The ±45° correlation interferometer as a means to measure phase noise of parametric origin

A radiofrequency interferometric detector is combined with the correlation-and-averaging technique in a new scheme for the measurement of the phase noise of a component. The method relies upon the assumption that the phase noise of the component being tested (DUT) exceeds the amplitude noise, which is consistent with the general experience in the field of wireless engineering. The new scheme is based on the amplification of the DUT noise sidebands and on the simultaneous measurement of the amplified noise by means of two mixers driven in quadrature, ± 45° off the carrier phase. The ±45° detection has two relevant properties, namely 1) the sensitivity is neither limited by the thermal energy k_BT₀, nor by temperature uniformity, and 2) the noise of the measurement amplifier is rejected, despite a single amplifier being shared by the two channels of the correlator. The article provides the theoretical background and experimental results. The sensitivity of the first 100-MHz prototype, given in terms of the S_φ(f) floor, is some 12 dB below k_BT/P₀, where P₀ is the carrier power. Using a dual carrier suppression scheme, the residual flicker is as low as -168 dBrad²/Hz at f=1 Hz off the carrier.