Frequency stability of an oscillating maser: Analysis of the effect of an external feedback loop

The noise temperature of the microwave cavity of an oscillating maser operated with an external feedback loop is given. The theoretical frequency stability of the signal delivered by the maser is deduced and the effect of the external feedback loop is discussed. Short- and mid-term frequency stabilities achievable with a conventional hydrogen maser with and without an external feedback loop and with a compact hydrogen maser operated either in active or in passive modes are given.