Shielded cable is pointless when no metal enclosure? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: November 12, 2021, 10:08:08 pm
content: Hi,From 7:00 to 8:06 of this PCB video, it says that the only purpose of the shield round a shielded cable is to continue the Faraday cage effect of the metal chassis that surrounds the PCB that the shielded cable connects to...From that, you would therefore conclude that when a PCB does not exist inside of a metal enclosure, then it is pointless to have a shielded cable connecting to it?

author: bob91343
author_rank: Super Contributor
timestamp: November 12, 2021, 10:59:35 pm
content: I wouldn't say it would be pointless but it's an incomplete job. If you are going to do it, do it properly.

author: David Hess
author_rank: Super Contributor
timestamp: November 13, 2021, 12:28:17 am
content: Even within a chassis, coaxial cable may be used to direct return currents exactly where they need to go, while also shielding a sensitive signal. Twisted pair with one side grounded also works. So it depends on how the coaxial cable is being used.

author: Feynman
author_rank: Regular Contributor
timestamp: November 13, 2021, 10:23:24 am
content: Well, EMI interactions are complex, but yes: Without enclosure chances are high that a cable shield is pointless or even doing more harm than good.

author: Faringdon
author_rank: Super Contributor
timestamp: November 13, 2021, 10:26:51 am
content: QuoteThanks, i recently took a device through radiated emissions, and adding shielded cable made a massive improvement. It was a metal chassis enclosed product, which was having a "daughter" product connected to it by a power cable....using a shielded power cable made massive improvement.....even though we did not use that "360 degree connection" type connection....we just "pig-tailed" the shield and fastened it to the earthed chassis.BTW, we found it was better to unconnect the shield from the daughter_product's side.(the "daughter product" comprised a metal chassis, but this was 'open' so that people could take pagers out of it)So my experiences here dont add up with what's in this video of the top post.

author: Bassman59
author_rank: Super Contributor
timestamp: November 15, 2021, 10:27:43 pm
content: Quote from: Faringdon on November 12, 2021, 10:08:08 pmThat is exactly the primary purpose of the cable shield.QuoteWhen there is no metal case, you still use the shield connection to the circuit board to direct where you want the noise that collects on the shield to be drained.Also note that in single-ended systems like consumer audio the cable shield also carries the signal return, and you definitely need that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 15, 2021, 11:01:47 pm
content: By opening an ever-wider hole in an enclosure, one can smoothly transform from a fully shielded enclosure, to a PCB with ground plane. This continuously worsens the EMI performance of the system (radiation passes through the hole, proportional to its size, roughly speaking, and give or take positioning of components/traces with respect to it and other nearby ground), but remains better than a random hairball of grounds and signal wires, for example.A key insight from such a thought experiment, is the finding that external noise currents are largely carried around the periphery of the board. Consider the board exposed to a magnetic field for example; when the board is oriented perpendicular to the field lines, it acts to block them, inducing an equal and opposite current around the perimeter. The board itself serves as shield, and therefore any cable shields must be terminated into it, preferably around the edges, and with very low impedance (multiple wide ground connections).Tim

author: thm_w
author_rank: Super Contributor
timestamp: November 16, 2021, 11:49:06 pm
content: Quote from: Faringdon on November 13, 2021, 10:26:51 amWhy do they not add up? He said 360 degree connection is the best, he never said a non-360 degree using pigtail will do nothing.Pigtail would have some additional inductance/resistance.

author: Faringdon
author_rank: Super Contributor
timestamp: November 18, 2021, 06:52:43 pm
content: QuoteThanks, why is it that usually you need to disconnect the shield at one end?

author: Bassman59
author_rank: Super Contributor
timestamp: November 18, 2021, 07:18:33 pm
content: Quote from: Faringdon on November 18, 2021, 06:52:43 pmYou don't.That is bad advice, and in any event will only work with balanced signals.

author: Faringdon
author_rank: Super Contributor
timestamp: November 19, 2021, 09:21:25 am
content: QuoteThanks, but last time we were in the radiated lab, we were failing, we then disconnected the shield at the one end, and we then passed.

author: Gyro
author_rank: Super Contributor
timestamp: November 19, 2021, 01:47:11 pm
content: It was still having a shielding effect then (one end connected). You just hadn't understood the return current path - and the loop you had created by soldering both ends.