Design of the flux-flow type Josephson oscillator inductively coupled to a stripline impedance transformer

In order to realize efficient coupling between a flux-flow type Josephson oscillator (FFO) and a stripline impedance transformer, the authors have made a theoretical study of the inductive coupling between FFO and a stripline. It is shown that by choosing the output impedance of the FFO Z_o smaller than the characteristic impedance of the coupling section Z₁, the frequency dependence of the output power at the end of the coupling section can be extremely flat over a large frequency band. A model simulation employing a NbN/Pbin Josephson junction and a Nb/NbN stripline demonstrates that in the case of optimum design Z₀=0.6 Z₁, the bandwidth of the coupling section, which is defined by the frequency range where the deviation is within +or-5%, is shown to be as large as 80% of the centre frequency.