Why the oscillator phase noise flat out when it is far away from the center frequency

why the oscillator phase noise flat out when it is far away from the center frequency instead of 1/f^2?

Does anyone has a good explanation?

Thanks,

One limit is the thermal noise floor. Also, you might see wideband noise from your active devices that isn't filtered by the resonator.

exactly. Another way to look at it...most of the source of noise (traps, 1/f, etc) are all AUDIO FREQUENCY noise effects. after a MHz or so, they are all done with....to those effects can not mix up to the carrier frequency and cause havoc much further than +/- 1 MHz from the carrier.