air core transformer
You do two spiral inductors on adjacent PC card planes. It is best if they are offset so that the mutual capacitance is less. To make the center tap on each, you actually do four spiral inductors on 4 planes and hook them up in pairs in series. This is less than ideal being air core because the mutual inductance is less than a ferrite core version, but it is the best you can do.
You can go one further with putting a hole in the center and a few holes around the outside circumfrence of the coils and put two sort of ferrite cup cores over the front and back of the board. This is relatively expensive.
The smaller mutual inductance will make the voltage and impedance ratios different than an ideal transformer.