Superconducting thin-film transformers at microwave frequencies

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