A Revisit To Phase Noise Model Of Leeson

Leeson´s is one of the most famous models for predicting the phase noise in feedback oscillator. But there are several limitations and drawbacks. Leeson equation involves some key parameters, and these parameters are often determined by the oscillator structure and the oscillator circuit itself. Then a directly application of the Leeson´s model without care would lead to erroneous results. Leeson´s also assumed that the amplifier gain is remained a constant versus the frequency close the carrier frequency, and the filter transfer function is considered symmetrical on both sides of the carrier frequency. For a state of the art crystal oscillator the flat noise floor should be about -180 dBc/Hz. The Leeson equation does not obviously describe these oscillators well, and the flat noise floor is about 15 dB lower than that would be expected from the Leeson model. In this paper, a detailed analysis is performed to enlighten the key parameters and try to make it valid for all oscillator circuits. It explicitly takes needed parameters into account for phase-noise calculation, especially for the detailed descriptions of the flicker corner frequency (f_c) and the load Q-factor of oscillator circuits. In this paper, a phase flat noise floor model for oscillator is also described. This model allows us to specify that the flat noise floor is about -180 dBc/Hz. An example of the phase noise of the colpitts quartz oscillator is also simulated and discussed.