Where to connect the shields of wiring and connector advice - Page 1
Forum section: electronics / rf-microwave

author: daqq
author_rank: Super Contributor
timestamp: January 11, 2016, 09:53:44 am
content: Hi guys,Where should I connect the shields of a shielded differential pair cable in the following situation:There are two metal boxes. Both are grounded locally, there is less than 1 meter between them. In the whole space around them there is a nasty pulsed RF field at around 10GHz that we can't do anything about (existing installation).There are 3 devices - one master (C), two slaves (A, B). There will be one connector board on the boundary of the first main enclosure. All of the signals are floating (transformer coupling on both sides). The crossed circles represent connectors. The signals are low power.The signal path will be as follows:Source device -> Transformer -> Connector -> Shielded cable -> Connector -> Connector PCB -> Connector -> Shielded cable -> Connector -> Transformer -> Target deviceMy question is: At which points should I connect the shields of the cables to the enclosures? Should It be one single, undisturbed connection, or should I connect part of the signal path to one enclosure, the other part to another enclosure? Using what way would I achieve the least RF interference getting into either enclosure and onto the transmitted signal?My guess would be that the best place to connect the shields would be at the connector board (3, 4, 5, 6) and no where else. Is this a reasonable assumption?Also, I was looking into using SATA cables - they are relatively cheap, well shielded, with a well defined impedance. Would you recommend their usage in this kind of situation?Thanks,David

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 11, 2016, 11:58:33 am
content: First preference: can you do this? (pic)If there's ANY distance between the enclosure entry and the PCB, you'll have radiation from those inside lengths. The distance between "B" and the grounding point, or any other point where a cable enters the PCB to the grounding point, also allows radiation. At low frequencies, we'd be talking stray inductances, but these will be full random fractions of wavelengths, even for tight construction, so you really need shielded connectors, coaxial cables (or fully shielded/screened), EMI spring stock stuff, and all that.Any kind of transformer isolation is simply a short circuit at those frequencies, and the immunity of a given device will be down to how well the CM filtering and rejection is, and what the susceptibility of the devices are.Tim

author: Rerouter
author_rank: Super Contributor
timestamp: January 11, 2016, 12:06:17 pm
content: Generally the shield is connected at the side with the lowest impedance, as hard tying it to a ground return can cause a loop picking up more noise rather than lessening it,The is obviously different if the shield is your ground return, but i thought i would give both cases,For both these cases my knowledge runs out somewhere close to the 10's of Mhz range, so for higher, you need an RF wizard to hop in,

author: KJDS
author_rank: Super Contributor
timestamp: January 11, 2016, 05:56:02 pm
content: I've used SATA connectors successfully for high speed data.I'd then move one box next to the other rather than leave it a metre away.

author: Ian.M
author_rank: Super Contributor
timestamp: January 11, 2016, 06:26:27 pm
content: At 10GHz, a joint between two plates with a 1.5cm section not making electrical contact looks like a resonant 1/2 wave slot antennae.I'd look at the possibility of running metal conduit (rigid or metal flexible with outer metal braid) between the two enclosures, with threaded or conductive compression end fittings to make them effectively a single Faraday cage. Depending on the possible earth fault currents, it may need a bonding strap between the cabinets. Then use screened cable within the conduit, with an anti-EMI ferrite ring right at the conduit end fitting in both cabinets, with its screen grounded at the source end to the source PCB. An extra ferrite right up against the connectors at each end would also help.I doubt anything less will keep out high energy local X band signals.What is being done to stop the EMI coming in on the other wiring?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 11, 2016, 11:37:27 pm
content: Quote from: Rerouter on January 11, 2016, 12:06:17 pmYeah, the hazard here is, okay, you might shield the outside microwaves, but is the environmentwithinthe enclosure noisy as well? If so, you might be worsening things, by inviting an internal ground loop.To solve that, you want the PCB (to which the cables attach -- the lettered points) pushed up as close to the enclosure as possible, and grounded to it with screws, or EMI fingers.You'll still have a ground loop for the internal board-to-board connections, but you did anyway, so this doesn't worsen anything.Note that, the enclosure itself can be used as a microstrip PCB, when the cables are routed tightly along the walls (where flux goes to zero, at least for lower frequencies). This is simply "proper lead dress", and goes a long way on its own.If it's quieter inside, then this absolutely produces an advantage, even without proper lead dress.Tim

author: daqq
author_rank: Super Contributor
timestamp: January 12, 2016, 01:35:36 pm
content: Hi guys,Thanks for your great insights!QuoteBasically, in the left enclosure there is a frequent current pulse, but the current path is well defined, so, no extra loops should form. Everything is isolated, controlled by isolated differential signals in shielded cables, so nothing nasty should form.QuoteYou mean the power? It's a full metal D Sub with the body connected to the chassis, connected to the cable shield. It does go further away though than the signal.QuoteThat's the worst case scenario, it'll be more along 30-45cm. Less with luck.QuoteWould connecting the shield of a SATA cable firmly to the enclosure on both ends do the trick? I assume that FB means ferrite beads?QuoteIn those cases it's transmission by capacitance rather than flux, isn't it?

