HI All,
I have designed a board (4-layer) which is using the w5500 and the modular jack 7499210121A from Wurth. We failed EMC pre-compliance because of a 225MHz and 275MHz frequency peaks. I identified that these emissions are coming from the ethernet part of our design. It looks we have impedance mismatching. We are re-designing the board and we’ve rerouted the differential pairs. The calculated differential impedance is 100.6 Ohm and the individual track impedance is 56ohm. I don’t think that the 56ohm will cause us any issue (50ohm wasn’t easy to achieve).
The total differential pair length for the RX is 12.93mm for RX+ and 12.88mm for RX- .
The total differential pair length for the TX is 20.12mm for TX+ and 20.11mm for TX-.
What is better to do with the ground plane? Should i route the differential pair to the modular jack having ground plane underneath it or I should stop the ground plane in the edge of the modular jack connector and continue the differential routing without it?
I have attached you a photo with the routing. Where is green there isn’t ground or power planes.
Thank you in advance.
