Leeson's equation is an empirical expression that describes an oscillator's phase noise spectrum.
Leeson's expression[1] for single-sideband (SSB) phase noise in dBc/Hz (decibels relative to output level per hertz) and augmented for flicker noise:[2] where f0 is the output frequency, Ql is the loaded quality factor, fm is the offset from the output frequency (Hz), fc is the 1/f corner frequency, F is the noise factor of the amplifier, k is the Boltzmann constant, T is absolute temperature, and Ps is the available power at the sustaining amplifier input.
The common misunderstanding, that Ps is the oscillator output level, may result from derivations that are not completely general.
In 1982, W. P. Robins (IEE Publication "Phase noise in signal sources") correctly showed that the Leeson equation (in the −20 dB/decade region) is not just an empirical rule, but a result that follows from a linear analysis of an oscillator circuit.
So for accurate results, fc must be determined from added noise measurements on the amplifier using R.F., with the actual circuit configuration to be used in the oscillator.