A low-flicker scheme for the real-time measurement of phase noise

The paper presents a new scheme for the measurement of phase noise in real time, based on carrier suppression and synchronous detection of the noise sidebands of the device being tested. In instruments of the interferometric type, the carrier is suppressed by adding an equal and opposite signal that must be adjusted with a phase shifter and an attenuator. The proposed scheme makes use of a dual adjustment of the carrier suppression, coarse and fine. The former is by-step, while the latter is continuous. Owing to the higher stability of the by-step adjustment, and to the lower weight of the continuous adjustment in the suppression circuit, the instrument performs intrinsically low residual flicker and low microphonicity. A prototype shows a residual flicker as low as -160 dB rad²/Hz at 1 Hz off the 100 MHz carrier. Applications include the noise characterisation of components and the design of innovative ultrastable oscillators