  The noise of a device under test (DUT) is measured simultaneously with two
instruments, each of which contributes its own background. The average cross
power spectral density converges to the DUT power spectral density. This method
enables the extraction of the DUT noise spectrum, even if it is significantly
lower than the background. After a snapshot on practical experiments, we go
through the statistical theory and the choice of the estimator. A few
experimental techniques are described, with reference to phase noise and
amplitude noise in RF/microwave systems and in photonic systems. The set of
applications of this method is wide. The final section gives a short panorama
on radioastronomy, radiometry, quantum optics, thermometry (fundamental and
applied), semiconductor technology, metallurgy, etc. This report is intended as
a tutorial, as opposed to a report on advanced research, yet addressed to a
broad readership: technicians, practitioners, Ph.D. students, academics, and
full-time scientists.
