Neither did I, it seems a little-known company “Intel” did a paper on this back in 2002 though. ;)

http://www.kip.uni-heidelberg.de/DCS-Board/datasheets/ethernet/MagneticlessEth.pdf

On a lot of ethernet resources on the web it does seem to refer to isolating/decoupling transformers used in the transmission line.
Why they had to call them “magnetics”, I haven’t worked out.

Uh, what? Most transformers/power isolation units are basically magnets or some form of electromagnet to protect your equipment. That is why this guy fried his units the first time. The idea is, if you don’t have the protection on a certain line, if say you get a power problem (lets say, power surge) on one device, that tiny amount can affect other devices or ethernet ports itself. I had an issue where the power in my house went out. My computer was on a battery backup, but xbox 360 wasn’t. Xbox 360 ethernet adapter has never worked, but the rest of my devices still do.

I used an app note once too. Do you want me to write it up and send you the link?

smoker_dave,
that’d be great. hopefully its an app note I’ve never read before – like this very one! looking forward to your submission.

@Matt
Despite being an electronic engineer for a good many years, I’ve never heard anyone refer to components that make use of electromagnetism as “magnetics” which is why I was questioning its origin. It’s incredibly broad and not at all useful in explaining what it actually is.
Inductors, just like capacitors, come in many sizes and costs.

@Matt Those RJ-45 connectors with integrated magnetics really have small transformers built into them. I’ve opened a couple and its there. Its a standard, don’t remember which one exactly.

@Sheldon
I would aggree with the term ‘magnetics’ perhapse you are from a different part of the world than I, but for those of us in the Digikey spectrum, refer to the section “Jacks with Magnetics” http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=1442739

Having designed several Ethernet interfaces in my career, I’ll chime in. I’ll assume we’re talking about BASE-T here, and not about classic coax Ethernet

The good ones all use magnetics (sorry, Sheldon, that’s what they’re called in the network biz) and transient suppressors. The cheap ones don’t. You can tell the good interfaces from the cheap ones by how they behave after a near miss during a lightning storm.

The magnetics are there to break ground loops and couple from an unbalanced source (like a 5V to gnd signal) to an unbalanced load (like a 120-ohm CAT5 twisted pair) and back again to the receiver. The fact that you’re putting a carefully engineered, balanced drive signal on the twisted pair goes a long way (literally) to keeping the error rate down and passing FCC emissions tests.

You can’t use opto isolators for this because they’re not fast enough. Check the specs. If you go optical, you use lasers.

The reason this isn’t done for more than on-board connections is because capacitive coupling has a nasty habit of coupling all sorts of noise onto the line. In audio lines with a shared ground, there’s an equalization “pop” on connection as the capacitor draws current to reach the equilibrium state, but that’s not the worst of it.

When you break the shared ground line and go differential, unlike a transformer, capacitors don’t protect you from common-mode noise. A shift in the ground potentials between the two ends will spike both ends of the differential pair – and while that won’t effect the differential receiver for small spikes, it’ll happily couple spikes large enough to cause damage.

Transformers’ natural resonance is a convenient bandpass filter and it also conveniently decouples the (variable) capacitance of the actual cabling. I.e. the circuit above is generally fine in the lab but will never work in the real world. Rest assured that a whole lot of really smart network engineers have been trying to get magnetics out of the equation for many years as they are the largest and most expensive component in more ways than one.

WTF, this person uses an app note and then gets a write-up like they actually did something?

LOL

Let’s see some creativity, people. This is not a ‘hack’!!!

Can someone explain in simpler terms why ethernet jacks require “magnetics”?