cable shield to PCB? - Page 1
Forum section: electronics / beginners

author: Zeyneb
author_rank: Regular Contributor
timestamp: May 04, 2024, 03:32:43 pm
content: Hi there,Maybe this topic is already discussed but I haven't been able to find it here.Considering a round multiple conductor cable with a shield. Either aluminum foil or that mesh of tiny non-insulated wires. What is a professional way to connect the shield specifically to a PCB? That PCB I'm going to design myself, so the options are open. The signal wires I know how to deal with.I like the design of shielded modular connectors like RJ-45/ethernet. Where you fold the shield back and then crimp a C shape conductor around it.Would there be some crimp terminals for this? The cable I have specifically has an OD of 5mm. For the PCB connection I'd prefer to have it as low-profile as possible.Thanks in advance for your suggestions!

author: radiolistener
author_rank: Super Contributor
timestamp: May 04, 2024, 04:05:59 pm
content: the shielding is not easy task. If you want cable with a good shielding, then it should use at least two isolated shield layers and RF chokes.It all depends on the frequency and interference type. The most complicated interference type is a near region magnetic field.The professional way to connect the shield to a PCB is to keep wave impedance between shield and central wire constant across connection and equals to the cable impedance. It allows to eliminate reflections, radiation and sensitivity to interference and can be done with using special geometry and dielectric with specific electromagnetic properties. If it's really important, you can use EM simulation software to design proper geometry for frequency and bandwidth of your needs.I recommend you to use existing standard connectors, because they already designed for specific impedance used in coax cables. For example if your RF power is less than 100W and you want a cheap and compact solution, you can use SMA connectors. For higher power you can use N connectors.

author: Zeyneb
author_rank: Regular Contributor
timestamp: May 04, 2024, 04:34:58 pm
content: Ok, but I meant in a more practical sense. What terminals can you buy to make that connection in the first place?

author: radiolistener
author_rank: Super Contributor
timestamp: May 04, 2024, 04:40:42 pm
content: Quote from: Zeyneb on May 04, 2024, 04:34:58 pmit depends on your needs. I like SMA connectors, because they are compact. But they have their cons, like small power limit and a smaller number of connection cycles. N type can handle more power and more reliable, but it is more massive. BNC is more easy to connect, but it is less reliable.So all connectors have their pros and cons and you're needs to select it according to your requirements.

author: mag_therm
author_rank: Frequent Contributor
timestamp: May 04, 2024, 05:03:08 pm
content: Search " Shielded Back Shell with metal pcb receptacle" .... Is that the style ?

author: Zeyneb
author_rank: Regular Contributor
timestamp: May 04, 2024, 05:55:31 pm
content: Quote from: mag_therm on May 04, 2024, 05:03:08 pmFor example consider my 5mm cable with 5 signal wires. And the shield of course. What product can you actually buy to connect it to that cable?I was more thinking about a ring terminal like Panduit PM6-3R-L. Just for the shield connection. With a rivet nut installed through the PCB and the signal wires to a right angle Molex microblade connector. Some idea?

author: radiolistener
author_rank: Super Contributor
timestamp: May 04, 2024, 06:33:02 pm
content: Quote from: Zeyneb on May 04, 2024, 05:55:31 pmDIN connector?

