  The measurement of the close-to-the-carrier noise of rf and microwave devices
is a relevant issue in time and frequency metrology and in some fields of
electronics, physics and optics. While phase noise is the main concern,
amplitude noise is often of interest. The highest sensitivity is achieved with
the bridge (interferometric) method, which consists of the amplification and
synchronous detection of the noise sidebands after suppressing the carrier by
vector subtraction of an equal signal. A substantial progress in understanding
the flicker noise mechanism of the interferometer results in new schemes that
improve by 20--30 dB the sensitivity at low Fourier frequencies.
  The article provides the complete theory and detailed design criteria, and
reports on the implementation of a prototype. In real-time measurements, a
background noise of -175 -180 dBrad^2/Hz has been obtained at f=1 Hz off the
100 MHz carrier. Exploiting correlation and averaging in similar conditions,
the sensitivity exceeds -185 dBrad^2/Hz at f=1 Hz. A residual noise of -203
dBrad^2/Hz at f=250 Hz off the carrier has been obtained, while the ultimate
noise floor is still limited by the averaging capability of the correlator.
This is equivalent to a S/N ratio of 2E20 with a frequency spacing of 2.5E-6.
  Applications include the measurement of the properties of materials and the
observation of weak flicker-type physical phenomena, out of reach for other
instruments. We measured the flicker noise of a by-step attenuator (-171
dB[rad^2]/Hz at f=1 Hz) and of the ferrite noise of a reactive power divider
(-173.7 dB[rad^2]/Hz at f=1 Hz) without need of correlation.
