A well designed transformer must have:

1. low series resistance in the primary and secondary windings

2. high magnetic coupling between the primary and the secondary

3. low capacitive coupling between the primary and the secondary

4. low parasitic capacitances to the substrate

An integrated transformer generally comprises two spiral inductors with strong magnetic coupling. To arrive at planar structure, we begin with a symmetric inductor and break it at its point of symmetry. Segments AB and CD now act as mutually coupled inductors. We consider this structure a 1 to 1 transformer because the primary and the secondary are identical.

The transformer structure suffers from low magnetic coupling, an asymmetric primary and an asymmetric secondary. To remedy the former, the number of turns can be increased but at the cost of higher capacitive coupling. To remedy the latter, two symmetric spirals can be embedded as shown but with a slight difference between the primary and secondary inductances. The coupling factor in all of the above structures is typically less than 0.8.

An integrated transformer can be viewed as two inductors having magnetic and capacitive coupling. The figure shows an example, where the primary and secondary are represented by the compact inductor model with the mutual coupling M and coupling capacitor Cf added.

Due to complexity of this model, it is difficult to find the value of each component from measurements or field simulations that provide only S or Y parameters for the entire structure.