author: dmills
author_rank: Super Contributor
timestamp: January 12, 2016, 04:22:02 pm
content: Shields should go directly to the chassis (with a 360 degree termination, NO PIGTAILS!).Do note that any resonant slots will radiate up there, and do consider the possibility of extending any openings you must have (cooling?) to form waveguides that are below cutoff at your interference frequency (You can get excellent attenuation this way).All lids and such should either be seam soldered or have EMC gaskets to reduce the leakage, and sometimes a double screening arrangement (box in a box) helps if you really need to keep the RF out.0402 or smaller caps and series terminating resistors to the plane will help, anything with leads will probably just appear inductive.I might actually be very tempted to go single ended out of the transformers on SMA or the like (Soldered to the box wall all the way around) and use good quality microwave SMA cables to hook things up (Sucoflex or similar).Regards, Dan.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 13, 2016, 05:51:53 am
content: Quote from: daqq on January 12, 2016, 01:35:36 pmProbably, and yes.I don't know how SATA cables are constructed, and can't seem to find a drawing of their build (and don't have any handy to sacrifice). If they are fully shielded (which would make them something like twinax for each lane, which is rather cool), then it's good.QuoteIn those cases it's transmission by capacitance rather than flux, isn't it?Yes, or even more precisely, the overlapping wires inside the transformer exhibit transmission line effects, which at low frequencies manifests as capacitance. But at frequencies on par with the electrical length of the windings, symmetry is broken and coupling occurs between CM and diff modes. (Shielded windings help, but these are rarely if ever used, especially in data signal transformers.)Tim

author: daqq
author_rank: Super Contributor
timestamp: January 13, 2016, 10:04:02 am
content: Thanks!QuoteThe signal pair cables themselves are fully shielded with a foil that is connected to three shielding cables that run the entire length of the cable. The differential impedance is 100Ohm. Yeah, they are cool and cheap for what they can do - certainly cheaper than anything similar I could cook up.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 13, 2016, 10:23:48 am
content: Foil counts more as screen than shield (which usually means braid, from what I've seen), but at these frequencies, that should be fine!Tim

author: daqq
author_rank: Super Contributor
timestamp: January 15, 2016, 07:13:11 am
content: Would using SMA connectors + coax cables be better? Say, have 2 feed throughs per differential signal pair on both sides. As I understand it, if both sides were connected to the enclosures, it might create some nasty induction in the shield, which would pass on to the inside of the cable.I've never done this sort of thing, could you please recommend some literature?

author: ConKbot
author_rank: Super Contributor
timestamp: January 15, 2016, 08:12:46 pm
content: Given that sata connectors arent shielded, and youre dealing with pretty high frequency interference, I'd avoid it. Run coax the whole way between the two enclosures. If youre doing low-ish frequencies you could use the bulkhead mount SMA/BNC adapters which just have a single center post for you to run a wire too. If you are doing higher frequencies, use a pass though SMA, so that it grounds to the case, and maintains 50 ohm though the connector, to the coax on the inside, to the PCB. Typical coax will be 30-50 dB of shielding, if you use low-loss LMR-195 (compatible with most RG-58 connectors) tack on another 20-30dB of shielding to that.If you need to pass lots of signals, that arent particularly high frequency, something like a mil-spec type 38999, a backshell adapter that supports connecting to a braid, and braided/foil shielded multiconductor cable, and some ferrite beads could work, but youre going to pay a good bit, especially for connector with EMI fingers. Tooling will get you too, some non-solder cup type pins can be soldered with care to make sure solder doesnt flow to naughty places, but an 8-indent crimper is much much easier. And if youre using a tinel-ring or band type braid termination, you need tooling for that too.Edit :d sub connectors can be emi shielded with proper back shells to terminate the braid to, and some connectors even have enhanced bumps or contact areas on the connector shell to make for better grounding to the mating connector, easier on the budget then a 38999 but not as sexy :p