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A substantial progress in understanding the flicker noise mechanism of the interferometric phase noise measurements results in new scheme that improves the sensitivity at low Fourier frequencies by 20-30 dB upon the previous instruments [Rev. Sci. Instr. 73 (6) 2445-2457 (2002)]. The new scheme also features automatic carrier suppression and simplified carrier suppression. Two prototypes have been constructed, designed for the carrier frequency of 100 MHz and 5 MHz, with similar performances. A residual noise of -175 to -180 dB dBrad2/Hz has been obtained at f = 1 Hz off the carrier in real-time measurements, while exploiting the correlation-and-averaging method in similar conditions the sensitivity is some -182 dBrad2/Hz (parallel detection), and -186 dBrad2/Hz (± 45° detection) at f = 1 Hz. A residual white noise of -203 dBrad2/Hz has been obtained at f = 250 Hz off the carrier, while the ultimate noise floor is still limited by the averaging capability of the correlator. The instruments have been tested measuring the noise of by-step attenuators and 180° hybrids, which is below -170 dBrad2/Hz at f = 1 Hz.