9
\$\begingroup\$

If I am going to use a PCB mount RJ45 connector for 100mbps Ethernet, does it need to be a shielded connector, or can I get away with using an unshielded one?

Unshielded RJ45 connector

Is there some standard which deals with this, or is it just my responsibility to get the thing EMC tested.

\$\endgroup\$
  • \$\begingroup\$ Well isn't the shield there so STP cables could have something to mate with? The idea, if I get it correctly, is to have shielded cable, shielded 8P8C jack, shielded receptacle and then have shield along the whole way. \$\endgroup\$ – AndrejaKo Apr 18 '12 at 22:25
  • 4
    \$\begingroup\$ @AndrejaKo - But most Ethernet cables that I have seen don't have shielded connectors. And most ethernet sockets seem to be shielded. Maybe they're shielded just in case you happen to use a shielded cable connector? \$\endgroup\$ – Rocketmagnet Apr 18 '12 at 22:29
  • \$\begingroup\$ I believe that it is so. How much would the shielded socket complicate the design? From what comes to my mind, you'd basically get full STP compatibility for pretty much small investment, so unless there is a good reason to save as much money as possible, people just put them there. \$\endgroup\$ – AndrejaKo Apr 18 '12 at 22:35
  • \$\begingroup\$ The reason is not money. I'm trying to squeeze the connector into a 15.5mm space. And I'm having trouble finding a shielded one that's less than 16.5mm wide. Un-shielded ones are available in 15mm, perfect. \$\endgroup\$ – Rocketmagnet Apr 18 '12 at 22:39
8
\$\begingroup\$

There does not need to be a shield. Standard ethernet cable has no shield, so even if the socket were shielded it wouldn't do much for the data on the cable.

I think the reason there are shielded RJ-45 jacks is so that you can make a reasonably tight chassis without a gap in it at the connector, but that is just speculation on my part. The way to deal with ethernet EMI is to use the right transformer. Good ones have a balun on the network side of the differential pairs. Sometimes that's enough by itself, and sometimes you need to put small caps, like 22 pF, on each line to ground. Note that the caps then limit the isolation voltage between the ethernet and your device. That may not matter, but you need to think about it. If necessary, use high voltage caps.

\$\endgroup\$
  • 4
    \$\begingroup\$ "the reason there are shielded RJ-45 jacks is so that you can make a reasonably tight chassis without effectively a gap in it at the connector" -- also my understanding. The shield is to block radiation from other parts inside your box from getting out through the hole in the chassis and to prevent unwanted radiation getting in to sensitive parts inside as well. \$\endgroup\$ – The Photon Apr 18 '12 at 23:26
  • \$\begingroup\$ "reasonably tight chassis" ? Is that right? Isn't it actually the Faraday cage effect of having the sensitive or high-emission components in a metal cage, not just a "tight fit?" \$\endgroup\$ – Craig Apr 22 '15 at 19:31
  • 1
    \$\begingroup\$ @Craig: Yes, it's a Faraday cage, but that's exactly what a reasonably tight chassis is. Faraday cages leak at frequencies where the holes are a significant fraction of the wavelength. It is the maximum dimension of a hole that matters. For example, a long thin slit is worse than a bunch of small round holes adding up to the same overall area. This is why you often see ventillation openings as a hexagonal array of small holes. At a few GHz, a hole the size of a ethernet connector can start to become significant. \$\endgroup\$ – Olin Lathrop Apr 22 '15 at 21:08
  • \$\begingroup\$ Oh, well, since you put it that way. ;-) So to minimize emissions from a device with multiple ports, you'd really want a metal plug in each of the unused ports, in addition to shielded connectors in the utilized ports? I presumed the shielded connectors were also about isolating the cable to the very end, and grounding it. \$\endgroup\$ – Craig Apr 23 '15 at 1:51
  • \$\begingroup\$ @Craig: Yes, metal plugs are sometimes used for this purpose. A common example is a tower PC chassis. These come with metal pieces covering up the unused holes where PCI cards would poke out. When you install a real card, you remove the metal piece first. \$\endgroup\$ – Olin Lathrop Apr 23 '15 at 12:08
10
\$\begingroup\$

One advantage of using shielded jacks is that the soldered shield tabs provide a very mechanically robust anchor to the PCB, reducing the chance of damage if the cable gets yanked.

\$\endgroup\$
  • \$\begingroup\$ Yes, that is a good point, and I would much rather have a shield. But I am starting to think that I won't be able to find one which is no more than 15.5m across. \$\endgroup\$ – Rocketmagnet Apr 18 '12 at 23:08
2
\$\begingroup\$

See my similar question over here:

safe to use an unshielded jack for 10Mbps Ethernet?

I've had no problems running a 10MBps link with an unshielded jack, but I don't know if there's any EMI emission from it. I'd definitely suggest using a shielded jack for 100/1Gbps just to be on the safe side.

Regarding EMC - if you're doing it for a personal project, probably don't worry about it. If you want to sell it (at least in the US) you need to get FCC Part 15 tested; similar rules apply for the EU.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.