What is Angle- and Order- Analysis?

When analysing rotating machinery, one may use time or rotation as the reference. A time signal can be resampled from a fixed time resolution, dt, to a fixed angle resolution with the help of the tacho signal. This is what the tacho pulse looks like when resampled from the time domain to a fixed angle resolution (see Figure 1):   Figure 1. A tachometer signal resampled from the time domain to the angle domain. One revolution is one square pulse period. The machine is speeding up, as can be seen in the square pulse width being shortened in the time record. (Click figure to enlarge) When...