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 Zo 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.
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