pair wires, and the receiving input cancels the noise/interference by taking only the DIFFERENCE between the pair. Twisted pair will also reduce the amount of signal "leaked" out of the cable. Although this is typically not a major consideration.
  2. Yes, shielding is of significant benefit to protect the signal from noise and interference. Yes that applies to both single-ended, unbalanced, and balanced pair.
  3. The shield is grounded so that the interference collected by the shield goes somewhere to be made inert. If you don't ground the shield, then it is just a great receiving "antenna" for collecting all your noise, interference, EMI, RFI, etc. And then it is a great method of caacitively coupling all this noise right into your signal wire(s). It is almost worse to have an ungrounded shield than to have no shield at all.
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  • \$\begingroup\$ You should note that shields are meant to be grounded to the chassis of a device via a low-impedance, preferably coaxial connection and to do otherwise creates the dreaded "pin 1 problem" -- on plastic-cased devices, an I/O reference slab or plane can be substituted for the chassis. \$\endgroup\$ – ThreePhaseEel Aug 7 '16 at 17:29
  • \$\begingroup\$ Mr @Richard Crowley I have a few follo up questions, if I may. 1. I am more concerned of the noise leaked, and the interference that it causes on other electronice equipments. So as a thumb rule, the twisted pair is better at stopping the noise going out of the cable? 3. So does that mean that if it's not grounded, the shield will leak more signal or absorb more signal? If I just want to stop the signal leaking and EMI then is it better to have grounded shield or non grounded shielf? \$\endgroup\$ – Jensen68 Aug 7 '16 at 17:46
  • \$\begingroup\$ If you are not actually transmitting "noise" through your cable, it is not clear why you are concerned with your signal leaking OUT? You will need to reveal the details of your circuit if you want a more precise response. An un-grounded shield is worse than no shield at all. What is the point of a shield if there is no where for the noise to go? \$\endgroup\$ – Richard Crowley Aug 7 '16 at 19:21
  • \$\begingroup\$ I am concerned for my signal leaking out becasue it can interfere with nearby electronics, so the EMI/RFI must be reduced as much as possible, that is why I ask. So basically if the shield is not grounded it will amplipy the leaking signal/noise instead of reducing it? \$\endgroup\$ – Jensen68 Aug 7 '16 at 22:06
  • \$\begingroup\$ You are asking questions that cannot be answered without revealing WHAT is your signal that you are so concerned about "leaking out". What is the frequency? What is the amplitude? What kind of cable are you talking about using? And WHY are you even asking about using shielded cable, but then not grounding the shield. Without some context or explanation, your questions don't make any sense! \$\endgroup\$ – Richard Crowley Aug 8 '16 at 1:18

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