  The frequency flicker of an oscillator, which appears as a 1/f^3 line in the
phase noise spectral density, and as a floor on the Allan variance plot,
originates from two basic phenomena, namely: (1) the 1/f phase noise turned
into 1/f frequency noise via the Leeson effect, and (2) the 1/f fluctuation of
the resonator natural frequency. The discussion on which is the dominant
effect, thus on how to improve the stability of the oscillator, has been going
on for years without giving a clear answer. This article tackles the question
by analyzing the phase noise spectrum of several commercial oscillators and
laboratory prototypes, and demonstrates that the fluctuation of the resonator
natural frequency is the dominant effect. The investigation method starts from
reverse engineering the oscillator phase noise in order to show that if the
Leeson effect was dominant, the resonator merit factor Q would be too low as
compared to the available technology.
