I'm finishing off the layout of a board in Eagle which has a LAN8710A 100Mbps Ethernet PHY on it. The SMSC documentation is really pretty good but I'm stuck on the important detail of how to do the 100\$\Omega\$ differential pairs for the Rx/Tx. I understand this is important, but what I don't quite get is how to actually calculate the balanced impedance.
From what I understand, I can use this PCB Impedance and Capacitance Calculator (Microstrip) to calculate the characteristic impedance of the individual traces to make them 50\$\Omega\$, then use this PCB Impedance and Capacitance Calculator (Differential Microstrip) to calculate the differential impedance of the two tracks routed together. If this is wrong, please somebody point out my error.
However, the bit I don't understand is that both of those calculations require a distance to the ground plane. This is fine for the tracks from the PHY to the magnetics, but the SMSC app notes recommend no planes beneath the tracks from the magnetics to the connector, so how is this supposed to be calculated?
I'm confused. If someone out there can offer me some pointers it'd be much appreciated.