I. Introduction
An oscillator consists of a resonator in closed loop with a sustaining amplifier that compensates for losses. The frequency stability is limited by the noise of the amplifier through the Leeson effect [1] and by the fluctuation of the resonator's natural frequency. The stability of the resonator's natural frequency is by far the most important parameter that limits the long-term stability. In turn, the resonator's natural frequency is affected by environmental parameters and aging. In contrast, the noise of the sustaining amplifier affects the phase noise and the short-term stability. When very-long-term stability is the most important parameter, as in timekeeping and in radionavigation systems, atomic resonances are the only viable frequency references. In this case, a flywheel oscillator is frequency locked to the atomic resonance. On the other hand, macroscopic-cavity resonators show several advantages versus the atomic resonators because of their simplicity, reliability, and power-handling capability. Higher power results in higher signal-to-noise ratio, and ultimately in low phase noise and high short-term stability. Ultimate stability in the range of 1 to s measurement time is of paramount importance in physical experiments involving long averaging and, of course, in radioastronomy. In this paper, we demonstrate for the first time a microwave oscillator based on a macroscopic resonator with a frequency stability at long integration times that is competitive with those of classical microwave atomic clocks.