Superconducting Nb cavities of high Q factor have been used to stabilise Gunn-effect oscillators. These stabilised oscillators have given an r.m.s. frequency deviation σ(N = 2, r = 1.004, τ = 10) of (1.2±0.1)×10−13 for an averaging time of 10s.
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Stein et al. (1972) studied this question.