A noisy amplifier or a noisy oscillator connected to a reference source will do whatever it takes to follow the reference source signal. This applies for slow reference noise only which means noise is within the loop bandwidth. Every amplifier or oscillator circuitry has a defined bandwidth. Now if the source is very low noise then the noise from the amplifier/oscillator will be reduced by the negative feedback action. But noise above the loop bandwidth will be unaffected.
Loop bandwidth! If you are driving your car a sunny Sunday morning in a rural country side at 30 mph you can easily follow the highway, this is within the loop bandwidth. On the other hand if you would try to drive your car through the desert at 100 mph this would be outside the loop bandwidth. And you will have troubles the rest of the day.
G?ran
G?ran Krusell