I'm working on a DC jack powered Ethernet design and I've downloaded many Ethernet Layout guidelines from many semi vendors with varying recommendations. I've read app notes recommending almost every possible termination resistor position, for example. Placing termination resistors at the PHY, at the Magnetics, the TX at the PHY and the RX at the magnetics, and visa versa. The most popular seem to be at the PHY, and this seems to make the most sense. Ethernet uses balanced differential pairs, which are typically terminated at the extremes to filter any common mode noise injected into the transmission lines, and the RX / TX traces on the board constitute part of the transmission line (these are being run at 100 ohm impedance to match CAT5 cable impedance).
The other controversy here is what to do with the ground plane. If this wasn't a DC jack powered app my life would be easier. Many app notes recommend no ground plane under the magnetics (which are built into the RJ45 connector in my case) to avoid coupling into the ground plane. But… that is exactly what I want. Better coupling into the ground plane then into the conformity testing antenna! A ground plane under the jack will help close the metal enclosure around the rest of the connector. I've read at least one example of anecdotal evidence on the net claiming better radiation performance with a solid ground plane in a DC jack application as opposed to a separate isolated Ethernet plane tied in with caps. So… I think I'm going to keep a solid plane under the RJ45 jack.
Some papers also recommend no plane under the RX / TX pairs. I can't make my mind up about this. I want to avoid coupling any ground noise into the RX and TX pairs but my experience seems to be any ground plane splitting / opening is usually based on hocus pocus type thinking instead of sound physics.
Does anyone here have any experience or suggestions related to Ethernet layout, specifically with regard to the RX / TX termination resistor placement and whether or not to use a ground plane under the RJ45 connector (with magnetics) as well as under the TX / RX pairs? Any suggestions greatly appreciated.
Best Answer
Look for application notes for your PHY and magnetics. The manufacturer would know best in regards to what works with their parts.
Generally there is no ground/power or routing under the magnetics and try to avoid ground/power under the TX/RX pairs. If you can't route the whole trace without a ground/power plane under it, leave the plane under it. It is worse if you go over a break in the plane.
For termination, check with manufacturers of the PHY and magnetics. Like you said, there are a few different schemes, the manufacturer should know best about their device.
We follow what I described above at work and don't have any problems with ethernet.