Integrated spiral transformers

We will discuss the operating principles, design, fabrication and measured properties of an inductively coupled monolithic spiral transformer Devices like this one can be integrated into module substrates, and are practical for RF wireless applications operating at frequencies above several hundred MHz. The common model for a transformer is a pair of mutually coupled inductors. At high frequencies, however, capacitive coupling between the windings leads to non-ideal behavior and resonance, limiting the device's useful operating range. We show that the wide-band characteristics of transformers is closely related to that of coupled transmission lines, and we can rake advantage of this feature to obtain good transformer behavior at higher resonant modes.