Superconducting thin-film transformers at microwave frequencies

We describe the use of thin-film superconducting transformers at frequencies up to 20 GHz. Transformers with turns ratios of 2, 4, and 7 were fabricated out of Nb and tested in a liquid-He Dewar. The measured S-parameters showed considerably less bandwidth than predicted based on consideration of the lumped element inductances of the circuit elements. We present an alternative model for the transformer where each part of the patterned circuit is treated as a distributed element (a transmission line). Despite the simplicity of the model, simulations using it agree well with the measured data without using any fitted parameters. These simulations take only a few minutes to compute on a typical workstation, making the model well suited as a design tool.