Mechanical signals produced by rotating or reciprocating machines are remarkably well modelled as cyclostationary processes. The cyclostationary family accepts all signals with hidden periodicities, either of the additive type (presence of tonal components) or multiplicative type (presence of periodic modulations). This happens to be the case for noise and vibration produced by gear mechanisms, bearings, internal combustion engines, turbofans, pumps, propellers, etc. The explicit modelling of mechanical signals as cyclostationary processes has been found useful in several applications, such as in noise, vibration, and harshness (NVH) and in condition monitoring. In the latter field, cyclostationarity has been found to generalize the envelope spectrum, a popular analysis technique used in the diagnostics of bearing faults. One peculiarity of rotating machine signals is that the period of the process is strictly linked to the "angle" of rotation of a specific component – the “cycle” of the machine. At the same time, a temporal description must be preserved to reflect the nature of dynamical phenomena that are governed by differential equations of time. Therefore, the angle-time autocorrelation function is used,  where formula_42 stands for angle, formula_43 for the time instant corresponding to angle formula_42 and formula_45 for time delay. Processes whose angle-time autocorrelation function exhibit a component periodic in angle, i.e. such that formula_46 has a non-zero Fourier-Bohr coefficient for some angular period formula_47, are called (wide-sense) angle-time cyclostationary. The double Fourier transform of the angle-time autocorrelation function defines the order-frequency spectral correlation,  where formula_49 is an "order" (unit in "events per revolution") and formula_50 a frequency (unit in Hz).