  Based on the principles of microwave circuit interferometry we have
constructed a Reduced Noise Amplifier (RNA) with power spectral density of
phase, as well as amplitude, fluctuations close to -170 dBc/Hz at 1 kHz offset.
The RNA has been incorporated with a cryogenic sapphire resonator as a loop
oscillator whose noise performance is governed by the Leeson's model. Following
this model and using the results of in-situ measurements of gain and phase
fluctuations of the RNA we inferred noise properties of the oscillator. In
particular, for a signal transmitted through the resonator we found that power
spectral density of its phase/amplitude fluctuations must be close to -185
dBc/Hz at offsets above 300 Hz. We discuss a few approaches that will allow
direct measurements of such low levels of noise.
