Electronic – How to connect CPU with RGMII pins to an LTE module that takes only SGMII signals

• First purpose of the magnetics on PHY is create a BALUN ( or interface BALanced line to UNbalanced IC and visa versa) This improves the Common Mode Rejection Ratio CMRR significantly over the full signal bandwidth.

• Secondary requirements are for impedance matching.

• Third requirements are by improving CMRR is to reduce radiated CM noise.

• Fourth is provide immunity to expected EM fields , ESD etc.

  1. When stray common mode magnetic fields are coupled to nearby unbalanced lines, it defeats the purpose. Due to inverse square law, couple after about twice the size of the magnetic core may be sufficient to achieve adequate CMRR but being unbalanced signal and ground impedances, making the path long exposes it to other sources of noise than be converted from CM to differential mode due to differences in coupling of different impedances.

  2. Magnetic core in 100MHz range and up tends to be conductive ceramic mix and also susceptible to conductive coupling of ESD as opposed to the more insulative LF high mu ferrite cores.

If your MCU has an ethernet PHY built in, you would normally use that. But many MCUs do not have that available. Even so, here are several reasons I can think of why you might not want to use an internal PHY.

1) Design reuse. If you have a previous design that used an external PHY (perhaps on an earlier MCU that didn't support an internal PHY), you may save on development time by using the same design and code.

2) Physical separation. You typically want to put the PHY as close as possible to the magjack. If your MCU has to be far from the jack, it may make sense to run the MII signals across the board.

3) Different physical medium: you might want to use a fiber optic PHY, either as standard or an option.

4) No PHY: For short-range communication (say between modules in a single chassis), you can even skip the PHY entirely and connect the RMII signals directly. In particular, if you are making something like a SOHO router, the on-board MCU might connect to the switch this way, avoiding going through two PHYs.