author: shabaz
author_rank: Frequent Contributor
timestamp: May 04, 2024, 06:34:48 pm
content: I think you're just generally asking how to terminate the braid and get it into a format that it can be 'connected' in some way.One typical simple way that it can be done, is with a wire-to-board connector. You then treat the braid like any of the cores, in that you create a pig-tail (separate out the braid and twist it like any other conductor) and either shove that in the connector, or, more neatly, place some heat-shrink on the braid first, or, trim the braid a bit, solder an insulated wire to it, and then protect the join with heat-shrink, and then optionally add more heat-shring to cover the exposed part where the braid originally came out of the cable. You can then insert that insulated wire into your connector which will contain all the signal wires.That method would also work for the case where you want to connect it to a ring terminal. It's really up to you, if you have no other electrical or mechanical requirements.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2024, 07:31:58 pm
content: As above, drain wire or braid can be brought to a header, but keep in mind, the uncoupled length reduces CMRR, proportional to uncoupled length and frequency of interference.For low to modest levels of interference (e.g. within a metal enclosure, mainly the equipment's own internal noise), this is acceptable. Example: USB headers from PC motherboard to panel-mount jacks (the shell is grounded to enclosure or IO backplate).It's also acceptable when bandwidth is low and filtering is ample; industrial gear sometimes needs shielding, but not so bad that a few inches of uncoupled length entering the PLC's IO board fouls it up.Optimal shielding is obtained when the shield is contiguous with the circuit's own shield (preferably enclosure, else ground plane). This dictates metallic shell connectors, some means to crimp or otherwise bond the shield to the shell, and bulkhead mounting of the connector (or at least many shield pins into plane).Note that RF shielding can be done independent of DC/galvanic grounding/earthing. If a DC "lift" happens to be needed, bypass capacitors can be placed across the shield boundary.Use many in parallel, as the ESL of just one or a few will likely not suffice -- again, uncoupled length matters!If you have the space, and don't mind stripping the jacket in multiple places (and any environmental concerns that may create), you can use clips like this:https://www.icotek.com/en-us/products/emc-cable-shield-clampsAgain, note that there is some uncoupled length: in this case, the height of the clip/clamp above enclosure (reference plane). It's not an ideal ground (compare: suppose you put two bulkhead connectors right beside each other, and joined the signals on the interior; then both incoming/outgoing shields terminate solidly into the enclosure, no direct path between them), but it can knock down the brunt of incident EMI, greatly relaxing the connector requirements. This also greatly lowers the shield impedance in the area, so that a ferrite bead has tremendously more effect (i.e. when placed between the shield clamp and a bulkhead connector, than it would on a free cable).Tim

author: coppercone2
author_rank: Super Contributor
timestamp: May 04, 2024, 07:37:40 pm
content: I highly recommend finding out if the drain wire is good enough for your application because its simply the best termination and most reliable compared to anything involving the braidand its how you don't have the cable assemblers wanting to kill youImo when its more then 1 wire, and you need to deal with the braid, its hard.and drain wire is way more fixable by whoever. the braid breaking = problem. its like a liability if you don't have people that know how to deal with it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2024, 09:42:07 pm
content: It's not...(again, uncoupled length).Cable repair isn't exactly something manufacturers care about (just get a new one..), and cables can be made more flexible if need be (finer stranding).Braid is usually crimped or swaged into a coaxial connector/housing, or taped over with mesh. Looking up several metal-shell connector systems should show typical assembly kits with such parts included.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: May 04, 2024, 10:29:23 pm
content: its definately higher performance if you terminate the braid at the connector no arguementbut for the foil, I thought you basically NEED the drain wire to ground it on. I would have zero confidence on a shield going into anything. Well maybe one exception, I have seen a connector that uses a rubber collet to terminate the shield, that might be OK for foil ,but still.. you basically KNOW thats going to tear with any use (but it is the wrong cable to use for a frequently moved application)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 05, 2024, 12:48:35 am
content: Foil yeah you don't have much choice. You can make do with mesh tape and whatnot, but it might not stick very well or whatever. It might also not fit into such connectors as well, and you get gaps and extra flex that way, etc.Tim

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 05, 2024, 05:33:51 am
content: Quote from: Zeyneb on May 04, 2024, 03:32:43 pmYour easiest and most economical option would be to use shielded CAT-5 cable (probably stranded) and the appropriate connectors.What's the brand and part number of your cable? Unless it's something standard, like CAT-5, you're not going to find a crimp connector or termination for it.You don't mention what kind of signals this cable will be carrying. The higher the frequency, the more difficult it becomes to do right.Raychem makes shield terminations, but they are damn expensive. You can do something similar with heatshrink and wire but it's more tedious: