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The measurement of the close-to-the-carrier noise of rf and microwave devicesis a relevant issue in time and frequency metrology and in some fields ofelectronics, physics and optics. While phase noise is the main concern,amplitude noise is often of interest. The highest sensitivity is achieved withthe bridge (interferometric) method, which consists of the amplification andsynchronous detection of the noise sidebands after suppressing the carrier byvector subtraction of an equal signal. A substantial progress in understandingthe flicker noise mechanism of the interferometer results in new schemes thatimprove by 20--30 dB the sensitivity at low Fourier frequencies.  The article provides the complete theory and detailed design criteria, andreports on the implementation of a prototype. In real-time measurements, abackground noise of -175 -180 dBrad^2/Hz has been obtained at f=1 Hz off the100 MHz carrier. Exploiting correlation and averaging in similar conditions,the sensitivity exceeds -185 dBrad^2/Hz at f=1 Hz. A residual noise of -203dBrad^2/Hz at f=250 Hz off the carrier has been obtained, while the ultimatenoise 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 theobservation of weak flicker-type physical phenomena, out of reach for otherinstruments. We measured the flicker noise of a by-step attenuator (-171dB[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.Comment: 30 pages, 15 figures, 35 reference

Advanced interferometric phase and amplitude noise measurements