Accurate long-term phase noise measurements are performed by a mixer, a personal computer, and a general-purpose interface. When absolute noise has to be characterized, it is possible to have measuring sessions virtually infinite in time, utilizing a simple discrete phase shifter realized by switching three coaxial cables of different lengths. Accurate phase retrievement is accomplished by antitransforming the ideal sinusoidal transfer function of the mixer, while its higher spurious responses are minimized by a careful optimization of its operating parameters. This configuration has been successfully used to estimate the Allan variances from 1 s up to 100,000 s of the phase comparison of two H-maser atomic standards, for almost nine days, without the need of any frequency offset between the two sources, which were used at the same time as timekeepers. A computer simulation is also presented to predict a priori the level of inaccuracy determined by the higher-order mixer nonlinearities and the final effect of each experimental parameter.
