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 a 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.